Research Relevant Conditions and Pathology in Nonhuman Primates

Abstract

Biomedical research involving animal models continues to provide important insights into disease pathogenesis and treatment of diseases that impact human health. In particular, nonhuman primates (NHPs) have been used extensively in translational research due to their phylogenetic proximity to humans and similarities to disease pathogenesis and treatment responses as assessed in clinical trials. Microscopic changes in tissues remain a significant endpoint in studies involving these models. Spontaneous, expected (ie, incidental or background) histopathologic changes are commonly encountered and influenced by species, genetic variations, age, and geographical origin of animals, including exposure to infectious or parasitic agents. Often, the background findings confound study-related changes, because numbers of NHPs used in research are limited by animal welfare and other considerations. Moreover, background findings in NHPs can be exacerbated by experimental conditions such as treatment with xenobiotics (eg, infectious morphological changes related to immunosuppressive therapy). This review and summary of research-relevant conditions and pathology in rhesus and cynomolgus macaques, baboons, African green monkeys, common marmosets, tamarins, and squirrel and owl monkeys aims to improve the interpretation and validity of NHP studies.

Introduction

Animal research has played a critical role in many medical advances that saved or improved millions of human lives. It is estimated that 12–27 million animal procedures are being carried out per year in the United States alone, wherein rats and mice are the most commonly used species.¹ Nonhuman primates (NHPs) represent a very small proportion of the total number of animals used in experiments. According to the new USDA figures, US scientists used 70 797 NHPs for research in 2018.² In the European Union, 8235 NHP were used in 2017.³ NHPs often serve as an important link between basic science and human clinical applications due to their phylogenetic proximity to humans and their similarity in responses to treatment and pathogenesis of disease as observed in humans.

Owing to the high cost, scarcity, ethical considerations, and being the most demanding species used in biomedical research, studies are typically comprised of few NHPs. Small numbers in experimental groups may add challenges to interpretation of data, particularly of the microscopic changes in tissues related to experimental conditions. It is important that the pathologists and laboratory animal clinical veterinarians are able to identify lesions caused directly by treatment or disease with high confidence and differentiate them from incidental changes. Incidental lesions not related to experimental conditions are often referred to as “background” changes, and these changes arise spontaneously.⁴ The background changes in tissues can occur either before or during the study and can be affected by the nature of genetic variations within NHPs (ie, due to geographic origin), environmental conditions to which the NHPs were exposed, exposure to infectious agents, or normal aging changes. In addition, background findings can be induced or exacerbated by experimental conditions. For example, nearly all Old World and New World NHPs harbor infectious agents such as cytomegaloviruses and lymphocryptoviruses as lifelong persistent latent infections. During experimental immunosuppression, these latent viruses may reactivate, causing progressive cytolytic or lymphoproliferative diseases, respectively.^5–8

In this review, background histologic lesions in commonly used NHP species are discussed, which include Old World primates: rhesus macaques (Macaca mulatta), cynomolgus macaques (M. fascicularis), baboons (Papio spp.), and African green monkeys (Chlorocebus spp); and New World primates: common marmosets (Callithrix spp.), tamarins (Saguinus spp.), squirrel monkeys (Saimiri spp.), and owl monkeys (Aotus spp.). It is impossible to capture all the possible spontaneous lesions that can occur in NHPs, and their incidence varies within a given population over time and between different laboratories and is highly dependent on the geographical origin. A lengthy description covering all the potential background lesions in NHPs is beyond the intent of this review. We refer readers to the excellent extensive reviews on this topic, including: Simmons 2016,⁹ Sato et al 2012,¹⁰ Chamanza et al 2010,¹¹ Kaspareit et al 2006,¹² David et al 2009,¹³ and the International Harmonization of Nomenclature and Diagnostic Criteria for Lesions Project on proliferative and nonproliferative lesions in NHPs (INHAND Nonrodent working groups , unpublished data).

Rhesus Macaque (Macaca mulatta)

Primates of the genus Macaca are one of the major lineages of the Old World monkeys. They originate from Asia to North Africa, where they live in complex medium to large groups and are highly adaptable to a wide range of habitats from grasslands to forest. All Macaca are characterized by medium body size, a dense short fur with little to no fur on their face, large pouches in their cheeks, and similar forelimb and hindlimb length.¹⁴

The rhesus macaque (Macaca mulatta) is native to South to Southeast Asia (from India to China). Compared with the 10 subspecies of M. fasciculata, M. mulatta has no recognized subspecies but displays extreme genetic and phenotypic diversity (ie, physiology, morphology, and behavior). Mitochondrial DNA analyses show that Indian rhesus have low genetic diversity, with the exception of 1 group originating from Burma, and they have diverged substantially from the heterogeneous Chinese rhesus group.^15–17 Single nucleotide polymorphism analyses corroborate high genomic divergence between Indian and Chinese origin rhesus, with numerous population-specific single nucleotide polymorphisms.¹⁸ In addition, based on mitochondrial DNA and microsatellite genotyping, Satkoski et al¹⁷ noted greater heterogeneity of captive-inbred Chinese rhesus compared with wild-caught, attributed to interbreeding in captivity of wild populations from the 3 main geographic regions: eastern, western (the most heterogeneous), and southern. Phenotypic variations are specific to the country of origin for the immune system, such as origin related difference in major histocompatibility complex or immune responses to Simian immunodeficiency virus (higher survival of Chinese origin compared with Indian origin)^19,20 or reproductive differences between Indian and Chinese origin.²¹

With regard to biomedical studies, the rhesus macaque from India, Vietnam, or China is one of the two most used primate models in non-clinical research. They are typically bred in variably sized, mixed-sex social groups in which females outnumber males. The rhesus macaque is important in research on infectious disease (ie, immunodeficiency virus), immunology, reproductive physiology, and aging.^22–24 It is also used in the preclinical safety testing, although the cynomolgus macaque (M. fascicularis) has become more prevalent in part due to its smaller size.²⁵  Tables 1 and 3 summarize research-relevant common conditions and diseases (background findings) in rhesus macaques and those exacerbated with conditions of immune suppression.

Table 1.

Tabulated Overview of Morphological Background Findings in Rhesus Macaques (Macaca mulatta)

Organ	Findings	Comments
Multisystemic
	Amyloid deposits82–84	Mostly in spleen, liver, gastrointestinal tract (causing amyloid enteropathy), adrenal, liver, kidney (medullary interstitium), lymph node, and blood vessels
	Lymphoma9,82,85	Especially in bone marrow and lymphoid organs but can be observed in multiple organs. Often indicative of lentiviral infection (Simian Immunodeficiency Virus- SIV).
Cardiovascular system
Heart	Hemorrhage83	Mainly subendocardial
	Inflammatory cell infiltrate, myocardium (myocarditis)9,86	Often focal and mononuclear cells (idiopathic). Multifocal in association with myocardial necrosis and mineralization and pulmonary edema, indicative of Encephalomyocarditis virus (EMCV).87
	Inflammatory cell infiltrate, perivascular (perivascular inflammation)86
	Degeneration/necrosis, cardiomyocyte (focal myocarditis)9,86	Often associated with mononuclear cell infiltration
	Hypertrophy, cardiomyocyte9,88	Normal aging change; also associated with left ventricular cardiac hypertrophy
	Fibrosis, interstitial82
	Mineralization, valvular9
	Septal defect, atrial and/or ventricular (septal defect)89	Rare congenital disease associated with right ventricular hypertrophy
	Aorta originating from right ventricle (double-outlet right ventricle)89	Rare congenital disease associated with subpulmonary stenosis, dilated pulmonary artery, and double-septal defect.
	Persistent truncus arteriosus90	Rare congenital disease associated with enlarged ventricle and often septal defect
	Cyst, squamous/epithelial cell91
	Ectopic thyroid tissue91
	Endocarditis, valvular9	Due to systemic bacterial or fungal infection
	Endocardiosis, valvular9	Age-related chronic nodular thickening of valve
	Hyperplasia, mesothelium92	Papillary thickening of mesothelium
Blood vessels	Vascular thickening (arteriosclerosis)9,83,84	Hyaline (mucopolysaccharide accumulation) or hyperplastic with thickening of vascular walls with loss of structural detail
	Degeneration/necrosis, media (vasculitis)86,93	Associated with inflammatory cell infiltrates
Endocrine system
Adrenal gland	Mineralization94,95
	Ectopic liver91
	Infiltrate, inflammatory cell83
	Vacuolation, cortical, increased91
	Hypertrophy, cortical cell91	Often associated with vacuolation
	Adenoma85
Thyroid gland	Infiltrate, inflammatory cell83
	Hyperplasia, C-cell83
	Ectopic thymus91
	Adenoma85
Pancreas (endocrine)	Adenoma, islet cell85
	Amyloid deposition, islet9	Islet-associated polypeptide is precursor protein; associated with glucose dysregulation and diabetes mellitus
Parathyroid gland	Ectopic thymus91
Pituitary gland	Adenoma85,96
Gastrointestinal System
Tongue	Infiltrate, inflammatory cell83
Teeth	Caries82
	Inflammation (gingivitis)82
Salivary gland	Mononuclear cell infiltrate83
	Adenoma85	Parotid gland
Stomach	Inflammation, acute (gastritis)82,83,91	Often associated with Helicobacter spp.
	Erosion/ulceration83	Ulceration not common and often associated with Helicobacter spp. infection
	Hemorrhage83
	Micro-abscess, glandular83
	Mononuclear cell infiltrate83
Small intestine	Dilatation, glandular9,91	Mostly of Brunner’s gland in duodenum
	Pigment, macrophage82,91	Tip of villi, especially in ileum
	Micro-abscess, glandular82,83,91
	Hemorrhage83
	Infiltrate, inflammatory cell83
	Hyperplasia, crypt with inflammation (proliferative enteritis)97	Due to Lawsonia intracellularis, associated with necrosis, mucosal ulceration, and presence of intracellular organisms
	Parasitic granuloma91	Associated with section through a nematode parasite, especially Oesophagostomum spp.
	Adenocarcinoma82,85
	Atrophy, villus82,83,91	In ileum secondary to chronic colitis. Due to Campylobacter spp. infection
Large intestine	Dilatation, glands9,82
	Microabcesses, glandular82,91	Occurs in the context of chronic colitis/chronic inflammatory bowel disease/lymphoplasmacytic, idiopathic chronic colitis as well as Campylobacter spp. or Shigella spp. infection
	Hemorrhage83
	Hyperplasia, mucosal83	Often associated with chronic colitis
	Infiltrate, inflammatory cell (lymphoplasmacytic, idiopathic chronic colitis)9,82,83,98
	Parasitic granuloma91,99	Associated with Trichuris trichiura and Oesophagostomum spp.
	Adenocarcinoma82,84,85	Common neoplasm in aging rhesus, frequently at ileocecal junction
Hematopoietic system
Bone marrow	Lymphoid follicle development83	Often associated with subclinical simian betaretrovirus (SRV) infection
Liver, gallbladder, and exocrine pancreas
Liver	Multinucleated hepatocyte82
	Vacuolation, lipid (diffuse lipidosis)91	Associated with negative energy balance, weight loss
	Vacuolation, lipid (focal)	Tension lipidosis
	Fibrosis91	Capsular or subcapsular. Most frequent in periportal lobe area
	Bile duct hyperplasia91	Mostly periportal and subcapsular
	Mononuclear cell infiltrate83
	Necrosis, coagulative91	Focal subcapsular; rare in macaques
	Pigment deposition91	Rare in macaques
	Infiltrate, inflammatory cell83	Often periportal and mononuclear in nature. Mixed nature when associated with liver damage
	Cystadenoma85
Pancreas	Infiltrate, inflammatory cell (pancreatitis)83
Integumentary system
Skin	Mineralization (calcinosis circumscripta)100
	Lipoma84
	Sebaceous adenoma84
Lymphoid system
Lymph node, mesenteric	Pigment, macrophage83
	Cellularity, increased, lymphocyte (hyperplasia, lymphoid)82,83	Especially of the lymphoid follicle which is prominent. Suspicion of SRV infection
	Cellularity, decreased, lymphocyte (lymphoid atrophy)83
Spleen	Cellularity, decreased, white pulp (lymphoid atrophy)83
	Extramedullary hematopoiesis83,91
	Cellularity, increased, lymphocyte (lymphoid hyperplasia)82,83	Especially of the lymphoid follicle, which is prominent. Suspicion of SRV infection
	Pigment, macrophage (hemosiderosis)
Thymus	Cyst, epithelial83
	Cellularity, decreased, lymphocyte (involution; atrophy)83
Musculoskeletal system
Bone	Decreased bone (osteoporosis)9
	Bone fracture91
	Dysplasia, physeal91	Associated with bone fracture
Muscle	Degeneration, skeletal muscle (myopathy)83	Associated with inflammatory cell infiltrate, Sarcocystis spp. parasite should be considered
	Cyst101	Sarcocystis spp. parasite cyst could be observed
	Vaccination-related inflammation (granuloma)83
Joint	Inflammation (arthritis)9	Primary due to age or secondary due to traumatic joint injuries
Nervous system
Brain	Mineralization, vascular, or bodies84,91	Mineralized bodies are often in globus pallidus or putamen and associated with iron deposit while vascular mineralization are more calcium deposit
	Mononuclear cell infiltrate, perivascular (perivasculitis)86	Around blood vessels in meninges
	Infarct9	Spontaneous in parietal and temporal lobes in aged atrophy/loss of cerebral parenchyma
	Pigment9,84,91	Either iron or lipofuscin accumulation
	Vacuolation, white matter84
	Degeneration, neuron84,91
	Gliosis84,91	Often associated with perivascular cuffing
	Amyloidosis, vascular102–104
	Meningioma105	Rare and mostly in old and female
Female reproductive system
Ovary	Cyst, follicular83,96
	Mineralization95,96
	Hyperplasia, tubulostromal epithelium96
	Ovarian granulosa cell tumor84,96	Rare
	Teratoma84,96
Uterus	Hemorrhage, endometrium96	Due to irregular uterine bleeding
	Endometriosis9,96	Can also involve ovary, colon, and urinary bladder
	Adenomyosis96
	Epithelial plaque96,106
	Polyp, endometrial9
	Hyperplasia, endometrial96
	Leiomyoma84,85,96
	Adenocarcinoma, endometrial96	Rare
Vagina	Inflammation (vaginitis)96	Lymphoid aggregates are not unusual and must be differentiated from inflammation
Uterus	Papilloma96	Generally at level of cervix
Mammary gland	Carcinoma, ductular85
Male reproductive system
Testis	Fibrous hypoplasia83
Epididymis	Hypoplasia (maturation arrest)
	Inflammatory cell infiltrate, perivascular (perivascular inflammation)83
Prostate	Hyperplasia9
	Carcinoma9	Rare
Respiratory system
Lung	Alveolar macrophage, increased9,83
	Mononuclear cell infiltrate, perivascular (perivascular inflammation)86
	Inflammation, chronic bronchioloalveolar9,82,91	Subpleural or at the tip of lobes, often associated with presence of mites (Pneumonyssus spp.)
	Subpleural bullae9	Common finding in older rhesus macaques
	Pigment, macrophages9,83,91	Often hemosiderin pigment (yellow-brown), usually perivascular. Black pigment in interstitium around bronchi due to inhalation of carbon particles
	Granuloma83,91,94	Focal and containing foreign body
	Emboli83,91	Especially following intravenous injection
	Pleuritis83,91
	Fibrosis83,91	Subpleural and focal
	Adenoma, alveolar85
Special senses
Eye	Cataract9
	Mononuclear cell infiltrate, ciliary body/choroid83,91
Urinary system
Kidney	Eosinophilic inclusion bodies (cytokeratin aggregates)82	At level of pelvis in transitional epithelium
	Fibrosis, interstitial91
	Multinucleated cells82,84	Along collecting tubules
	Mononuclear cell infiltrate, interstitium (interstitial nephritis)82,83,91	From focal to diffuse
	Mononuclear cell infiltrate, perivascular (perivascular inflammation)86
	Glomerulonephritis, chronic84	Associated with age
	Tubular degeneration83,91	Associated with mononuclear cell infiltration, to be called tubulo-interstitial nephritis
	Pigment, tubular83
	Vacuolation83,91	In tubule or transitional epithelium
	Fibrosis, interstitial83
	Mineralization83
	Glomerulopathy, mesangioproliferative9,91	Mesangial cell proliferation
	Glomerulonephritis, membranous83,91	Thickening of basal membrane to increased glomerular matrix
	Glomerulosclerosis9,91	Deposition of eosinophilic material within mesangium (fibrocollagen confirmed by trichrome stain or other methods)
	Transitional cell adenoma84,85
	Carcinoma,tubular85
Urinary bladder	Mononuclear cell infiltrate, perivascular (perivascular inflammation)86
	Eosinophilic inclusion bodies (cytokeratin aggregates)82

Table 3.

Tabulated Overview of Morphological Findings Usually Associated with Conditions of Immune Suppression in Macaques (Macaca spp.)^a

Organ	Findings	Comments
Multisystemic
	Vesicular and ulcerative epithelial lesions with intranuclear inclusion bodies (INIB) and synctia. Necrotizing lesions with INIB in solid organs108	Macacine alphaherpesvirus 1 (Herpes B virus), associated only with non-viral immunosuppression
	Vesicular and ulcerative, hemorrhagic epithelial lesions with syncytial giant cells and eosinophilic INIB. Necrotizing hemorrhagic lesions in solid organs5,109–113	Cercopithecine alphaherpesvirus 9 (Simian varicella virus) associated with post transplantation immune suppression.
	Necrotizing and/or neutrophilic inflammation with cytomegalic cells and intracytoplasmic and intranuclear inclusion bodies (ICIB, INIB)114	Macacine betaherpesvius [Cytomegalovirus (CMV)], predilection for nerves, vessels, testis, meninges, small intestine, lung, kidney
	Microgranuloma with multinucleated giant cells and diffuse histiocytosis with intracellular 2–4 micron yeast115,116	Histoplasma capsulatum capsulatum, disseminated histoplasmosis (rare)
	Histiocytic and microgranulomatous inflammation6,8,115,117	Disseminated Mycobacterium avium complex (MAC)
	Necrotizing pancreatitis, gastroenteritis, hepatitis, nephritis118	Adenovirus
	Pyogranulomatous inflammation119	Spironucleus spp., predilection for mesenteric lymph nodes, may cause abdominal abscesses, infection may be systemic
	Posttransplant Lymphoproliferative Disorders (PTLD)120,121	Associated with Macacine herpesvirus 4 [Lymphocryptovirus (LCV)]
	Lymphoma122,123	Most common in lymphoid organs and gastrointestinal tract but can be observed in multiple organs. Usually associated with retroviral infection ie, SIV or SRV and gammaherpesviruses ie, LCV and rhesus rhadinovirus (RRV) co-infections
Cardiovascular system
Heart	Lymphohistiocytic and necrotizing myocarditis124–126	Trypanosoma cruzi, Chagas disease recrudescence associated with retroviral immune suppression
	Endocarditis, valvular127	Simian immunodeficiency virus (SIV)
	Myocarditis127	SIV
	Dilated cardiomyopathy127	SIV
Blood vessels	Proliferative arteriopathy128	SIV
	Pulmonary artery thrombosis128	SIV
	Cytomegaly, endothelial cell84,114	CMV. Endothelial cells with ICIB/INIB. Other cell types can be affected in heart, lung, and kidney
Gastrointestinal system
Oral cavity	Hyperkeratosis with yeast and pseudohyphae115,129	Candida albicans
	Ballooning degeneration of epithelial cells with INIB130	LCV, oral hairy leukoplakia
	Gangrenous necrosis and bone denudation8,108	Noma associated with SRV
	Proliferative inflammation in pharynx and tonsil with superficial organisms8,131	Cryptosporidium spp.
Salivary gland	Lymphofollicular aggregates, paraductular132,133	Nonspecific, increased with SIV and SRV
	Duct mucosal hyperplasia and inflammation8,131	Cryptosporidium spp.
Esophagus	Hyperkeratosis with yeast and pseudohyphae115,129	Candida albicans
	Ballooning degeneration of epithelial cells with INIB130	LCV, Oral hairy leukoplakia
Stomach	Lymphoplasmacytic gastritis with trichomonad trophozoites within lumens of gastric glands134	Tritrichomonas spp. SIV
	Necrosuppurative gastritis containing intralesional flagellates135	Tritrichomonas spp.
	Mucosal epithelium, INIB108	Adenovirus
Small intestine	Atrophy, villus5	Enteropathogenic (attaching and effacing) E. coli
	Atrophy, villus5	Cryptosporidium spp.
	Enteritis with INIB99,136	Adenovirus
	Histiocytic inflammation6,8,115	MAC
	Myenteric plexitis, lymphocytic124	Chagas disease
	Myenteric plexitis, lymphohistiocytic137	SIV
	Discrete proliferative and hemorrhagic enteritis with neutrophilic inflammation and ICIB and INIB138	CMV, causes serpiginous, mass-like proliferations
Large intestine	Discrete proliferative and hemorrhagic enteritis with neutrophilic inflammation and ICIB and INIB138	CMV, mass-like proliferations
	Histiocytic colitis115,139	MAC
	Colitis140	Flagellates
	Colitis with INIB99	Adenovirus
Peritoneum	Proliferative serositis141	Enterocytozoon bieneusi
	Mesenchymal proliferation arising from the mesentery5	Retroperitoneal fibromatosis associated with rhadinovirus, Retroperitoneal Fibromatosis-Associated Herpesvirus (RFHV) (Macacine gammaherpesvirus 8) in context of retroviral immune suppression, primarily SRV associated
Integumentary system
Skin	Maculopapular rash142–144	Inguinal and axillary regions, SIV Diffuse, Morbilliviruses (eg, Measles virus and canine distemper virus). Morbillivirus infections are not specific to immune suppression.
	Hyperkeratosis144	Canine distemper virus
Liver, gallbladder, and exocrine pancreas
Liver	Proliferative cholangiohepatitis145,146	Cryptosporidium spp.
	Proliferative cholangiohepatitis146	Enterocytozoon bieneusi
	Necrotizing hepatitis108,114	Adenovirus
	Hemozoin pigment in Kupffer cells5	Plasmodium spp.
Gallbladder	Proliferative, eosinophilic inflammation with superficial protozoa145	Cryptosporidium spp.
	Proliferative, eosinophilic inflammation with extruded epithelial cells146	Enterocytozoon bieneusi
Pancreas (exocrine)	Necrotizing pancreatitis centered on acini with INIB108	Adenovirus
	Proliferative, eosinophilic inflammation in ducts with superficial protozoa145	Cryptosporidium spp.
Lymphoid system
Lymph node	Necrotizing and pyogranulomatous lymphadenitis, mesenteric lymph nodes119	Spironucleus spp.
	Histiocytic lymphadenitis and lymphangitis115	MAC
	SIV giant cell disease8,108	SIV
	Cytomegalic cells with INIB in subcapsular sinuses; necrotizing and neutrophilic inflammation with cytomegalic cells with ICIB and INIB114	CMV
	Lymphoma108	B cell lymphomas associated with LCV
	Follicular hyperplasia133	SIV related changes
	Follicular involution with normal or expanded paracortex133	SIV related changes
	Diffuse follicular and paracortical depletion133	SIV related changes
	Giant cells and granulomatous inflammation133	SIV related changes
	Lymphoproliferative disorder133	SIV related changes
Thymus	Depletion133
Spleen	Follicular hyperplasia133	SIV related changes
	Follicular involution with normal or expanded paracortex133	SIV related changes
	Diffuse follicular and paracortical depletion133	SIV related changes
	Giant cells and granulomatous inflammation133	SIV related changes
	Lymphoproliferative disorder133	SIV related changes
	Lymphoma147,148
	Multifocal microgranulomatous inflammation with acid fast bacilli115	MAC
Bone marrow	Lymphoma122,123
	Erythrocyte precursor INIB5,149	Simian parvovirus (Primate erythroparvovirus 2)
	Multifocal microgranulomatous inflammation with acid fast bacilli115	MAC
Nervous system
Brain	Lymphohistiocytic/granulomatous inflammation with syncytial giant cells133	SIV giant cell encephalitis
	Progressive multifocal leukoencephalopathy (PML) with demyelination with amphophilic INIB150,151	Macaca mulatta polyoma virus [Simian virus 40 (SV40)]
	Meningoencephalitis with amphophilic INIB151	SV40
	Neutrophilic, histiocytic meningitis with cytomegaly and ICIB/INIB152	CMV
Spinal cord	Neutrophilic meningitis, radiculoneuritis with ICIB and INIB152	CMV
Reproductive system
Testis	Necrotizing orchitis with ICIB/INIB108	CMV
Respiratory system
Trachea	Inflammation with squamous metaplasia153	Cryptosporidium spp.
Lung	Granulomatous pneumonia154	Mycobacterium tuberculosis, M. bovis, not specific to immune suppression; may disseminate, involve tracheobronchial lymph nodes, liver, spleen
	Granulomatous interstitial pneumonia with giant syncytial cells133,155	SIV giant cell pneumonia
	Interstitial pneumonia with cytomegalic and syncytial cells and ICIB/INIB114,156,157	CMV
	Proliferative interstitial pneumonia with amphophilic INIB108	SV40
	Alveolar histiocytosis, pyogranulomatous pneumonia158	Pneumocystis spp.
	Infarction, hemorrhagic128	Infarcts secondary to proliferative arteriopathy SIV
	Necrotizing bronchiolitis with bronchiolar squamous metaplasia153	Cryptosporidium spp.
	Microgranulomatous interstitial pneumonia with acid fast organisms (often nodular)115,159	MAC
	Bronchioalveolar pneumonia160	Adenovirus
	Bronchiointerstitial pneumonia with INIB/ICIB161	Measles (Measles morbillivirus)
Urinary system
Kidney	Tubulointerstitial nephritis108,157,162	Adenovirus
	Interstitial nephritis with INIB	SV40
	Individual cytomegalic tubular epithelial cells with INIB and ICIB108	CMV Not associated with inflammatory response in kidney
	Glomerulonephritis163	SIV related
	Nonspecific lymphofollicular aggregates133
Special senses (eye and ear)
Eye	Conjunctivitis164	Cryptosporidium spp. Morbilliviruses

ICIB = intracytoplasmic inclusion body; INIB = intranuclear inclusion body.

Cynomolgus Macaque (M. fascicularis)

The crab-eating macaque (M. fascicularis), also known as the long-tailed macaque and the cynomolgus macaque in laboratories,²⁶ is a cercopithecine primate native to Southeast Asia. It is long lived and continues to closely interact with humans (synanthropic) in many countries.²⁷ Because of this, it has variably been seen as an agricultural pest,²⁸ a sacred animal,²⁹ an invasive species and a threat to biodiversity,²⁶ and, more recently, the subject of medical experiments.²⁷ In the research setting, it has surpassed the once dominant role of the rhesus macaque (M. mulatta), mainly due to its smaller size and easier availability, which has made handling and housing easier and overall costs lower. However, the availability and lower cost of these macaques has changed drastically in the past 5 years. The cost of an individual animal, for example, has doubled in the past year due to low availability, primarily due to an export ban from China as part of a larger ban on the sale of all wildlife due to the COVID-19 outbreak.³⁰

It is now generally seen as one of the most important NHP animal models in basic and applied biomedical research. Cynomolgus macaques are now considered the standard NHP model for preclinical safety assessment of biopharmaceuticals.³¹ Several commercial breeding centers in Indonesia, China, the Philippines, and Mauritius are now able to provide sufficient numbers of captive-bred, long-tailed macaques originating from wild-trapped founders. In addition, Cambodia has recently emerged as an important origin, and there are suppliers in Vietnam and Laos as well. In the research setting in particular, it is important to appreciate that the wide geographic distribution of and considerable interspecies hybridization (with M. mulatta) in shared habitats points to populations that are genetically and phenotypically quite diverse.³² Conversely, Mauritian cynomolgus macaques are descended from a small founder population, resulting in limited major histocompatibility complex diversity compared with Asian cynomolgus and rhesus macaques and are preferentially utilized for transplantation experiments.³³ Thus, genotyping and phenotypic characterization of animals is desirable prior to studies aimed at predicting the safety of novel medicines in humans.

In the wild, troops of M. fascicularis are matrilineal with a female dominance hierarchy,³⁴ with male members leaving the group when they reach puberty.³⁵ It is an opportunistic omnivore³⁶ and has been documented using tools to obtain food in Thailand and Myanmar.³⁷  Tables 2 and 3 summarize research-relevant common conditions and diseases (background findings) in M. fascicularis (cynomolgus macaques).

Table 2.

Tabulated Overview of Morphological Background Findings in Cynomolgus Macaques (Macaca fascicularis)

Organ systems	Findings	Comments
Multisystemic
	Lymphoplasmacytic cell infiltrates	Digestive tract: stomach, colon, cecum107
	Lymphoplasmacytic cell infiltrates	Kidney, heart, liver, spleen, lung11
Cardiovascular system
Heart	Eosinophilic granulocyte infiltrates107	Scattered and occasionally seen in epicardial adipose tissue
	Granulomata107	Small granulomas surrounded by inflammatory cells, predominantly eosinophils in epicardial adipose tissue
	Minimal focal fibrosis, focal myocardial mucin deposition, ectopic epithelial structures, blood-filled valvular cysts107	Rare
	Inflammatory cell foci10
	Focal myocarditis,10 myocardial necrosis10
	Myocardial degeneration/fibrosis11
	Karyomegaly11
	Mucinous change11
	Endocardiosis11
	Coronary intimal thickening11
	Arteritis/periarteritis11
	Mineralization11
	Squamous/epithelial cysts or plaques11
	Mesothelium hyperplasia/metaplasia10	Proliferation and squamous metaplasia of epicardial mesothelium
	Hemorrhage10	In endocardium
	Arterial sclerosis10
	Infarction10
Blood vessels	(Aorta) mucinous change11
	Intimal thickening/degeneration11
	Mineralization11
	Arteritis in the mural artery11
Endocrine system
Adrenal gland	Mineralization11
	Cortical cell vacuolation11
	Pigment11
	Adrenohepatic fusion/adhesion11
	Accessory adrenocortical tissue10
	Nodular hyperplasia of cortical cells10
	Decreased lipid in cortical cells10
	Mineralization of corticomedullary junction10
	Inflammatory cell foci10
Thyroid gland	Ectopic thymus/salivary gland11
	Cystic/ultimobranchial cysts11
	Dilated/cystic follicles10
	Lymphocytic thyroiditis10
	Focal C-cell hyperplasia10
	Macrophage infiltration in follicle10
	Hydropic degeneration of follicle cells10
	Fatty infiltration10
Parathyroid gland	Ectopic thymus11
	Congenital cysts11
	Fatty infiltration10
	Increased oxyphil cells10
	Focal hypertrophy of chief cells10
	Inflammatory cell foci10
Pituitary gland	Inflammatory cell foci11
	Cysts11
	Focal anterior pituitary cell hypertrophy10
Pancreas (endocrine)	Hypertrophy, islets10
	Angiectasia in islets10
	Hemorrhage in islets10
	Fibrosis in islets10
	Amyloidosis in islets10
Gastrointestinal tract
Tongue	Inflammatory cell foci11
	Inflammation/subepithelial myositis11
	Erosion10
	Edema/reticular degeneration10
	Foreign body granuloma10
	Regeneration muscle fiber10
Esophagus	Inflammatory cell infiltration11
	Focal muscle atrophy/degeneration11
Salivary gland	Inflammatory cell infiltrations11	Submaxillary and parotid
	Mineralized duct contents11
	Focal fibrosis10
	Hydropic degeneration of acinar cells10
Stomach	Helicobacter heilmannii-like organisms107
	Gastritis11
	Inflammatory cell infiltration11
	Lymphoid hyperplasia10
	Parasitic granuloma10
	Gastric infarction/necrosis10
	Erosion10
Small intestine	Trichuris trichiura  107
	Inflammatory cell infiltration11
	Pigmented macrophage, lamina propria11
	Inflammation lacteal ectasia/edema10
	Diffuse goblet cell hyperplasia10
	Diverticulum/glandular herniation10
	Ectopic pancreatic tissue10
	Increase in goblet cells10
	Pigmented macrophages107
Large intestine	Balantidium coli  11	Ciliated protozoa
	Trichuris trichiura  11	Nematode
	Inflammatory cell infiltration11
	Syncytial cells, GALT11
	Inflammation/colitis11
	Vasculitis/perivasculitis11
	Glandular microherniation11
	Parasitic granuloma/fibrosis/mineralization11
	Crypt dilatation10
	Pigmented macrophages10
Hematopoeitic system
Bone marrow	Lymph follicle formation10
	Pigment deposition10
Liver, gallbladder, and exocrine pancreas
Liver	Centrilobular glycogen107
	Binucleated hepatocytes107
	Pigment in gallbladder submucosa107
	Tension lipidosis107
	Inflammatory cell foci11
	Hepatocyte vacuolation/lipidosis11	Diffuse
	Tension lipidosis/focal lipidosis11
	Glycogen vacuolation11
	Focal necrosis/single cell necrosis11
	Subcapsular hemorrhage/fibrosis11
	Bile duct hyperplasia/periportal fibrosis11
	Pigment11
	Ectopic adrenal gland11
	Eosinophilic bodies in hepatocytes10
	Fatty change10
	Microgranuloma10
Gallbladder	Inflammatory cell infiltrations11
	Pericholangitis10
Pancreas (exocrine)	Inflammatory cell infiltrations11
	Acinar cell atrophy/duct cell hyperplasia11
	Acinar cell degranulation10
	Accessory spleen10
	Hydropic degeneration10
	Apoptosis of acinar cells10
	Decrease in zymogen granules10
	Chronic pancreatitis10
Integumentary system
Skin	Dermatitis11
	Hair follicular atrophy11
	Epidermal hyperplasia, penile11
Lymphoid system
Lymph node, mesenteric	Sinus histiocytosis11
	Eosinophil infiltration11
	Lymphangiectasia11
	Increased number of pigment-laden macrophages in the sinuses10
Lymph node, submandibular	Pigmented macrophages11
	Granulocytic infiltrates11
Spleen	Pigment deposition107	Brown, finely granular pigment deposition (presumably hemosiderin) in splenic macrophages
	Lipidic granuloma107
	Extramedullary foci of hemopoiesis107
	Increased granulocytosis (myeloid hyperplasia) in red pulp107	Granulocytosis was mainly composed of neutrophils and some eosinophils and was more prominent near capsule than center of parenchyma
	Focal lymphoid hyperplasia11
	Capsular fibrosis11
	Hyalinized germinal centers11
	Nodular hyperplasia (lymphoid follicle/red pulp)10
Thymus	Atrophy11
	Cystic tubular hyperplasia11
	Myoid cells/muscle tissue11
	Involution10
	Cyst10
	Ectopic parathyroid tissue10
	Ectopic muscle tissue10
	Follicle formation in medulla10
	Proliferation of epithelium10
	Thymoma10
Musculoskeletal system
Bone	Digital fractures10,11
	Physeal lesions10,11
Muscle	Inflammatory cell infiltrates10,11
	Histiocytic infiltration/vaccine granuloma10,11
	Myositis10,11
Nervous system
Brain	Inflammatory cell foci, meningeal/choroid plexus11
	Pigment11
	Perivasculitis, meningeal10
	Perivascular cuffs10
	Mineralization, thalamus10
	Focal gliosis/glial scar10
Spinal cord	Perivasculitis, meningeal11
	Pigment, perivascular10
Sciatic nerve	Inflammation, perineural11
	Perivasculitis11
	Renaut body10	Alcian blue positive, subperineurial structure composed of loosely arranged and randomly oriented collagen fibers in fine fibrillary material, seen in normal nerve and certain pathologic states
	Inflammatory cell foci10
	Focal fibrosis10
Female reproductive system
Ovary	Cysts (follicular/paraovarian/rete ovarii)11
	Mineralized atretic follicles11
	Aberrant corpora luteum10
	Mineralization10
	Paroophoritic cyst10
	Mesonephric cyst10
	Hyperplasia of rete ovarii10
	Mucinous cystadenoma10
Uterus	Adenomyosis10
	Melanin pigment deposition10
	Focal inflammatory cell infiltration, endometrium10
	Dilatation of endometrial gland10
	Papilloma96	Generally at level of cervix
Vagina	Melanin pigment deposition10
Male reproductive system
Testis	Hypoplasia11	Seminiferous tubules
	Inflammatory cell foci11
	Immature/prepubertal10
	Segmental dilatation10	Seminiferous tubules
	Eosinophilic changes in Sertoli cells10
	Spermatocele10
	Corpus amylaceum10
	Increased stromal collagen10
Epididymis	Inflammatory cell foci/inflammation11
	Arteritis/periarteritis11
	Immature10
Prostate	Inflammatory cell infiltrates11
	Immature10
Seminal vesicle	Immature10
	Spermatocele10
	Mineralization of secretions10
Respiratory system
Nasal cavity	Lymphoid follicle hyperplasia10
	Mucosal erosion of turbinate10
Lung	Pigmented macrophages in periarteriolar location and filled with black granular intracytoplasmic pigment that strongly suggested anthracosis107
	Foamy macrophages, multifocal intra-alveolar distribution107
	Focal to multifocal pleural fibrosis, sporadic finding107
	Inflammatory cell infiltration107
	Interstitial inflammation107
	Alveolar macrophage accumulation107
	Pigment deposits11
	Focal pleural fibrosis/pleuritis11
	Foreign body granuloma11
	Hyperplasia of BALT11
	Bronchiolitis11
	Emboli11
	Vasculitis/vascular degeneration11
	Focal alveolar hemorrhage10
	Anthracosis10
	Thrombus
	Focal hyperplasia of alveolar epithelium10
	Pulmonary acariasis10
	Osseous metaplasia10
Special senses (eye and ear)
Eye	Focal inflammatory cell infiltration in the conjunctiva, ciliary body10
	Cataract10
	Retinal dysplasia10
Lacrimal gland	Focal inflammatory cell infiltration10
	Pigment10
Urinary system
Kidney	Cytoplasmic inclusions10,107	Occasionally seen in urothelium of pelvis, ureters, and vesical bladder
	Glomerular degeneration107	Deposition of amorphous eosinophilic substance, decreased cellularity, and generally expanded volume of glomerulus, sometimes associated with periglomerular fibrosis
	Immature/non-developed glomeruli107	Small size, basophilic appearance, many cuboidal cells composing visceral surface of Bowman epithelium, no expanded capillaries and no red blood cells
	Mineralization in papilla107
	Papillary edema and fiber deposition107	Edema was represented by myxoid appearance of tip of papilla, whereas fiber deposition consisted of pale, fibrillar, eosinophilic substance with low cellularity and slight Masson’s Trichrome positivity
	Multinucleated cells10,107	Inner medullary collecting ducts
	Inflammatory cell infiltrations11
	Interstitial nephritis11
	Mineral deposits/mineralization10
	Glomerulonephritis/sclerosis10
	Tubular degeneration/regeneration10
	Pigment in tubular epithelium10
	Osseous metaplasia10
	Focal hyperplasia, tubular epithelium10
	Atypical hyperplasia, tubular epithelium10
	Nephrosclerotic syndrome10
	Edema, renal papilla10
	Pyelitis10
	Pyelonephritis Immature glomeruli10
	Glomerulonephritis10
Urinary bladder	Inflammatory cell foci91
	Focal mineralization, adventitial remnants10
	Cystitis/eosinophilic cystitis10

Baboons (Papio spp)

Baboons, Papio spp, are native to Africa and the Arabian Peninsula. They are relative newcomers to biomedical research, and the first research colony in the United States was established in 1960 at the Southwest Foundation in San Antonio, TX. Baboons are larger than many laboratory NHP species, increasing housing space required, but are hardy and highly fecund in captivity. Their larger size allows for certain procedures to be conducted that would be difficult to achieve in the smaller macaques.

Baboons are considered a single species, Papio hamadryas (s.l.), with 5 subspecies: sacred baboons (P. h. hamadryas Linnaeus, 1758), yellow baboons (P. h cynocephalus Linnaeus, 1766), chacma baboons (P h ursinus Kerr, 1792), red baboons (P. h. papio Desmarest, 1820), and olive baboons (P. h. anubis Lesson, 1827). Most of the founders of baboons in the United States were P. h. anubis trapped in East Africa, and the remainder were P. h. cynocephalus, also trapped in East Africa.³⁸ Hybridization between some subspecies occurs in the wild as well as in captive research colonies. Significant genetic differences exist between the subspecies, and they should not be used interchangeably in biomedical research.³⁹

Baboon chromosomes are very similar to those of humans, and baboons are used widely for study of the genetics of susceptibility to complex diseases. They are the first NHP for which a framework genetic linkage map was established.⁴⁰ Baboons resemble humans in cholesterol metabolism, early stages of atherosclerosis, and alcoholic liver disease. Baboons are also used for reproductive research and as models for schistosomiasis, obesity, and type 2 diabetes.

Baboons have many enzootic viruses, bacteria, and parasites, some of which can confound biomedical research results and pose zoonotic risk. A specific pathogen-free colony of baboons was developed at the University of Oklahoma Health Sciences Center⁴¹ and is now housed in Texas at MD Anderson’s Keeling Center for Comparative Medicine and Research. Several reviews of spontaneous pathology of baboons have been published.^42,43  Table 4 summarizes research-relevant common conditions and diseases (background findings) in laboratory baboons (Papio spp).

Table 4.

Tabulated Overview of Morphological Background Findings in Baboons (Papio spp)

Organ systems	Findings	Comments
Multisystemic	Secondary systemic amyloidosis165	As in other NHP species
	Lymphoma/leukemia165–167	Simian T-lymphotropic virus type 1 (STLV1) infection. Predilection for lung, skin, spleen, liver, heart, lymph nodes, bone marrow (leukemia) Predominately T cell malignancies
	Herpesvirus papio 2 (HVP2) (Papiine alphaherpesvirus 2)168–170	Enzootic in baboons, >85% prevalence, latency; Primarily causes ulcers at genital, oral, mucocutaneous junctions; vesicular lesions that can be secondarily infected by bacteria. Scarring of urogenital tract can result in obstruction; chronic lesions can also lead to neoplasia. More severe in females. Can cause fatal pneumonia in infants/neonates
Cardiovascular system
Heart	Myocarditis, lymphoplasmacytic, +/− protozoal amastigotes171,172	Outdoor-housed animals in the southern US, especially Texas, are naturally exposed to triatomine vectors and can be infected with Trypanosoma cruzi, which can lead to lymphoplasmacytic myocarditis +/− visible protozoal amastigotes (Chagas disease)
	Necrotizing myocarditis, nonsuppurative173	Encephalomyocarditis virus. Pulmonary congestion and edema, hydropericardium, hydrothorax, ascites, lymph node and splenic hypertrophy, and pale white-to-tan mottled hearts.
Blood vessels	Arteriosclerosis/Atherosclerosis174–176	Associated with diet and age, model of human disease, similar to other NHPs
Endocrine system
Pituitary gland	Pituitary adenoma177
Thyroid gland	Thyroid adenoma, carcinoma177
	Thyroiditis177
Adrenal gland	Pheochromocytoma177
	adrenal hyperplasia177
	adenoma177
Pancreas (endocrine)	Amyloid177	Islets; subsequent diabetes mellitus
	Neuroendocrine tumors (somatostatinoma, insulinoma, glucagonoma; neuroendocrine carcinoma)178	Similar features as in humans, generally benign, can express hormones, many nonfunctional
Gastrointestinal system
Salivary gland	Neoplasia (adenoma, adenocarcinoma, undifferentiated carcinoma)179	Uncommon
Esophagus	Hyperplasia, epithelial, +/− inflammation, lymphocytic180	Gastroesophageal reflux disease (GERD); similar to GERD in humans
	Lymphocytic esophagitis181	In animals without reflux
Stomach	Gastric hyperplasia182	Possibly immunologic disease
	Lymphocytic gastritis183	Helicobacter-like, but no etiology identified
	Acute gastric dilatation42
	Trichobezoar184	Also seen in small intestine, large intestine, and esophagus. Associated with group housing; can be fatal
	Gastrointestinal stromal tumor (GIST)185	CD117+
Small intestine	Enteritis43	Many etiologies, endemic
Large intestine	Colitis43	Many etiologies, endemic
	Adenocarcinomas186	Cecum is common site
Liver, gallbladder, and exocrine pancreas
Liver	Hepatitis187	Associated with gallbladder disease
	Granulomas188,189	Associated with parasite infection, including Hepatocystis spp.
	Hepatic neuroendocrine carcinoma190	Case report, n = 1
Gallbladder	Cholelithiasis187,191	Associated with diet; most commonly composed of cholesterol (as in humans); affects older animals, adult and geriatric
	Inflammation (cholecystitis)187	Follows obstruction of gallbladder by choleliths
Integumentary system
Skin	Ulceration	See HVP2 above
	Dermatitis/cellulitis43,192	Trauma, +/− secondary bacterial infection
	Dermatitis, granulomatous (histoplasmosis)193	Histoplasma capsulatum var. duboisii
	Pediculosis (Pedicinus obstrusus)194	Sucking lice
	Squamous cell carcinoma195
Sex skin	Squamous cell carcinoma195
	Myxoma196
Musculoskeletal system
Bone	Osteosarcoma197	Appendicular skeleton and skull, age 8–13 y
	Spondylosis42	Common over 14 y
	Osteoarthritis198	Age and trauma related
Nervous system
Brain
	Glioblastoma multiforme199	Progressive and fatal
	Epilepsy/seizures200,201	Probable genetic basis, undetermined etiology
	Meningoencephalomyelitis202,203	Orthoreovirus, endemic
	Hydrocephalus204	Congenital
	Marinesco bodies205	Intranuclear inclusion bodies in substantia nigra, found in NHPs and humans
	Pallido-nigral spheroids206	Associated with increased cellular stress
Male reproductive system
Testis	Atrophy207
	Orchitis/abscess207
	Hypospermia/aspermia207
	Cryptorchid207
Epididymis	Degeneration207
Prostate	Atrophy207
	Inflammation207
Female reproductive system
Ovary	Neoplasia177,208	Granulosa cell tumors are common, also other epithelial-stromal and sex cord-stromal tumors; associated with aging, generally found at necropsy
	Ovarian cysts177
Uterus	Endometriosis/adenomyosis165	Progressive disease, potentially fatal complications
	Polyps209	Endometrial and cervical
Mammary gland	Invasive ductal carcinoma210
Respiratory system
Lung	Pneumonia43	Various causes—foreign body, tuberculosis, mycotic (Coccidioides immitis), bacterial, viral (HVP2)
Air Sac	Air sacculitis165,211	Bacterial infection (mixed bacteria); male sex predilection; characterized by epithelial hyperplasia or hypertrophy, necrosis, fibrosis, cellular infiltrates
Special Senses
Eye	Blindness212,213	Congenital
Urinary system
Kidney	N ephritis207	Ascending infection secondary to HVP2 lesions and obstruction
	Nephrocalcinosis207
	Pyelonephritis207
	Cysts207
	Amyloidosis207
	Hydronephrosis
	Glomerulonephritis207	Associated with chronic Staphalococcus aureus infections
	Nephroblastoma214
Urinary bladder	Cystitis207

African Green Monkeys (Chlorocebus spp)

The African green monkeys are a group of African-origin Old World monkeys that belong to the genus Chlorocebus. They have a greenish-golden coat with a white ventrum and black to dark-blue skin on their face. The males have blue scrotum and red penis.⁴⁴ Six species have been recognized, including C. sabaeus (green monkey), C. aethiops (grivet monkey), C. djamdjamensis (bale monkey), C. tantalus (tantalus monkey), C. pygerythrus (vervet monkey), and C. cynosuros (malbrouck monkey). African green monkeys are a popular species for use in biomedical research as they are small, easily handled, easily bred in captivity, non-endangered, and evolutionarily close to humans, and health and safety risks are fewer compared with macaques.⁴⁵ The research colonies of Chlorocebus in the United States are primarily derived from St. Kitts and Nevis islands of the Caribbean populations.^46–48 The availability of genetic linkage maps for C. a. sabaeus⁴⁹ makes Chlorocebus a popular research model for identification of genetic factors in disease and behavior. Some research areas include investigations of Acquired Immunodeficiency Syndrome,⁵⁰ vaccinology,⁵¹ hypertension,⁴⁵ neurological disease,⁴⁷ psychology and social behavior,^52,53 atherosclerosis and metabolic syndrome,^54,55 African trypanosomiasis,⁵⁶ and leishmaniasis.⁵⁷ Vero cells are derived from the kidney epithelial cells of African Green monkeys and are one of the commonly used cell lines in biomedical research.⁵⁸  Table 5 summarizes research-relevant common conditions and diseases (background findings) in Chlorocebus spp, although there are fewer published reports compared with some other NHP species.

Table 5.

Tabulated Overview of Morphological Background Findings in African Green Monkeys (Chlorocebus spp)

Organ systems	Findings	Comments
Multisystemic	Multisystemic abscesses215
Cardiovascular system
Heart	Spontaneous ventricular septal defects216	Congenital
	Left ventricular hypertrophy217
Blood vessels	Spontaneous aortic aneurysm218
	Fatty streaks in aorta in adult animals219
	Aortic coarctation216	Congenital
Gastrointestinal system
Stomach	Gastric ulcers220	Stress induced
Liver, gallbladder, and exocrine pancreas
Liver	Hypertrophy and hyperplasia of hepatic stellate cells221	Due to vitamin A toxicity
Nervous system
Brain	Beta-amyloid plaques222	In aged animals
Urinary system
Kidney	Oxalate nephrosis223	In 93% of animals in a colony
	Horseshoe kidneys (renal fusion)224	Congenital

Common Marmoset (Callithrix jacchus)

Primates of the family Callitrichidae originate from South America, where they live in complex groups and inhabit the forest. All Callitrichidae are characterized by small body size, a dense short fur, and phalanges showing claws instead of nails. In the genus Callithrix (also known as true marmosets), incisors are enlarged whereas canine teeth are reduced in size. With regard to biomedical studies, the common marmoset (C. jacchus) is by far the most frequently used species in this genus.

The common marmoset is used in research on infectious, inflammatory, neurologic, metabolic, and skeletal diseases. It is also occasionally used in the preclinical safety assessment of drugs.⁵⁹ Similar to other laboratory animal species, the European Union (EU) has defined minimum requirements concerning enclosure floor area and heights for housing marmosets in laboratories. Since 2013, common marmosets may only be used for biomedical studies in the EU where animals are the offspring of marmosets bred in captivity; there are multiple breeders for common marmosets in the United States and the EU. Marmosets are typically bred in small monogamic groups in which only the dominant female reproduces. Interest in the common marmoset in biomedical research has grown recently, in part due to the successful demonstration of transgenic marmosets.⁶⁰

Table 6 summarizes research-relevant common conditions and diseases (background findings) in common marmosets. Life-limiting and research-limiting conditions affecting the kidneys and the gut have been particularly problematic in laboratory-housed common marmosets.⁶¹

Table 6.

Tabulated Overview of Morphological Background Findings in Marmosets (Callithrix spp)

Organ systems	Findings	Comments
Multisystemic
	Lymphoma (lymphosarcoma)13,61	Large numbers of immature lymphoblasts (described in kidney, lung, spleen, lymph node, pancreas, liver, intestine)
	Amyloid13,61	Pale eosinophilic, extracellular material (described in spleen, kidney, liver, intestine, stomach, adrenal)
Cardiovascular system
Heart	Fibrosis, myocardium13,61,225–227
	Inflammatory cell infiltrate, myocardium (chronic myocarditis)61,225,226
	Degeneration, cardiomyocyte (cardiomyopathy)13,225
	Pigment, myocardium (lipofuscinosis)227	Intracytoplasmic golden-brown pigment; stains positive with PAS
Endocrine system
Adrenal gland	Ectopic tissue, adrenocortical (extracapsular cortical tissue)225
	Extramedullary hematopoiesis13,225,226
	Hyperplasia, cortical cell13,225	Focal or diffuse
	Infiltrate, inflammatory cell225,227	Typically at corticomedullary junction
	Vacuolation, cortical, increased225	Focal or diffuse
Pituitary gland	Cyst225
	Persistent Rathke’s pouch225
Thyroid gland	Mineralization (calcification)225
	Cystic follicle13
	Dilatation, follicular, diffuse (colloid goiter)225–227	Follicular distension with low epithelium
	Ectopic tissue, thymus225,226
	Hyperplasia, c-cell225,226
	Hypertrophy, follicular cell225
	Infiltrate, inflammatory cell (chronic thyroiditis)225–227
	Adenoma, follicular cell13
Parathyroid gland	Cyst225
	Hypertrophy13
	Hyperplasia13
Gastrointestinal system
Tongue, oral cavity, teeth	Infiltrate, inflammatory cell225
	Abscess61	Tooth root abscess as consequence of traumatic fracture and bacterial infection
Salivary gland	Atrophy225
	Fibrosis225
	Infiltrate, inflammatory cell (chronic sialoadenitis)225,226
Esophagus	Edema225
	Hyperplasia, squamous cell225
	Infiltrate, inflammatory cell (esophagitis)225,227
Stomach	Atrophy225
	Dilatation, glands225
	Infiltrate, inflammatory cell (chronic gastritis)13,225–227
Small intestine	Atrophy225
	Dilatation225
	Edema225
	Erosion/ulcer225
	Hemorrhage225
	Hypertrophy225
	Infiltrate, inflammatory cell (chronic lymphocytic enteritis)13,61,225–227	Shortening of villi with hyperplasia of crypt epithelium, accompanied by a lymphocytic infiltrate that expands and replaces the normal lamina propria
	Adenocarcinoma61	Mucinous or diffuse pattern affecting duodenum and/or proximal jejunum (often with metastasis to regional lymph nodes and lung; mostly associated with erosion and ulceration)
Large intestine	Atrophy225
	Edema225
	Hemorrhage225
	Hypertrophy225
	Infiltrate, inflammatory cell (colitis)13,225	Inflammatory cell infiltrate associated with distortion and reduction in crypt size was associated with isolation of enterohemorrhagic E. coli carrying eae gene228
Hematopoietic system
Bone marrow	Angiectasis (congestion)225
	Cellularity, decreased, bone marrow (depletion hematopoietic cell)225
	Hyperplasia, hematopoietic225
	Myelofibrosis13
Integumentary system
Skin	Hyperplasia, adnexal, apocrine gland (dilatation; proliferation)225
	Hemorrhage225
	Infiltration, inflammatory cell (chronic dermatitis)13,225,226
	Erosion/ulcer226
Liver, gall bladder, and exocrine pancreas
Liver	Congestion225
	Degeneration, cystic13,225,226
	Extramedullary hematopoiesis13,225
	Fatty change13,225
	Fibrosis225
	Glycogen accumulation225
	Bile duct hyperplasia225,227
	Hypertrophy, Kupffer cell225
	Infiltration, mononuclear (hepatitis)13,225–227	Often at portal triads
	Necrosis13,225–227
	Pigment deposition225	Hepatocytes and/or Kupffer cells
	Pigment deposition, hemosiderin (hemosiderosis)13,226,229	Granular brown pigment in hepatocytes (primarily centrilobular and midzonal) and Kupffer cells
	Hyperplasia, Ito cell (vacuolation of sinusoidal cell)225
Gallbladder	Cholecystitis13,226,227
Pancreas (exocrine)	Accumulation, adipocytes (infiltration adipose tissue)225
	Infiltrate, inflammatory cell (pancreatitis)13,225
	Hyperplasia, ductal cell225
	Atrophy, acinar cell13,227	Often associated with fibrosis
	Adenoma13	Acinar or ductal cell
Lymphoid system
Lymph node, mesenteric	Cellularity, increased, macrophage, intrasinusoidal (sinus catarrh; histiocytosis)13,225
	Pigment, macrophage (increase in pigmentophages)13,225
	Cellularity, increased, lymphocyte (hyperplasia, lymphoid follicle)13,225,226
	Inflammation, granulomatous	Granulomatous lymphadenitis associated with nontuberculous mycobacteria230
	Necrosis225
	Extramedullary hematopoiesis225
Thymus	Cyst, epithelial225
	Ectopic tissue, parathyroid225
	Cellularity, increased, epithelial cell (hyperplais, thymic epithelium)225
	Cellularity, decreased, lymphocyte (involution; atrophy)225,226
Spleen	Cellularity, decreased, red pulp (atrophy)225
	Cellularity, decreased, white pulp (atrophy, lymphoid follicle)225
	Congestion225
	Extramedullary hematopoiesis13,225,226
	Erythrophagocytosis (hemorrhage)225
	Cellularity, increased, stromal cell (hyperplasia, stromal cell)225
	Necrosis, lymphocyte (necrosis germinal center)225
	Pigment, macrophage (hemosiderosis)13,227	Associated with hemolytic anemia
Musculoskeletal system
Bone	Fibrous osteodystrophy225,227	Increased numbers of osteoclasts and osteoblasts; osteolysis; fibrosis
	Decreased bone (osteomalacia)13,61,231	Osteoclastic bone resorption, primarily involving endocortical bone (TRAP-stain)
Skeletal muscle	Degeneration, skeletal muscle (myopathy)13,226,227,232	Segmental necrosis, hyalinization and loss of fiber striations (may be associated with pyogranulomatous pansteatitis in vitamin E deficiency)
Joint	Inflammation (arthritis)227	Typically associated with septicemia
Nervous system
Brain	Mineralization (calcification)13,225
	Extramedullary hematopoiesis225,226	Hematopoietic cells in the choroid plexus
	Inflammation (encephalitis)227
	Hydrocephalus227
Reproductive system
Testis	Atrophy, tubular13,225–227
	Hemorrhage225
	Hypoplasia (maturation arrest)225
	Necrosis, tubular225
Epididymis	Hypoplasia (maturation arrest)225
	Sperm granuloma225
Ovary	Atrophy225
	Hyperplasia, tubulostromal225
	Pigment225
	Cyst, follicular226
Uterus	Inflammation, myometrium (metritis)13,227
Respiratory system
Trachea	Inflammation, chronic (chronic tracheitis)226
Lung	Alveolar macrophage aggregation (accumulation of foam cell) 225,226
	Congestion, pulmonary225
	Inflammation (inflammatory cell infiltrate, pneumonia)13,225–227
	Megakaryocytes in alveolar wall225
	Inflammation, chronic bronchioloalveolar (lymphoid cuffing airways)226
	Thrombosis227
Urinary system
Kidney	Basophilia, tubule225
	Crystals (calcium deposition)13,225
	Casts225	Hyaline or granular
	Cyst225
	Dilation, Bowman’s space225
	Dilatation, tubule225,226
	Extramedullary hematopoiesis225,226
	Fibrosis225,233
	Hypertrophy, tubule225,233
	Infiltrate, inflammatory cell (interstitial nephritis)13,225–227,233,234
	Accumulation, pigment225
	Vacuolation, tubule225
	Glomerulopathy, mesangioproliferative (mesangial hyperplasia)13,233–236	IgM-mediated mesangioproliferative nephropathy; mesangium stains positive with PAS; may be associated with tubulointerstitial lesions

PAS- Periodic Acid-Schiff, TRAP-Tartrate-Resistant Acid Phosphatase

Conditions associated with rapid body weight loss and muscle wasting historically have been referred to as Marmoset Wasting Syndrome (MWD). It is likely that it represents a group of entities and etiologies with similarities to human inflammatory bowel disease. The term lacks diagnostic specificity, and the term chronic lymphocytic enteritis has been introduced.⁶² Very recently, a gastrointestinal disease characterized by duodenal dilation was described.⁶³

Tamarins (Saguinus spp)

Tamarins are small New World monkeys in the family Callitrichidae and subfamily Callitrichinae (similar to Marmosets). Adults weigh less than 0.5 kg. Tamarins of the genus Sanguinus are native to northwestern Colombia, the Amazon basin, and the Guianas. There are multiple species in genus Sanguinus.⁶⁴ Although research use of marmosets is increasing, tamarins are not commonly used primarily due to presence of few established breeding colonies and declining wild populations. However, tamarins particularly have been informative in ulcerative colitis/colon cancer,⁶⁵ inflammatory bowel disease,⁶⁶ and Crohn’s disease⁶⁷ studies. The cotton-top tamarin (S. oedipus) particularly has been studied as a model of chronic colitis and colon cancer.^13,68 Currently they are a critically endangered species and uncommon in research, however, the red-bellied tamarin (S. labiatus) and the mustached tamarin (S. mystax) are used in research on viral hepatitides.⁶⁹ In addition, mustached tamarins are a promising model for cardiomyopathy.^70,71  Table 7 summarizes common conditions and diseases (background findings) in cotton top and mustached tamarins.

Table 7.

Tabulated Overview of Morphological Background Findings in Tamarins (Saguinus spp)

Organ systems	Findings	Comments
Moustached tamarins (S mystax)
Cardiovascular system
Heart	Interstitial myocardial fibrosis13,70
	Intracardiac thrombosis associated with congestive heart failure secondary to cardiomyopathy71
	Aortic dissecting aneurysms71
	Cardiomegaly13
Gastrointestinal system
Colon	Colitis cystica profunda (CCP)- mucin-filled epithelial down growths and cysts in colonic submucosa70,237
Liver, gallbladder, and exocrine pancreas
Liver	Hepatocellular vacuolation70	Associated with anorexia
Respiratory system
Lung	Pulmonary alveolar proteinosis238
Urinary system
Kidney	Membranoproliferative glomerulonephritis70
Cotton-top tamarins (S oedipus)
Gastrointestinal system
Colon	Colitis with progression to colonic adenocarcinoma13

Owl Monkey (Aotus spp)

Owl monkeys are neotropical primates belonging to the genus Aotus. Aotus spp are native to primary and secondary forests of South America, ranging from Panama to northern Argentina. They are the only genus of simian that is completely nocturnal and are characterized by large eyes, small external ear pinnae, thick fur, and a prehensile tail. They are socially monogamous, living in small family groups consisting of a monogamous pair and immature offspring, with both sexes involved in parental care.⁷² The taxonomy has undergone significant changes and is still subject to debate. Once classified as a single species (Aotus trivirgatus), up to 11 different species are recognized, often more easily characterized by karyotype. Research-relevant differences among species and karyotypes are recognized. Examples include karyotype variations in susceptibility to Plasmodium falciparum,⁷³ susceptibility to the vitamin E-responsive hemolytic anemia and idiopathic eosinophilia syndromes,^74,75 and in calcium-phosphorus and electrolyte metabolism.⁷⁶

Owl monkeys have been used extensively in malaria research since the 1960s and in research of various other infectious diseases as well as in vision research. Common spontaneous diseases in captive Aotus are glomerulonephritis, cardiomyopathy, hemolytic anemia, and aortic dissection. Additionally, Aotus are highly susceptible to 2 herpesviruses that are enzootic and usually subclinical in squirrel monkeys. In Aotus species, Saimiriine alphaherpesvirus 1 (herpesvirus tamarinus or Herpes T) can cause mucocutaneous ulceration, severe multisystemic necrosis, and mortality, and Saimiriine gammaherpesvirus 2 causes lymphoproliferative disease (lymphoma/leukemia). Table 8 summarizes common conditions and diseases in owl monkeys.

Table 8.

Tabulated Overview of Morphological Background Findings in Owl Monkeys (Aotus spp)

Organ	Diagnosis (synonyms)	Description
Multisystemic	Lymphoma239	Lymphoma and/or leukemia develop as result of Saimiriine herpesvirus 2 infection
	Necrosis and ulceration (herpesvirus)240,241	Ulceration and necrosis in multiple organ systems result of Saimiriine alphaherpesvirus 1 and/or Herpes simplex virus 1 (Human alphaherpesvirus 1)—skin, oral mucosa, gastrointestinal tract, lung, liver. Characteristic herpesvirus intranuclear inclusion bodies
	Eosinophilia, multisystemic242,243	Some Aotus species have eosinophil counts higher than others, eosinophilia in multiple organs was reported in 1 A vociferans, a species not typically associated with elevated eosinophil counts
Cardiovascular system
Heart	Hypertrophy, left ventricular (hypertrophic cardiomyopathy)244–246	Older animals. Concentric hypertrophy results in increased left ventricular wall thickness to chamber radius ratio; increase in mass of heart overall
	Dilation, left ventricle244,246	Less common than hypertrophic. Older animals. Decreased wall thickness to chamber radius ratio, increase in left ventricular mass. Myocyte atrophy and loss with fibrosis
Blood vessels	Dissection, aorta247–249	Can be acute or chronic, may rupture leading to hemothorax. Chronic often leads to “double-barrel” aorta. Described as aneurysms in older literature, but dissection usually more accurate term
	Infiltrate, eosinophilic249	Associated with aortic dissection in 1 case
	Vascular thickening, hyaline (arteriolosclerosis)250	Hyaline; likely plasma protein accumulation secondary to chronic hypertension; seen in multiple organs, including heart, kidneys
Hematopoietic system
Bone marrow	Eosinophilic myelocytoma251	Case report, neoplastic cells also found in mediastinum, lymph nodes, kidneys
	Increased erythropoiesis (regenerative response)252	With hemolytic anemia
	Hemolytic anemia252,253	Vitamin E-responsive
Gastrointestinal system
Tongue	Ulceration, necrosis240	Herpesviruses – Aotus susceptible to Herpes simplex virus (Human alphaherpesvirus 1); similar lesions seen with Saimiriine herpesvirus 1
Small intestine	Enteritis, necrotizing and neutrophilic (Pseudotuberculosis)254	Yersinia enterocolitica reported as enzootic in Aotus colony. Lesions also in liver and spleen
	Acute catarrhal or hemorrhagic enteritis255	Various enteric bacteria cultured
Liver, gallbladder, and exocrine pancreas
Liver	Hemosiderin, hepatocytes and Kupffer cells (hemochromatosis)248,252	May be associated with hemolytic anemia but also seen in many non-anemic animals
	Inflammation, lymphoplasmacytic, periportal (cholangitis)248
	Necrosis, coagulative, centrilobular252	Associated with hypoxia due to hemolytic anemia
Gallbladder	Cholelithiasis248,256	Composed primarily of cholesterol, similar composition to those in humans
	Thickening, wall248
Integumentary System
Skin	Trauma75
	Decubital ulcers75	Pressure sores, result of poor husbandry
Musculoskeletal system
Skeletal muscle	Necrosis (necrotizing myopathy)253,257	May be associated with hemolytic anemia; may be related to Vitamin E metabolism
Respiratory system
Air Sac	Inflammation (air sacculitis)258	Klebsiella pneumoniae is most common organism isolated; also Streptococcus, Proteus mirabilis, E coli
Lung	Inflammation, giant cell248	Giant cell pneumonia
	Edema, pulmonary248	Resulting from heart failure
	Inflammation, granulomatous259	Mycobacterium tuberculosis and M. kansasii were isolated in natural tuberculosis outbreak in Panama; granulomas also found in pericardium, heart, liver, and spleen
Special senses
Eye	Keratitis260	Alpha herpesviruses (Herpes simplex virus and Saimiriine alphaherpesvirus 1)
	Dyscoria241	Alphaherpesvirus; intranuclear inclusion bodies and syncytia seen in epithelial cells
Urinary system
Kidney	Chronic nephropathy246,248,252,261,262	Older animals, very common; more specific lesions below
	Degeneration/fibrosis, tubular248,252
	Fibrosis, interstitium248,252
	Infiltrate, lymphoplasmacytic, eosinophilic, interstitium246,248,252
	Casts, hyaline248,252
	Casts, cellular246,248,252
	Sclerosis, atrophy, glomerulus248,252,262
	Pigment, bile, tubular epithelium248,252
	Glomerulonephritis, membranoproliferative252,261,262

Squirrel Monkey (Saimiri spp)

Squirrel monkeys are neotropical primates belonging to the genus Saimiri. They are endemic to the Amazon basin of South America, with small, isolated populations in Central America. The taxonomy of Saimiri is historically complex. Once considered to be a single species (S. sciureus), there are now at least 4 recognized species and 9 subspecies.⁷⁷ The South American species can be divided into 2 phenotypically distinct groups based on the shape of the patch of nonpigmented hair above the eyes: Saimiri boliviensis (Roman type) group and Saimiri sciureus (Gothic type) group. The 3 subspecies most commonly used in biomedical research are S. sciureus sciureus (Guyanese squirrel monkey), S. boliviensis boliviensis (Bolivian squirrel monkey), and S. boliviensis peruviensis (Peruvian squirrel monkey). Karyotypic differences in the number of acrocentric chromosome pairs between species and subspecies may render hybrids subfertile, and hybridization between species occurred in some research colonies before differences were widely recognized. Exportation of wild-caught squirrel monkeys was limited starting in the 1980s, and captive breeding resources are the most important sources of Saimiri spp in contemporary research.

Correct taxonomic identification of squirrel monkeys is recognized as particularly relevant to malaria and gallstones, reviewed in Abee⁷⁸ and Vanderberg et al.⁷⁹ Specifically, S. boliviensis are susceptible to malaria (Plasmodium falciparum Indochina I) and develop similar lesions as in humans, making them excellent models for malaria vaccine research, while S sciureus are resistant to infection and therefore less useful as models.⁸⁰  S. sciureus, but not S. boliviensis, are susceptible to gallstones.⁸¹ Other species/subspecies physiological variations that have been reported include incidence of glomerulonephritis, aspects of growth and development, and immunoglobulin structure.⁷⁹

Their small body size (<1 kg), preference for group housing, relatively short time to sexual maturity (2.5–3 years), low zoonotic risk, and adaptability to the captive environment make squirrel monkeys desirable candidates for biomedical research.⁷⁸ Early uses in biomedical research were primarily in the study of space flight, atherosclerosis, artificial insemination, and malaria vaccine development. They continue to be used to study genetically based social behaviors, aging, and transmissible spongiform encephalopathies, among other conditions. Table 9 summarizes common conditions and diseases of Saimirii spp.

Table 9.

Tabulated Overview of Morphological Background Findings in Squirrel Monkeys (Saimirii spp.)

Organ	Diagnosis (synonyms)	Description
Multisystemic	Toxoplasmosis263,264	Necrosis, intracellular organisms,
	Encephalitozoonosis265	Granulomatous inflammation in multiple organs, with vasculitis, perivasculitis, and intralesional protozoa. Transplacental infection, also in young animals (<9 months)
	Leukemic histiocytic sarcoma266	Saimiriine gammaherpesvirus 2, Saimiri sciureus lymphocryptovirus 2, and squirrel monkey retrovirus coinfection—more research is needed
	Lymphoma, histiocytic type267	Mediastinal mass with infiltration but no metastasis
	Mycobacteriosis268	Mycobacteria microti granulomas in mesentery, lymph nodes, skeletal muscle, vertebrae (n = 3)
Cardiovascular system
Heart	Dilative cardiomyopathy269,270	Congestive heart failure; geriatric animals; common cause of death
Blood vessels	Atherosclerosis271,272	Generally diet-induced
	Arteriosclerosis273
	Aortic dissection273,274	Naturally occurring
	Aortic aneurysm274	Saccular and fusiform, found in animals fed atherogenic diets
Gastrointestinal tract
Teeth	Periodontal disease275
Small intestine	Necrotizing enterocolitis276	Yersinia enterocolitica serovar O8; necrotizing enterocolitis and necrosis or abscesses in multiple organs
Liver, gallbladder, and exocrine pancreas
Liver	Subcapsular cysts/cystic intrahepatic bile ducts (CLH - personal communication)	von Meyenberg complexes; apparently incidental
Gallbladder	Cholelithiasis78,277	Bacteria-associated; also diet-induced cholesterol stones
Integumentary system
Skin	Atypical mycobacteriosis278
	Subcutaneous leiomyosarcoma279,280
	Alopecia281	Age-related in females; chronic telogen effluvium
Musculoskeletal system
Bone	Chondrosarcoma282
Nervous System
Brain	Beta-amyloid283	Senile plaques or cerebral amyloid angiopathy; monkeys >12 y
Reproductive system
Ovary	Granulosa cell clusters284	Non-neoplastic, common in older females
Uterus	Endometrial adenocarcinoma285
Vagina	Pelvic organ prolapse286,287
Urinary system
Kidney	Glomerulonephropathy/glomerulonephritis288,289	Mesangioproliferative, proliferative, membranous, or sclerosing; associated with IgM deposits in some cases
	Interstitial fibrosis289	Associated with chronic glomerulonephritis
	Polycystic kidneys/renal cysts290
	Pyelonephritis289	±Urinary calculi

Summary

NHPs continue to provide important insights into disease mechanisms and therapeutic interventions that impact human and animal health. Efforts to improve recognition and interpretation of research-relevant background conditions and lesions, as well as efforts to improve recognition and reporting of geographic origins and associated genetic variations, aim to improve the interpretation and validity of NHP studies.