Atresia Ani with Imperforate Anus in a Common Marmoset (Callithrix jacchus)

Abstract

In November 2015, an 83-d-old juvenile male common marmoset (Callithrix jacchus) in good body condition was found dead in his family cage with no previous premonitory signs. Necropsy revealed a gas-distended abdomen, feces-distended large bowel, and a full-thickness distal colonic perforation resulting in fecal peritonitis. The distal colon ended in a blind pouch at 7 mm prior to the expected anal opening, consistent with atresia ani. Here we present this case, briefly discuss the human and veterinary literature regarding correction of anorectal malformations, and highlight the importance of identifying such devastating congenital defects in breeding colonies while limiting the disruption and handling of seemingly healthy, young NHP raised in a complex social setting.

Common marmosets, Callithrix jacchus, are small, highly vocal New World NHP with unique biologic traits and increasing species popularity and demand for use throughout biomedical research. Compared with Old World NHP species, including macaques and baboons, marmosets are considerably safer and easier to handle, house, and care for in laboratory settings.⁸ In addition, the species is highly parous, with females capable of giving birth twice in a single year.

Furthermore, common marmosets are relatively short-lived, enabling rapid colony expansion and ease of use for reproduction, growth, development and aging studies. Indeed, over the past several decades, the use of marmosets has evolved considerably to include fields ranging from infectious disease, toxicology, and drug development to advanced behavior, cognition, and neuroscience.¹¹ With a published whole-genome sequence¹² and the creation of transgenics,¹⁶ the use of common marmosets as a highly translational NHP model will likely continue to expand.

Colonies of breeding common marmosets are typically housed in extended family units, where a single cycling dominant female suppresses ovulation in subordinates¹ and older siblings and family members assist in the rearing of offspring. Neonatal marmosets strongly cling to their caretakers’ backs, gradually exploring their environment as they become increasingly independent and eventually are completely weaned at approximately 3 mo of age.¹⁹ Most common marmosets give birth to twins, but litter sizes ranging from 1 to 5 offspring have been documented.²⁰ Infant mortality ranging from 25% to 40% is common, with an increased propensity for neonatal death in smaller animals from larger litters.²⁰ Several hand-rearing and supplemental feeding protocols for infants from litters of 3 or more have been established,^6,26 however potentially adverse consequences ranging from insufficient infant growth and development to atypical behavior during adulthood may result and thus should carefully be considered prior to implementation. Often, the time and complexities associated with alternative rearing strategies for litters larger than 2 is avoided through colony management or mother-driven selection,²⁰ where the smallest or less fit of the litter are euthanized shortly after parturition.

Due to recent and continued expansion of the use of marmosets in research, it is especially important in the existing founder breeding colonies that careful attention is paid to ensure appropriate fecundity while maintaining optimal animal welfare of neonates and the colony as a whole. Included in these breeding strategies should be the prevention of inbreeding and careful physical exam, observation, and documentation of any potential deleterious congenital defect that could have a genetic basis and thus be propagated within the colony if these animals are allowed to reproduce. Here we identify and describe such a potential case.

Case Report

A 96.6-g, 83-d-old, juvenile male marmoset of good body condition (2.5/5) was found dead in his group enclosure. From birth through the time of death, the animal remained consistently comparable in physical condition to his female twin and demonstrated no group social or individual behavioral abnormalities suggestive of underlying clinical complications. Comprehensive, full-body necropsy revealed a markedly distended and perforated bowel (Figure 1 A) that appeared secondary to a completely imperforate anus (Figure 1 B). Further dissection revealed that the colon terminated in a blind-ended rectal sack approximately 7 mm anterior to the expected anal opening (Figure 1 C and D). No other anomalies or relevant pathology was identified.

Figure 1.

(A) Distended large bowl with perforation (insert). (B) External male genital with absence of external anal opening (arrow). (C) Termination of the large bowel within the abdominal cavity. The dashed line indicates the distance between the blind pouch and expected anal opening (7 mm). (D) Blind-ended rectal sack (arrow). In each panel, cranial is to the left (A) or top (B through D).

The affected juvenile and his female twin were the only progeny derived from a breeding pair comprising a 3.3-y-old multiparous dam and a 4.5-y-old first-time sire. In May 2017, the dam and an unrelated sire successfully reproduced, but only 1 of the 2 offspring born was viable; the stillborn exhibited marked intracranial hemorrhage at necropsy but lacked any congenital defects. The sire of the animal we present here was euthanized in December 2016 due to unmanageable inflammatory bowel disease and metabolic bone disease (that is, Marmoset Wasting Syndrome)³ and has only one surviving decedent, the presented animal's apparently healthy twin sister.

All marmoset breeding protocols and supporting internal practices were reviewed and approved by the Johns Hopkins University IACUC in accordance with the Animal Welfare Regulations² and the Guide for the Care and Use of Laboratory Animals.¹⁴

Discussion

The absence of fibrosis and scarring along the juvenile's blind ended rectal sack is most consistent with the congenital deformity atresia ani with secondary bowel rupture. Atresia ani is one of several anorectal malformations, deformities occurring in approximately 1 in 5000 live births in humans and encompassing a broad spectrum of diseases that result in defects that range in severity and prognosis.⁹ The etiology of these malformations, which can be complicated by the involvement of the urinary and genital tracts, is likely multifactorial, but there is strong evidence that at least some types have a genetic and thus heritable component.⁴ In newborn humans, early management and decision making for correction are key. Assessment of the anorectal malformation informs whether to address the primary defect immediately or perform a protective procedure and divert the gastrointestinal tract temporarily to allow for a definitive repair once the baby is older, larger, and less fragile.²⁵ Protective procedures involve creating a colostomy in the lower left quadrant of the abdomen, with a proximal stoma bypassing the final segment of the colon and draining the gastrointestinal tract to the body's surface. A distal stoma, called a mucous fistula, is created to allow drainage of mucous buildup from the small distal section of colon. If the baby is growing well after the procedure, the primary defect usually is repaired through a posterior sagittal approach¹⁷ approximately 1 to 2 mo later, and the stomas can be disassembled and the intestine returned to continuity.⁹ The functional outcome of such surgeries varies widely by the degree of the defect and extent of required reconstruction, with constipation and soiling being the most commonly encountered postoperative problems.²⁵

In the veterinary literature, atresia ani is the most commonly identified anorectal malformation in small animals and is known to be hereditary in swine and cows.¹⁵ In swine, genome-wide association studies have uncovered regions of chromosomes 1, 3, 9, and 12 that show significant linkage with this syndrome.¹⁸ However, the actual incidence of this defect in various species is difficult to determine from the veterinary literature, because many newborns with this condition are euthanized without surgical correction, and the condition is not reported. Anatomically, atresia ani has been organized into 4 types in order of increasing severity: type I, characterized by congenital stenosis of a patent anus; type II, imperforate anus alone; type III, presence of an imperforate anus with the colon terminating further cranially; and type IV, characterized by a normal-ending terminal rectum and anus and a cranial colon that terminates blindly within the pelvis.¹⁵ The case we presented here is classified as type III. Animals with anorectal malformations can appear clinically normal for the first 2 to 4 wk after birth, with clinical signs of tenesmus, anorexia, depression, and abdominal distention only appearing later.¹⁵ Although it is highly unusual for a marmoset to live to be 83 d of age without any apparent way to void digesta, there is at least one case of a kitten that was diagnosed at 3 mo of age.²³ However, in this kitten, a small secondary rectovaginal fistula had opened between the colon and the vagina, allowing some feces to pass.

In the veterinary literature, correction of anorectal malformations has been reported, including in the kitten mentioned earlier.²³ In addition, a balloon dilation procedure has been used successfully to treat type I atresia ani in several cats and a dog;²¹ however surgical correction is required in cases with no rectal patency. Unlike in human medicine, most published reports of atresia ani correction in animals center around immediate surgical correction of the primary defect by using anoplasty or rectal pull-through procedures, with protective procedures involving temporary colostomies and secondary surgeries for correction at a later date reported only rarely.²²

Relatively few cases of atresia ani in NHP have been reported. Documented cases include a 10-d-old squirrel monkey with multiple congenital anomalies⁵ and a baboon (details not specified).¹⁰ Although cases of atresia ani in common marmosets have not been reported previously, other congenital defects including a trisomy 13-like syndrome in one animal,¹⁸ an imperforate vagina in another,¹³ and congenitally fused labias in several animals⁷ have been reported. Importantly, the cases of fused labias were found in 12 animals that came from 2 genetically related colonies. These animals were physically unable to copulate, and the trait appeared to be genetically linked and passed recessively.⁷

In light of these reports and because the atresia ani case we presented here may have a genetic component, given the similar congenital abnormalities reported in humans and other animals, early physical examination for such congenital defects should always be included in the standard captive breeding colony care regimen for neonatal marmosets. Identifying such congenital defects and ensuring that these animals do not reproduce will help to limit the propagation of genetic disorders and maintain genetic diversity as the use of marmosets in biomedical research continues to expand.