Overview of Coronaviruses in Veterinary Medicine

Abstract

Coronaviruses infect humans and a wide range of animals, causing predominantly respiratory and intestinal infections. This review provides background on the taxonomy of coronaviruses, the functions of viral proteins, and the life cycle of coronaviruses. In addition, the review focuses on coronaviral diseases in several agriculturally important, companion, and laboratory animal species (cats, cattle, chickens, dogs, mice, rats and swine) and briefly reviews human coronaviruses and their origins.

Coronaviruses get their name from their morphology, as the spike proteins present in the envelope of the virus give the virions a crown-like (‘corona’) appearance when viewed by electron microscopy. Coronaviruses are enveloped, spherical or pleomorphic viruses ranging from 120 to 160 nm in diameter. Coronaviruses have the largest nonsegmented positive-sense single-stranded RNA viral genome (25 to 32,000 nucleotides).^22,69 The genome is polyadenylated and capped. Coronavirus genomes can serve as mRNAs, and purified genomic RNA is infectious. The arrangement of genes is similar for all coronaviruses, with the nonstructural replicase gene encoded in the 5′ two thirds of the genome, and the genes for the structural proteins and several small accessory proteins encoded in the 3′ third of the genome.

Taxonomy

The 4 genera of Coronaviridae are Alphacoronavirus, Betacoronavirus, Deltacoronavirus, and Gammacoronavirus (Table 1).^17,19,30 The genus Alphacoronavirus includes 14 subgenera and 19 species. Several coronaviruses that infect dogs (canine enteric coronavirus), cats (feline infectious peritonitis virus [FIPV], feline enteric coronavirus [FECV]) and swine (transmissible gastroenteritis virus [TGEV], porcine respiratory coronavirus [PRCV], and porcine endemic diarrhea virus [PEDV]) are grouped in the species Alphacoronavirus 1, as their replicase genes are at least 90% homologous. Two other swine, 2 human, and 11 bat coronavirus species also belong to the genus Alphacoronavirus. The genus Betacoronavirus includes 5 subgenera and 14 species. Three coronaviruses that infect cattle (bovine coronavirus [BCoV]), swine (porcine hemagglutinating encephalomyelitis virus [PHEV]) and humans (human coronavirus OC43) are grouped in the species Betacoronavirus 1. Three human, one rodent (mouse hepatitis virus [MHV]/rat coronavirus [RCV]), and 7 bat coronavirus species also belong to the genus Betacoronavirus. The genus Deltacoronavirus includes porcine deltacoronavirus HKU15 and several coronaviruses of wild birds. The genus Gammacoronavirus includes coronaviruses that infect several domesticated avian species (chickens, geese, pheasants, quail, turkeys, and ducks) as well as wild avian species.

Table 1.

Coronavirus taxonomy

Genus	No. of species	Subgenus	Relevant viruses
Alphacoronavirus	19
		Tegacovirus	Alphacoronavirus 1 (canine enteric coronavirus, feline infectious peritonitis virus, feline enteric coronavirus, transmissible gastroenteritis virus, porcine respiratory coronavirus)
		Duvinacovirus	Human coronavirus 229E
		Pedacovirus	Porcine endemic diarrhea virus
		Rhinacovirus	Swine enteric alphacoronavirus
		Setracovirus	Human coronavirus NL63
		5 additional subgenera	Bat coronaviruses (11 species)
Betacoronavirus	14
		Embecovirus	Betacoronavirus 1 (bovine coronavirus, Human coronavirus OC43, porcine hemagglutinating encephalomyelitis virus)
		Embecovirus	Human coronavirus HKU1
		Embecovirus	Murine coronavirus (mouse hepatitis virus, rat coronavirus)
		Merbecovirus	Middle East respiratory syndrome coronavirus
		Sarbecovirus	Severe acute respiratory syndrome coronavirus, severe acute respiratory syndrome coronavirus 2
		4 additional subgenera	Bat coronaviruses (7 species)
Deltacoronavirus	7
		Buldecovirus	Porcine deltacoronavirus
Gammacoronavirus	5
		Igacovirus	Avian coronavirus (infectious bronchitis virus)
Compiled from reference 30

Coronavirus structural proteins

All coronaviruses encode the following 4 structural proteins: spike (S), envelope (E), membrane (M) and nucleocapsid (N) proteins.^22,69 Some coronaviruses also encode a fifth structural protein, the hemagglutinin–esterase (HE) protein. The S glycoprotein forms the characteristic spikes protruding from the envelope of the virus and is the largest coronaviral structural protein (1128 to 1472 amino acids). It forms homotrimers, is highly glycosylated, and is cleaved into 2 domains (S1 and S2) by host proteases.⁶³ S proteins bind to cell surface receptors to mediate viral entry into the cell. Coronaviruses use several different receptors to bind to cells and initiate infection (Table 2). The specificity of the receptor-binding domain of the S protein for the cell surface receptor is the primary determinant of tissue tropism and host range. For example, MHV uses murine carcinoembryonic antigen-related adhesion molecule as its receptor.⁷⁰ MHV infects only mice, and no other coronavirus has been shown to bind to murine carcinoembryonic antigen-related adhesion molecule.¹² In contrast, several alphacoronaviruses use aminopeptidase N (APN, also known as CD13) as a receptor. Canine enteric coronavirus, FIPV, TGEV, and human coronavirus 229E can infect cells expressing feline APN.^61,62 Similarities in the viral-binding region of the APN proteins in cats, dogs, swine and humans account for these cross-species infections. Porcine deltacoronavirus (PDCoV) also uses APN as a receptor; PDCoV can infect cells expressing porcine, feline, human, or chicken APN, and young chickens and turkeys can be experimentally infected with PDCoV.^9,44 Cross-species transmission plays a major role in the emergence and evolution of new viruses. The ability to use similar receptor molecules in a new host is believed to have resulted in the emergence of severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and SARS-CoV2 when a coronavirus jumped from an animal host into humans.^17,19,30

Table 2.

Coronavirus receptors

Genus	Virus	Receptor	Reference
Alphacoronavirus	Canine enteric coronavirus, feline infectious peritonitis virus, feline coronavirus	Aminopeptidase N	61
	Human coronavirus 229E	Aminopeptidase N	81
	Human coronavirus NL63	Angiotensin converting enzyme 2	56
	Transmissible gastroenteritis virus, porcine respiratory coronavirus, porcine endemic diarrhea virus	Aminopeptidase N	20
Betacoronavirus	Bovine coronavirus	N-acetyl-9-O-acetylneuraminic acid	66
	Human coronavirus HKU1	N-acetyl-9-O-acetylneuraminic acid	29
	Human coronavirus OC43	N-acetyl-9-O-acetylneuraminic acid	35
	Middle East respiratory syndrome coronavirus	Dipeptidyl peptidase 4	50
	Mouse hepatitis virus	Carcinoembryonic antigen-related adhesion molecule	70
	Porcine hemagglutinating encephalomyelitis virus	N-acetyl-9-O-acetylneuraminic acid	54
	Severe acute respiratory syndrome coronavirus	Angiotensin converting enzyme 2	43
	Severe acute respiratory syndrome coronavirus 2	Angiotensin converting enzyme 2	27
Deltacoronavirus	Porcine deltacoronavirus	Aminopeptidase N	44
Gammacoronavirus	Infectious bronchitis virus	⟨2,3-linked sialic acid	71

The S protein is a class I fusion protein and after binding of the S protein to the cell-surface receptor, S protein-induced fusion of the viral envelope with the host cell surface or endosome membranes releases the viral genome into the cytoplasm of the cell.⁷ S protein that is not incorporated into virions during viral assembly can be inserted into cell surface membranes, where it mediates cell–cell fusion leading to the production of giant multinucleated syncytia. The S protein induces both neutralizing and cell-mediated immunity. Almost all neutralizing antibodies are specific for the epitopes in the amino-terminal half of the S protein. Antigenic cross-reactivity among coronaviruses is limited to closely related species in the same genus.⁷

The E protein is a small pentameric integral membrane protein (74 to 109 amino acids) that is a minor component of the virion (approximately 20 copies per virion).⁵³ It has ion channel (viroporin) activities. Newly synthesized E protein is localized primarily in the endoplasmic reticulum–Golgi intermediate complex, where it is involved in the assembly, budding, and intracellular trafficking of infectious virions.⁵³

The M protein is the most abundant protein in the viral envelope. It is an integral type III membrane glycoprotein (218 to 263 amino acids), with 3 transmembrane domains, and is cotranslationally inserted into the membranes of the endoplasmic reticulum–Golgi intermediate complex.⁶³ Interactions between M proteins are believed to form a lattice-like structure on which assembly of virions occurs. Exclusion of host proteins from the membrane by M proteins creates space for the insertion of S and HE proteins into the viral envelope. Anti-M protein antibodies can neutralize coronaviruses in the presence of complement.⁶³

The N phosphoprotein is a basic RNA-binding protein (349 to 470 amino acids) that coats the RNA genome to form the helical nucleocapsid.^22,69 The N protein interacts with nonstructural genes to tether the viral genome to the replication–transcription complex and with the M protein during assembly of virions; the N protein also has RNA chaperone activity. The N protein of some coronaviruses can act as a type I interferon antagonist and can activate NF-KB. The N protein is the dominant coronaviral antigen, and although anti-N antibodies are not protective, they are commonly used in serodiagnostic tests.^22,69

The HE protein is present in some betacoronaviruses and forms a spike that is shorter than the S spike in the envelope of the virus.^22,69 The HE protein is a homodimeric type I glycoprotein with sialic acid binding and acetylesterase activities; HE may enhance S protein-mediated cell entry and spread within infected tissues. Antibodies to the HE protein can inhibit virion binding to O-acetylated sialic acids and inhibit release of virus from the cell via inhibition of the HE protein’s sialate-O-acetylesterase activity.^22,69

Life cycle

After binding of the S and HE proteins in the viral envelope to cell surface receptors, the virus enters the host cell, where the RNA genome is uncoated in the cytoplasm. Coronavirus replication and assembly takes place in the cytoplasmic compartment of the cell. The 2 large overlapping open reading frames (ORF1a and ORF1b) of the replicase gene are translated into a large polyprotein via a ribosomal frameshift.^22,69 The polyprotein is co- and post-translationally cleaved to produce 15 to 16 proteins with RNA-dependent RNA polymerase, proteinase, helicase, single-stranded RNA binding, exoribonuclease, endoribonuclease, ribose methyltransferase, ADP ribose phosphatase, and interferon antagonist activities. These enzymes initiate the transcription of negative-strand copies of the genome. From these new negative strand copies, new full-length genomes and a nested set of 3′ coterminal mRNAs are transcribed. In general, only the 5′ end of each mRNA in the nested set is translated. This large genome size, together with an RNA-dependent RNA polymerase that does not have a proofreading function, means that during coronavirus infection, a population of variant viruses is produced, each with one or more changes to their genome. In addition, coronaviruses undergo both homologous and nonhomologous recombination that is linked to the strand-switching ability of the RNA-dependent RNA polymerase. Polymerase errors and recombination play an important role in the evolution of coronaviruses.^22,69

Animal Coronaviruses

Coronaviruses infect a wide range of animal species, targeting primarily the intestinal and respiratory tracts (Table 3). These viruses cause a spectrum of diseases ranging from subclinical to fatal infections. Coronaviruses cause diseases in agriculturally important species (chickens, cattle, and swine) as well as in companion and laboratory animals (cats, dogs, mice and rats).

Table 3.

Target tissues for coronavirus infections

Virus	Intestine	Respiratory tract	Lymphoid tissue	Nervous system	Liver	Kidney	Salivary/ lacrimal gland	Gonad
Bovine coronavirus	x	x
Canine coronavirus	x	x
Feline enteric coronavirus, feline infectious peritonitis virus	x	x	x
Human coronaviruses (229E, HKU1, NL63, OC43)		x
Infectious bronchitis virus	x	x	x			x	x	x
Middle East respiratory syndrome coronavirus		x
Mouse hepatitis virus	x	x	x	x	x
Porcine deltacoronavirus	x
Porcine epidemic diarrhea virus	x
Porcine hemagglutinating encephalomyelitis virus		x	x	x
Porcine respiratory coronavirus		x
Rat coronavirus		x	x				x
Severe acute respiratory syndrome coronavirus		x
Severe acute respiratory syndrome coronavirus 2		x
Swine enteric alphacoronavirus	x
Transmissible gastroenteritis virus	x

Chickens

‘Infectious bronchitis of baby chicks’ was described in 1931 and was determined to be caused by a virus in 1937. In chickens, IBV causes a respiratory disease with nasal discharge, sneezing, coughing, tracheal rales, and lethargy and predisposes chickens to secondary respiratory infections.¹⁰ IBV is transmitted via aerosols and fomites. IBV initially replicates in ciliated and mucus-secreting cells of the respiratory tract but can spread to the alimentary tract, kidney, testes, oviduct, Harderian glands, and bursa of Fabricius. Morbidity and mortality are highest in young chicks and vary among chicken breeds. The economic effect of IBV infection is large, as it causes growth retardation in meat birds and decreases the quantity and quality of eggs even after the infection has been resolved in egg-laying birds. IBV infection of chicks younger than 2 wk can be persistent, with reactivation of viral shedding around 5 mo of age when egg production begins. Some IBV strains infect epithelial cells of the collecting ducts, collecting tubules, distal convoluted tubules, and the loops of Henle in the kidney, causing nephritis. There are more than 50 serotypes of IBV that are poorly cross-protective, therefore several serotypes can cocirculate in a region. Vaccination of 1-d-old chicks with attenuated live virus via aerosols, intranasal inoculation, intraocular inoculation, or in drinking water has been used for over 50 y to prevent IBV infections.^32,60 Inactivated virus and subunit vaccines have also been developed. The USDA licenses 57 vaccines for IBV alone or in combination with other viruses.

Cattle

The first bovine coronavirus (BCoV) was identified in 1971. BCoV, human coronavirus OC43, and PHEV comprise a single species, Betacoronavirus 1 (Table 1). BCoV infects the respiratory and intestinal tracts of cattle and wild ruminants⁵² and is transmitted via contact with infected animals or exposure to feces or contaminated equipment. BCoV infects cattle of all ages and is endemic worldwide, as most adult cattle are BCoV seropositive. BCoV causes 3 different clinical syndromes: calf diarrhea, winter dysentery, and bovine respiratory syndrome/shipping fever.⁵² BCoV isolated from cattle with all 3 syndromes comprise a single serogroup.

Calf diarrhea occurs in young calves (1 d to 3 mo of age), with most infections occurring at 1 to 3 wk of age when maternal antibodies in the milk decline.⁵² Viral replication in the small and large intestine of calves produces villous atrophy and crypt hyperplasia, causing severely diminished absorptive and digestive capacities of the intestine and leading to dehydration and malnutrition. Diarrhea generally resolves in 2 to 8 d, but BCoV infection can be fatal in a minority of infected calves with severe diarrhea.

Winter dysentery occurs in adult dairy and beef cattle housed in crowded conditions during the winter months and is characterized by dark bloody diarrhea.⁵² Intestinal lesions resemble those of calf diarrhea but with extensive necrosis of the colonic crypt cells and intestinal hemorrhage. Cough, nasolacrimal discharge, fever, depression, and anorexia can also occur. A dramatic drop in milk production is seen for several months in dairy cattle infected with BCoV and can lead to substantial economic losses. Although morbidity is high (20% to 100%) in herds with winter dysentery, mortality is low (1% to 2%).

Bovine respiratory disease complex/shipping fever occurs in 6- to 10-mo-old feedlot cattle and is the result of interactions between BCoV, respiratory bacteria, stress, and environmental factors.⁵² It is characterized by fever, dyspnea, bronchopneumonia, weight loss and sometimes death. Shipping and comingling of cattle from many sources creates stress, and BCoV infection frequently occurs shortly after arrival of cattle in a feedlot. Subsequently, infection of the lung with commensal bacteria from the nasal cavity results in bovine respiratory disease complex. In addition, other bovine viruses (bovine respiratory syncytial virus, parainfluenza virus 3, bovine herpesvirus, bovine viral diarrhea virus) alone or in combination with BCoV can lead to bovine respiratory disease complex. The upper and lower respiratory tracts of 2- to 6-mo-old calves can be infected with respiratory strains of BCoV, leading to mild to moderate disease characterized by coughing, rhinitis, fever, dyspnea, and diarrhea.⁵² Intranasal vaccines against BCoV can be administered to cattle just prior to calving or to 1-to 4-d-old calves.⁶⁰

Swine

Six coronaviruses are known to infect swine. The first porcine coronavirus, TGEV (also known as Alphacoronavirus 1) was described in 1946.²¹ Transmission occurs via the fecal–oral route. TGEV replicates in the intestinal epithelium, causing villous shortening and resulting in malabsorption and maldigestion; death occurs 3 to 6 d after infection. Initially, TGEV caused acute epizootics of neonatal diarrhea and vomiting with dehydration and high mortality on breeding farms, recurring every 2 to 3 y as herd immunity waned. Disease was most severe in swine younger than 2 wk that were suckling TGEV-seronegative sows.²¹ As swine breeding intensified on large farms, enzootic subclinical infections became common in weaned nursery swine. PRCV, a variant of TGEV that has a 200 amino-acid deletion in the amino-terminus of the S protein, was described in 1984 in Belgium and causes an asymptomatic to mild respiratory disease with tachypnea, polypnea, dyspnea, sneezing, coughing, hyperthermia, anorexia, and delayed growth.⁵² Mortality is negligible. The deletion in the S protein of PRCV removes the sialic acid binding activity of the virus, preventing PRCV from binding to mucins and mucin-like glycoproteins in the intestine and changing the tissue tropism of the virus to the upper and lower respiratory tract.⁵⁵ Immunity to PRCV protects against TGEV infection, thus leading to a decline in TGEV outbreaks during the last 30 y.¹¹ Although modified live and inactivated TGEV vaccines exist, they are used infrequently because the prevalence of TGEV is low and declining. An oral recombinant-corn–based vaccine produces a mucosal immune response.⁶⁰

PEDV, an alphacoronavirus in the subgenus Pedacovirus, was recognized as the cause of ‘epidemic viral diarrhea’ of pigs in 1977.^34,41 PEDV infects all ages of swine, and the severity of disease is inversely proportionate to the age of the swine. PDEV causes watery diarrhea, anorexia, depression, dehydration, and vomiting primarily in newly born piglets. Clinical signs in piglets last for 3 to 4 wk, and PDEV is shed in the feces for as long as 4 wk. Mortality is 50% to 100% in newly born piglets. In older swine, PDEV infection causes minimal clinical signs, but infection can decrease growth rates in weaner and finisher swine. PDEV is transmitted via the fecal–oral route or via fomites. PDEV is highly enterotropic, infecting small intestinal enterocytes, causing severe diffuse atrophic enteritis, villous necrosis and sloughing of the villous lamina propria. The slow enterocyte turnover rate in newly born swine results in maladsorptive and maldigestive diarrhea. PDEV infections have caused substantial economic losses in the pork industry in Asia and North America. For example, in 2013, when PDEV emerged in North America to cause a yearlong epidemic, more than 10% of the US pig population was lost. During endemic infections, newly born piglets that do not receive adequate maternal antibodies can permit a recurrence of epidemic infection with high mortality. In addition, the emergence of PDEV variants can lead to new epidemics. Attenuated and inactivated PEDV vaccines have been developed for use mainly in pregnant sows but have only low to moderate effectiveness.^60,67

PHEV (also known as Betacoronavirus 1) emerged in nursery swine in 1957 in Canada.⁴⁶ PHEV was so-named because it hemagglutinates chicken, rat, and mouse erythrocytes, and in brains (mesencephalon, pons, medulla oblongata) from infected piglets it produces evidence of nonsuppurative viral encephalomyelitis, including perivascular cuffing with mononuclear cells, neuronal degeneration, and gliosis. PHEV infection is also known as ‘vomiting and wasting disease,’ with viral replication in the vagal sensory ganglion causing the vomiting.⁴⁶ Lesions in the myenteric plexus may cause gastric stasis and delayed emptying of the stomach. Piglets that are born to virus-naïve sows and are younger than 4 wk develop clinical signs that include reluctance to nurse, sneezing, fever, anorexia, constipation and severe progressive emaciation. Neurologic signs follow, including ataxia, tremors, paddling, hyperesthesia, vomiting, and paresis, and death occurs 2 to 3 d after initial clinical signs. Lactogenic immunity can protect piglets from clinical disease. Infection of adult swine is subclinical. The primary site of PHEV replication is the nasal mucosa and tonsils, with dissemination to the peripheral and CNS. PHEV is shed in oronasal secretions, and transmission is primarily from naïve sows to neonates or from comingling of swine at weaning.

Infections with PDCoV, a novel swine enteropathogenic coronavirus, were first reported in the United States, Canada, and South Korea in 2014. PDCoV causes acute watery diarrhea and vomiting 4 to 5 d after infection, followed by lethargy, dehydration, and death in piglets.³³ Diarrhea is present in piglets that survive for 5 to 10 d, and viral shedding ceases by 3 wk after infection. Transmission occurs via the fecal–oral route. PDCoV infects the villous epithelial cells of the small and large intestines. Infected epithelial cells in the small intestine undergo acute necrosis, and marked villous atrophy occurs. Like the diarrheas due to TGEV and PEDV, PDCoV-associated diarrhea is believed to be due to the loss of villous enterocytes and their ability to reabsorb water and electrolytes. In addition, the loss of brush-border digestive enzymes may contribute to diarrhea. PDCoV has cocirculated with PEDV in US pig populations for the last 6 y, causing a high number of deaths in piglets and substantial economic losses.³³

SeACoV (also known as swine acute diarrhea syndrome coronavirus and porcine enteric alphacoronavirus) was discovered in China in 2017, when outbreaks of severe diarrhea occurred in suckling piglets of 4 commercial pig herds, killing more than 24,000 swine.⁴⁷ SeACoV is a member of the subgenus Rhinacovirus and is believed to have evolved after transmission of Rhinolopus bat coronavirus HKU2 from bats to swine. SeACoV has a broad species range in vitro, infecting human, monkey, bat, mouse, rat, hamster, gerbil, and chicken cells.⁷⁸ The pathogenesis of SeACoV after experimental infection of piglets revealed that different strains of SeACoV cause different severity of disease, ranging from subclinical infection to severe diarrhea.⁷⁹ Strains that cause severe diarrhea result in a large accumulation of fluid in the cecum and colon, inflation of the intestine, and thinning of the intestinal wall. Intestinal villous atrophy and necrosis were evident, and mortality occurred between 5 and 12 d after inoculation.⁷⁹

Cats

In the field of companion animal medicine, feline coronaviruses are of high concern. They infect both domestic and wild cats and, along with closely related canine enteric coronavirus, TGEV, and PRCV, are known as Alphacoronavirus 1.⁵⁹ From 20% to 90% of domestic cats show feline coronavirus seropositivity. The 2 biotypes of feline coronaviruses are FECV and FIPV. The majority of FECV infections are subclinical or cause only mild diarrhea, although severe enteritis occurs occasionally. Mild upper respiratory symptoms may also occur. FECV infects the apical epithelium of the intestinal villi in the lower small intestine and cecum. FECV transmission is fecal–oral, and most cats become infected from their mother as kittens. FECV infections are persistent, with detectable FECV in the feces of infected cats for several months.

In contrast to FECV infections, FIPV infections are usually fatal.⁵⁹ Most cases of feline infectious peritonitis occur in cats younger than 2 y. Approximately 5% to 10% of cats persistently infected with FECV develop feline infectious peritonitis. Feline infectious peritonitis is characterized by fibrinous and granulomatous peritonitis with the production of protein-rich serous exudates into the peritoneal and pleural cavities and/or the production of pyogranulomas. Based on the presence or absence of effusions, the clinical forms of FIPV are characterized as wet (with effusions), dry (without effusions), or mixed. In natural infections, the wet form is most common. The different clinical forms are believed to be dependent on the type and strength of the host responses generated. A strong antibody response with a weak cellular immune response has been associated with the wet form of the disease. FIPV replicates in and activates monocytes. These infected monocytes, which accumulate around the blood vessels of the omentum and serosa, produce enzymes that cause endothelial barrier dysfunction resulting in exudates. Immune complexes can accumulate in the glomeruli causing glomerulonephritis. Stress and coinfections with feline leukemia virus or feline immunodeficiency virus increase the risk for feline infectious peritonitis. FIPV is not transmitted between cats but evolves from FECV in the monocytes of the persistently infected cat. At present, the mutations that cause this change in cell tropism and virulence are unknown. Because serum antibodies can increase the severity of disease, conventional vaccines are not used for FIPV. A modified live (temperature-sensitive) intranasal vaccine that replicates at the lower temperature of the upper respiratory tract is licensed by the USDA.⁶⁰ This vaccine induces a local IgA response in the nose and oropharynx that prevents initial replication of FECV but does not induce serum antibodies.

Dogs

Canine enteric coronavirus (also known as Alphacoronavirus 1) infects enterocytes, resulting in villous atrophy.²⁵ It causes a mild self-limiting enteric disease and is transmitted via the fecal–oral route. More severe disease occurs when dogs are coinfected with canine parvovirus or canine adenovirus. Canine respiratory coronavirus is a betacoronavirus with high sequence homology to BCV.²⁵ It replicates in respiratory epithelium, causes a mild upper respiratory disease, and is transmitted via aerosols. In co-infections with canine adenovirus, canine parainfluenza virus, canine herpesvirus, canine pneumovirus, Bordetella bronchiseptica, Streptococcus equi subsp. zooepidemicus, or Mycoplasma spp., canine respiratory coronavirus causes a polymicrobial respiratory disease known as ‘kennel cough.’²⁵

Rodents

In the field of laboratory animal medicine, the rodent betacoronaviruses—MHV and RCV—are of the highest concern.¹⁴ MHV and RCV are considered to be the same viral species, Murine coronavirus. They naturally infect only a single rodent species: mice or rats.³⁰ The primary tropism of classic prototype MHV strains (MHV-JHM, 1, 3, and A59) is the respiratory tract of mice. However, because these strains disseminate to other organs, including liver and brain, they are considered to be hepatotropic, neurotropic, or polytropic.⁴ MHV-JHM was isolated in 1949 from a mouse with flaccid paralysis.¹ Widespread demyelination of nerves in the CNS and focal necrosis in the liver were seen. After intranasal inoculation, MHV-JHM initially replicates in the nasal mucosa and invades the CNS via the olfactory nerves.³ The virus can be detected in the liver, lung, bone marrow, spleen, and lymph nodes, but necrotic lesions are seen primarily in neurons, astrocytes, and oligodendrocytes in the olfactory lobes, hippocampus, and meninges. Mice that recover from acute infection can become persistently infected, with small foci of demyelination seen for as long as 16 mo.⁵⁷ MHV-JHM infects all strains of mice, with the exception of SJL/J mice, because SJL mice express an allele of the viral receptor (murine carcinoembryonic antigen-related adhesion molecule) gene that does not serve as a viral receptor.⁵

MHV1, 3, and A59 were isolated between 1951 and 1961 from mice with hepatitis. After intraperitoneal inoculation, MHV1 causes mild hepatic lesions in weanling and adult mice and acute fatal hepatitis in neonatal mice.²⁴ MHV1 antigen is present in the liver, respiratory tract and lymphoid organs.⁴ MHV3 causes 3 disease patterns, depending on the strain of mouse infected.⁴⁹ After intraperitoneal infection of 6- or 12-wk-old BALB/c, C57BL/6, DBA/2, or NZB mice with MHV3, fulminant fatal hepatitis occurs.⁴⁹ Necrotic lesions were seen in the liver, spleen, thymus, lymph nodes, and Peyer patches. Six-wk-old C3H, CBA, and AKR mice developed fatal hepatitis, whereas mice inoculated at 12 wk of age were chronically infected, with wasting and progressive neurologic signs (inactivity, incoordination, and paresis of one or more limbs) occurring at 3 to 6 wk after inoculation and death at 1 to 12 mo after infection.⁴⁹ In contrast, 6- and 12-wk-old A/J and A/Orl mice were resistant to MHV3-induced disease. MHV3 targeted the lung and lymphoid tissues (spleens, thymus, lymph nodes and bone marrow) in neonatal Swiss mice inoculated intranasally with MHV-3.⁴⁹ MHV-A59 causes acute fatal hepatitis via most routes of inoculation due to the destruction of liver parenchymal and Kupffer cells.³⁹ MHV-A59 antigen is present primarily in the liver and lung.⁴ Intracerebral inoculation of weanling mice with MHV-A59 causes meningoencephalitis or subacute spastic paralysis due to demyelination in the brain and spinal cord.^40,77 After the emergence of SARS-CoV, intranasal inoculation of A/J mice with MHV1 or of C57BL/6 mice with MHV-A59 were proposed as models of SARS-CoV lung infections in humans.^18,80

Whereas most experimental MHV studies have used prototype MHV strains, the MHV strains that cause most infections in laboratory animal facilities are enterotropic.²⁸ Replication of these MHV strains is restricted to the intestines, they are transmitted via the fecal–oral route, and they do not cause clinical disease in adult mice.¹⁴ Although the titers of MHV produced in the intestine of all ages of mice infected with enterotropic strains of MHV are similar, only neonatal mice nursing MHV-seronegative dams develop diarrhea. In neonatal mice experimentally infected with enterotropic strains of MHV, intestinal epithelial degeneration, necrosis, and viral syncytia occur with 48 h, with the distal small intestine, cecum, and ascending colon having the most severe lesions.² Enterotropic MHV-Y was cleared from adult immunocompetent mice in 2 to 3 wk.¹³ In contrast, adult B-cell–deficient mice infected with MHV-Y had a chronic subclinical intestinal infection that lasted more than 7 wk.¹³ MHV-Y caused a disseminated infection in adult T-cell–deficient mice with chronic viremia, and all tissues examined at 4 wk after infection had detectable MHV antigens.¹³ Chronic wasting disease occurs in nude mice infected with enterotropic MHV strains, and IFNγ-deficient mice infected with MHV develop fatal peritonitis and pleuritis.^6,15,23

The original isolates of RCV, sialodacryoadenitis virus, and Parker’s rat coronavirus as well as newer isolates of RCV all infect the lung, nasopharynx, salivary glands, lacrimal glands, and cervical lymph nodes of rats.^31,48 Mild RCV infections can be clinically silent but outbreaks in RCV-seronegative rats can be associated with anorexia, weight loss, photophobia, rhinitis, and sneezing, and cervical lymph node enlargement and red-tinged discharges from the lacrimal glands, are frequently seen.^8,31 Initial RCV replication occurs in the nasopharynx and results in epithelial necrosis. Infection then spreads to salivary and lacrimal glands, where necrosis of ductal epithelium and edema occurs. Ocular inflammation, an indirect effect of impeded tear production, can lead to corneal opacities and ulcers.³⁶ Inflammatory edema and focal necrosis occurs in the cervical lymph nodes.³¹ Mild tracheitis and interstitial pneumonia can occur with some RCV strains.⁷² Most rats recover from RCV infection in 2 to 3 wk, but newborn rats can develop fatal pneumonia. Disruption of estrus, a decline in birth rates, and inadequate nursing can occur.^64,65 RCV is shed in lacrimal and oronasal discharges for about 1 wk in immunocompetent rats, and transmission occurs by contact and via aerosols. In contrast, infection in athymic rats is chronic, with prolonged viral shedding.^26,68

Evolution of human coronaviruses

Seven human coronaviruses have been identified. Two alphacoronavirus species (Human coronavirus 229E and Human coronavirus NL63) and 2 betacoronavirus species (Human coronavirus OC43 and Human coronavirus HKU1) cause self-limiting, mild, upper and lower respiratory tract infections (the common cold) in children and adults. These coronaviruses are transmitted via sneezing, coughing, and contact with contaminated surfaces.¹⁶ HCoV-229E and HCoV-NL63 are believed to have originated in bats and become endemic in the human population.^16,58 In contrast, endemic HCoV-OC43 (Betacoronavirus 1) and HCoV-HKU1 are believed to have evolved from a rodent coronavirus.¹⁶

Three highly pathogenic human coronaviruses, SARS-CoV, MERS-CoV and SARS-CoV-2, have emerged in the last 18 y. SARS-CoV emerged in humans in 2002 in China. SARS-CoV infects human epithelial cells in the lung, causing mild respiratory disease to severe respiratory distress. The SARS-CoV epidemic caused 778 deaths, mostly in the elderly.⁷⁶ Because SARS-CoV was transmitted inefficiently between humans, the epidemic was restricted primarily to households and healthcare facilities and resolved within a year. SARS-CoV is believed to have moved from Himalayan palm civets or raccoon dogs to humans. Because civets in the wild had no evidence of SAR-CoV infection whereas several bat coronaviruses have high sequence homology to SARS-CoV, the virus likely originated in bats, with civets providing an intermediate host.³⁸

MERS-CoV emerged in humans in 2012 in Saudi Arabia. Like SARS-CoV, MERS-CoV causes mild respiratory disease to severe respiratory distress. MERS-CoV has caused 858 deaths.⁷⁵ MERS-CoV has limited human-to-human transmission but is zoonotic, with dromedary camels providing a reservoir for the virus.⁵¹ Transmission of MERS-CoV from dromedary camels into the human population continues to occur sporadically. In camels, MERS-CoV replicates primarily in the upper respiratory tract, where the level of the viral receptor, dipeptidyl peptidase 4, is highest. In contrast, dipeptidyl peptidase 4 expression is highest in the alveolar epithelium of humans, resulting in a lower respiratory tract–based infection. Bat coronaviruses with high sequence homology to MERS-CoV have been found, suggesting that bats are also a reservoir for MERS-CoV.⁴²

SARS-CoV-2 emerged in late 2019 in China and causes ‘coronavirus disease 2019’ (COVID-19).⁷⁴ A detailed description of COVID-19 pathogenesis and a discussion of the role that animals are playing in our understanding of SARS-Cov2 pathogenesis are presented later in this special topic issue. Briefly, SARS-CoV-2 causes asymptomatic to fatal respiratory disease with systemic complications. Efficient human-to-human transmission of SARS-CoV-2 has resulted in a worldwide pandemic. SARS-CoV2 has caused over4.1 million deaths, with most deaths in the elderly and in those with underlying conditions, such as diabetes, cardiovascular disease, and lung disease. According to sequence homology, SARS-CoV2 is believed to have originated as a bat coronavirus that used Malayan pangolins as an intermediate host.³⁷

Bats have also served as reservoirs or sources for swine coronaviruses. Of the 20 species of alphacoronaviruses and betacoronaviruses recognized by the International Committee of Viral Taxonomy, 11 are found only in bats, and these bat coronaviruses are believed to be the source of many of the pathogenic coronaviruses that affect other species.⁷³ Bats persistently infected with coronavirus, without any signs of illness, can produce populations of variant viruses. When one of the variant viruses gains the ability to infect a new host, then a novel coronavirus can emerge. As human populations are exposed increasingly frequently to bats or other wild animals that are harboring coronaviruses, other coronaviral epidemics may occur.^17,19,45

Summary

Coronaviruses infect humans and a wide range of animals, causing predominantly respiratory and intestinal infections. Subclinical to fatal infections can occur and young animals frequently sustain more severe disease. Cell surface viral receptors are the primary determinant of species specificity and tissue tropism. The large coronavirus genome, an error prone polymerase and recombination results in populations of variant viruses which fuel the evolution of coronaviruses.