Coronaviral enteritis affects pigs of all ages and typically manifests as an acute watery diarrhea. Multiple coronaviruses cause enteric disease in pigs, and clinical differentiation is difficult. Ancillary diagnostic tests (ie, PCR assays or immunohistochemistry) are needed to confirm the etiology of the disease.
Three members of the family Coronaviridae are associated with enteric disease in pigs worldwide: transmissible gastroenteritis virus (TGEV), porcine epidemic diarrhea virus (PEDV), and porcine deltacoronavirus (PDCV). Although all three viruses belong to the same family; cross-protection does not occur. TGEV and PEDV are members of the alphacoronavirus genus, and other novel alphacoronaviruses have been described in association with enteric disease in Asia; however, their global impact is not yet known. Coronaviruses are transmitted directly via infected pigs and indirectly via virus-contaminated fomites and transport trucks.
Coronaviruses infect and destroy villus enterocytes of the jejunum and ileum, resulting in severe, segmental villous atrophy. Reduced absorptive surface area in the gut results in malabsorption, osmotic diarrhea, dehydration, and metabolic acidosis. The incubation period is <24 hours. Massive amounts of virus are shed in the feces of infected pigs, and infection is transmitted rapidly by aerosol or contact exposure. Severe epidemics are more common during winter because the virus survives at lower temperatures.
The main clinical sign of porcine coronaviral enteritis in all age groups is watery diarrhea. Piglets infected during the first week of life often die within 3 or 4 days of infection, and mortality can exceed 60% in naive populations. In growing and finishing pigs, clinical signs are typically limited to diarrhea, which may be profuse, that resolves in a few days. Vomiting occurs occasionally. Morbidity is high; however, mortality is negligible in older pigs. Moderate severity may be observed in sows and gilts with no preexisting immunity, and these dams may exhibit varying combinations of anorexia, vomiting, diarrhea, depression, and agalactia.
Severity of coronaviral enteritis also varies depending on the strain of virus and any preexisting immunity. Some strains of PEDV have generally lower virulence, particularly the S-INDEL variant strain; prior infection with porcine respiratory coronavirus, a mutant of TGEV, may reduce the severity of disease after TGEV infection.
Typical gross lesions include thin-walled, small-intestinal segments filled with watery contents. Suckling piglets may have undigested milk in the colon. Microscopically, there is severe segmental villus blunting and fusion, with near complete loss of villi in severe disease. Neutrophilic infiltration of the lamina propria may be observed in areas where enterocytes are sloughed.
Clinical differentiation among coronavirus infections is difficult. Epidemic forms of coronaviral enteritis cause characteristic rapidly transmitted diarrhea in pigs of all ages, with high mortality in neonates. Although clinical signs and gross lesions are often highly suggestive of coronaviral enteritis, additional laboratory testing, including histopathologic evaluation, may be required for a definitive diagnosis.
PCR assays are available for PEDV, TGEV, and PDCV and can be applied to feces and population-level samples, such as oral fluids, for rapid detection of coronaviruses. Affected pigs shed high quantities of virus in feces, and viral nucleic acid is often detectable by PCR assay even after clinical signs have resolved. Direct detection assays (eg, immunohistochemical tests and in situ hybridization) may also be of value on fixed tissue specimens. As novel coronaviruses emerge, direct examination of feces by electron microscopy, as well as next-generation sequencing, may be helpful. Numerous serologic tests can be used to demonstrate rising antibody titers to known porcine enteric coronaviruses and may be a useful surveillance tool.
No specific treatment is available for porcine coronaviral enteritis. Measures taken during an outbreak are of a general nature and include supportive care. Increasing farrowing room temperature to minimize loss of body heat and providing electrolyte solutions to combat dehydration are helpful measures. Pigs with diarrhea should have free access to water, and finishing pigs should have feed withheld for 1–2 days.
Protective immunity depends on the presence of antibody in the small intestine. Passive protection of piglets is provided by continual nursing of immune sows. Active, protective immunity develops after coronaviral infection of the intestinal mucosa, resulting in a secretory IgA response. A variety of commercial and autogenous coronavirus vaccines containing inactivated or attenuated viruses are available in different countries. Vaccination of gestating sows to boost lactogenic immunity may be sufficient to protect neonates and is particularly useful in endemically infected herds. Vaccination of pigs in herds without a recent history of coronaviral disease may not be economical, because vaccines do not induce complete immunity, and there does not appear to be appreciable cross-protection among different enteric coronaviruses.
Because enteric coronaviruses are easily transmitted during epidemics by humans, animals, and fomites, special care should be taken to prevent transmission to unexposed groups of pigs and to neighboring herds. Strict sanitation and biosecurity Biosecurity of Animals The tenets of biosecurity have been long recognized by veterinarians. However, throughout the past decades, interest in biosecurity as a scientific discipline has surged because of 1) disease... read more protocols are essential to prevent introduction of coronaviruses into swine premises. Avoiding commingling of sources and ensuring that transport vehicles are thoroughly washed, disinfected, and dried before loading pigs can help reduce transmission of enteric coronaviruses.
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