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Viral Enteritis in Horses

Viral infection of the small intestine (viral enteritis) in horses normally affects only the superficial layers of the small intestinal mucosa, particularly the enterocytes of the absorptive villi. Clinical symptoms of viral enteritis tend to be mild and transitory, and commonly include lethargy, anorexia, and fever. Low-grade gastrointestinal symptoms such as diarrhea and colic may also occur. On the rare occasion, viral enteritis results in more serious disease such as dehydration, malnutrition, septicemia, mucosal necrosis and encephalopathy. Coronavirus has been associated with outbreaks of disease among groups of adult horses, while both rotavirus and coronavirus can affect foals. Other viruses, such as Equine Viral Arteritis (EVA), can also cause enteritis, however the predominant clinical features of these viruses do not tend to include intestinal dysfunction.

Etiology and Pathophysiology of Viral Enteritis

Coronavirus and rotavirus are the two viruses most commonly associated with the development of viral enteritis in horses. Transmission of virus occurs by the fecal-oral route. Coronavirus may also possibly be transmissible through respiratory secretions (Nemoto 2014).

Herd outbreaks can persist for up to 3 weeks (Pusterla 2013), however not all of the horses or foals will necessarily get sick. Unaffected horses can shed virus and act as subclinical sources of infection.


Rotavirus is a species-specific, double-stranded, non-enveloped RNA virus that is a well-recognized cause of infectious diarrhea in foals (Magdesian, Bailey 2013). Rotavirus is categorized into groups from A to G, with only group A affecting horses (Magdesian 2013). Rotavirus can be a problem in intensive breeding facilities, due to its highly infectious nature, and also because the virus can persist in the environment for up to 8 months (Magdesian 2013).


Coronavirus is a single-stranded, enveloped RNA virus, which causes respiratory disease and enteric disease in a wide variety of species. The coronaviruses are classified into alpha, beta, delta and gamma groups based on genetic and serologic differences ( Coronavirus only causes enteric disease in horses, and the equine virus comes from the beta group.

Viral Enteritis in Foals

Rotavirus causes destruction of the superficial enterocytes at the tips of the intestinal villi. These enterocytes normally have a brush border (microvilli) that produces the milk-digesting enzyme lactase. When the brush border is damaged, there can be insufficient lactase production and the foal is unable to digest the lactose in milk normally. Undigested lactose can draw body water into the lumen of the bowel, by osmotic attraction, resulting in diarrhea and dehydration. Fever from the viral infection can cause lethargy and inappetence, and the foal can rapidly become depleted in energy and electrolytes. Neonatal foals are particularly susceptible to the effects of viral enteritis as they are less able to compensate for these nutritional derangements. Colic is rare, unless the maldigestion of milk leads to bloat. The rapidly dividing cells from the intestinal crypts quickly replace the damaged enterocytes; therefore intestinal dysfunction tends to be temporary and self-limiting.

Rotavirus can be the cause of enteritis and diarrhea in foals in up to 20-50% of cases (Frederick 2009). The incubation period for rotavirus is 12-24 hours. Protein leakage is not a feature of this disease (Knottenbelt 2004). Adult horses do not show clinical signs of disease with rotavirus, however have been shown to seroconvert with exposure to the virus (Magdesian 2013).

In foals, coronavirus causes similar disease to rotavirus, however coronavirus is less common.

Viral Enteritis in Adult Horses

In adult horses, outbreaks of disease have been reported in groups of racing horses and show horses, and in boarding facilities in the United States, Japan and Europe. The affected horses are usually horses older than 2 years of age, and the outbreaks tend to occur in the colder months of the year (Cornell University Fact Sheet). The common symptoms of infection are lethargy, inappetence and fever, with some horses developing colic and diarrhea. As in foals, the disease tends to be mild and self-limiting; however there have been reports of severe acute fatal disease, with horses developing necrotic enteritis, septicemia and/or neurologic symptoms. Fecal viral loads have been found to be significantly higher in these horses (Fielding 2015).

In the horses that developed neurologic symptoms, high blood ammonia levels were often found. One theory is that the viral infection triggers an overgrowth of urease-producing bacteria, which causes increased systemic absorption of ammonia, leading to encephalopathy (Fielding 2015).

Giannitti (2015) reported on the postmortem findings of three equids that died due to coronavirus infection. One horse and a donkey had severe diffuse necrotizing enteritis, with marked villous attenuation, epithelial cell necrosis at the tips of the villi, neutrophilic and fibrinous extravasation into the small intestinal lumen (pseudomembrane formation), as well as crypt necrosis, microthrombosis, and hemorrhage. Another horse had hyperammonemic encephalopathy with Alzheimer type II astrocytosis throughout the cerebral cortex.

Clinical Signs of Viral Enteritis in Horses

In Foals

  • Affects foals between the ages of 2-253 days (Frederick 2009).
  • Normally mild and transient outbreaks of disease, with lethargy, anorexia, fever and diarrhea.
  • Diarrhea lasts for 3 days, on average. Virus may be shed for 3-4 days after diarrhea has resolved. (Fielding 2015). Colic is not common.
  • Neonatal foals are more likely to become debilitated by maldigestion and malabsorption of milk, causing dehydration, electrolyte derangements and energy deficits.
  • Viral infection may lead to secondary bacterial infections.
  • Once recovered, the foals do not tend to be susceptible to recurrence of disease or reinfection (Magdesian 2013).

In Adults

  • Affected horses are usually older than 2 years.
  • Normally mild, self-limiting infections that can result in fever (usually less than 104 °F (40 °C)), lethargy, anorexia, low-grade colic (laying down, looking at flank) and/or diarrhea.
  • The symptoms usually resolve in 1-4 days.
  • Rare complications include dehydration, endotoxemia, septicemia, encephalopathy, and bowel necrosis.
    • Encephalopathy – depression, ataxia, circling, head pressing, nystagmus, recumbency (Fielding 2015).
    • One horse was reported to develop anemia, with mucosal and lingual ulceration (Fielding 2015).

Diagnosis of Viral Enteritis in Horses

Blood work:

  • Hematology. Both foals and adult horses are frequently leukopenic (neutropenic, lymphopenic) with only mild changes to inflammatory markers such as Serum Amyloid A. Foals with viral enteritis tend to have lower sepsis scores than those with bacterial enteritis.
  • Blood ammonia is worth analyzing with any signs of encephalopathy, and may be considered even earlier during outbreaks of disease where coronavirus enteritis is suspected.

Fecal Analysis:

  • Rotavirus in foals can be detected using one of a variety of in-house/stall-side tests that utilize either ELISA, latex agglutination and immunochromatographic technology. To completely rule out the presence of rotavirus requires a series of three negative tests results.
  • Electron microscopy was once considered the gold standard diagnostic test for rotavirus and may still be available in some laboratories.
  • Real-time PCR panels generally include rotavirus and coronavirus, as well as the other common infectious agents that can cause intestinal disease. These PCR panels can be the most efficient way of making a diagnosis, although false negatives may occur.
  • Cornell University recommends that the feces should be tested as soon as possible, and packaged in unbreakable leak-proof containers, with ice packs, to prevent the bacterial overgrowth that can inhibit PCR analysis.

Treatment of Viral Enteritis in Horses

Treatment, if required, involves supportive care:

  • Replacing fluid and electrolyte deficits.
  • Nutritional support – parenteral nutrition, especially for inappetent foals.
  • Lactase enzyme supplementation – to assist foals with milk digestion, dose recommended 600 FCC* units/50kg (Magdesian 2013).
  • Gastric ulcer prevention – i.e. omeprazole, +/- sucralfate.
  • +/- Antibiotics for secondary bacterial infections.

* FCC = Food Chemical Codex

Treatment for encephalopathy and hyperammonemia may include intravenous fluids and oral antibiotics that combat ammonia-producing bacteria (i.e. neomycin, possibly metronidazole). Lactulose syrup, administered orally, can help trap ammonia in the intestinal lumen and decrease systemic absorption.

Prevention if Viral Enteritis in Horses

A vaccination against rotavirus is available for pregnant mares that can boost colostral antibody concentrations. Studies have shown that vaccinating mares decreases the incidence and severity of viral enteritis in foals. The general recommendation is to administer a three-dose series of intramuscular injections to the mare at 8, 9 and 10 months gestation (American Association Equine Practitioners guidelines).

Quarantine and biosecurity measures should be instigated to try and minimize the spread of infection. Property owners should presume the virus is being shed for up to 3 weeks (Cornell University Fact Sheet), and that the virus can persist in the environment for up to 9 months (Magdesian 2013) if not properly disinfected.

Disinfectants that are active against rotavirus and coronavirus include:

  • Some phenolic disinfectants,
  • Ethanol,
  • Formalin,
  • Peroxygen and accelerated hydrogen peroxide

Bleach is not effective. (Magdesian 2013).


  • Bailey, K.E. et al. (2013) Equine rotaviruses–current understanding and continuing challenges. Vet Microbiol. 167:135-44.
  • Cornell University. Equine Enteric Coronavirus: Fact Sheet.
  • Fielding, C.L. et al. (2015) Disease Associated with Equine Coronavirus Infection and High Case Fatality Rate. J Vet Intern Med 29:307–310.
  • Frederick, J. et al. (2009) Infectious Agents Detected in the Feces of Diarrheic Foals: A Retrospective Study of 233 Cases (2003–2008). J Vet Internal Med. 23:1254-60.
  • Giannitti, F. et al. (2015) Necrotizing Enteritis and Hyperammonemic Encephalopathy Associated With Equine Coronavirus Infection in Equids. Vet Pathol 52(6):1148-56.
  • Knottenbelt, D. et al. (2004) Equine Neonatology, Medicine and Surgery. Elsevier Health Sciences pp. 232.
  • Magdesian, K.G. (2013) Viral Diarrhea; in Equine Infectious Diseases, ed. Sellon, D.C., and M. Long. Elsevier Health Sciences pp. 198-202.
  • Nemoto, M. et al (2014) Experimental inoculation of equine coronavirus into Japanese draft horses. Arch Virol 159:3329-34.
  • Oue, Y. et al. (2012) Epidemic of equine coronavirus at Obihiro Racecourse, Hokkaido, Japan in 2012. J Vet Med Sci. 75(9):1261-5.
  • Pusterla, N. et al. (2013) Emerging outbreaks associated with equine coronavirus in adult horses. Vet Microbiol. 162:228-31.

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