Rotaviral diarrhea is a life-­threatening disease and the most common cause of foal diarrhea. It also causes serious diarrhea in other animals and can be deadly in infants and children worldwide.

The rotavirus family of viruses is large and categorized into Groups A-D and F-J based on genome sequence. They tend to infect in a species-specific fashion. Until recently, researchers considered Group A rotaviruses to be the only ones responsible for equine infections, though they can also infect other animals. The other groups were known to infect humans and animals besides horses. Rotaviruses are considered zoonotic (can pass between animals and humans). It’s important to consider any animal with diarrhea a zoonotic risk.

Infection

Rotaviruses cause infection via the fecal-oral route, with disease occurring about 12 to 24 hours after ingestion. Diarrheic animals can shed up to 100 billion virus particles per gram of diarrhea, and only small numbers of particles are required to cause disease. The virus can persist in the environment for up to nine months and can easily transfer between animals via equipment, tools, and contaminated hands. As such, outbreaks can develop quickly after the first case of diarrhea on a farm. Recovering foals can shed the virus in their feces for about three days after their diarrhea stops.

Pathology

Rotaviruses damage the digestive tract, but recovered animals don’t typically suffer long-term effects or get reinfected. Rotaviruses, once ingested, invade the cells at the tips of the small intestine villi. These are microscopic fingerlike projections in the lumen of the small intestine that increase the gut’s surface area and are integral to digestion and absorption. Inside these cells the viruses hijack the cellular machinery, multiply rapidly, and release into the lumen as the cell ruptures. This destruction blunts the villi, causes inflammation, and vastly reduces the small intestine’s digestive and absorptive capacities. Notably, it destroys the lactase enzymes on the tips of the villi that allow the foal to digest the milk sugar lactose. The persistence of these undigested substances draws water into the gut, can create an environment favorable to pathogenic (disease-causing) bacteria, and further contributes to the diarrhea resulting from maldigestion and malabsorption.

Disease manifestation and medical care

Infection severity depends on the foal’s age and immune status, the virulence (disease-causing capacity) of the virus, and number of virus particles ingested. The disease affects younger foals more severely, and veterinarians typically don’t observe it in foals older than 6 months. Foals born to mares vaccinated against rotavirus A tend to develop mild, self-limiting diarrhea around 90 to 120 days of age. These foals usually recover quickly with supportive care. Foals born to unvaccinated mares are more likely to develop the clinical syndrome within the first few days of life. These neonates stop nursing and develop gut stasis (lack of motility, known as ileus) and colic and severe watery diarrhea with dehydration and electrolyte derangements that rapidly become life-threatening.

Neonatal foals need prompt and intensive medical care, including intravenous (IV) fluid therapy and gastric ulcer prevention. Because such young foals are at risk of bacteremia (bacteria circulating in the bloodstream), treating veterinarians usually place them on antibiotics, as well. Preventing an affected foal from nursing can, anecdotally, reduce colic signs and the duration of diarrhea. However, that necessitates round-the-clock hospital care to provide IV fluids and nutrition.

Diagnosis

A variety of reliable rotavirus A tests are available (bench-top kits and laboratory PCR tests) for horses. However, in a 2021 outbreak of contagious neonatal foal diarrhea, Feng Li, DVM, PhD, and colleagues at the University of Kentucky’s Gluck Equine Research Center and Veterinary Diagnostic Lab discovered a novel equine rotavirus B and developed a diagnostic qPCR test for it. As with rotavirus A, some asymptomatic foals were able to shed the virus during the outbreak.

Control

Vaccination and well-designed biosecurity and management protocols (see bit.ly/2Y3C5rx) are essential to reducing the risk of a rotavirus outbreak. Peroxygen, accelerated hydrogen peroxide, or phenolic compounds can be effective disinfectants when used correctly. Bleach is not effective in conventional barn environments due to porous surfaces and organic matter. The rotavirus vaccine is available to administer to pregnant mares during Months 8, 9, and 10 of gestation. Breeders should vaccinate their mares during every pregnancy to protect the foal via colostral immunity. The Li and Wang labs at the Gluck Center are working on equine rotaviruses and hope to develop a vaccine for equine rotavirus B.