Viruses appear to cause diarrhea in foals but rarely affect adult horses. Rotavirus is the main cause of viral diarrhea in foals; however, other viruses (eg, coronavirus) have been implicated. Diarrhea induced by rotavirus is characterized by depression, anorexia, and profuse, watery, malodorous feces. It is usually seen in foals <2 mo old; younger foals typically have more severe clinical signs. The diarrhea usually lasts 4–7 days, although it can persist for weeks.
Rotavirus destroys the enterocytes on the tip of the villi in the small intestine, which results in malabsorption. Lactase becomes deficient, so lactose passing into the large intestine induces an osmotic diarrhea. Diagnosis is made by identification of virus in the feces by electron microscopy or commercial immunoassay kits designed for detection of human rotavirus. Requesting that the laboratory test specifically for rotavirus, collecting feces early in the course of disease, and sampling several foals improve the chances of virus detection.
Treatment is generally supportive. Certain farm management practices and disinfection techniques have effectively limited the spread of rotavirus during outbreaks. Sick foals are highly contagious and should be isolated in the stall in the barn in which the foal originally became ill or moved to a designated isolation facility. Personnel should wear disposable gloves and cleanable boots and wash their hands with soap before and after handling diarrheic foals. Foot dips containing phenolic disinfectants outside the stalls of sick foals should also be used. Specific stall-cleaning equipment should be designated only for cleaning the stalls of diarrheic foals. Once the stall has been vacated, it should be cleaned of particulate material, washed with detergent, and then disinfected with phenolic compounds that meet EPA standards. Bleach, chlorhexidine, and quaternary compounds do not appear to be effective disinfectants for rotavirus. Fecal material of sick foals removed from stalls should not be spread on pastures used for horses and foals, and care should be taken to avoid fecal contamination of alleyways. All stall-cleaning equipment should be disinfected. Stalls with dirt floors are difficult to adequately clean and disinfect. Removal of the top layers of dirt may be required.
Arriving horses and foals, including those returning from veterinary hospitals, should be isolated for ≥7 days before being introduced to the resident population. A vaccine for pregnant mares to induce colostral antibodies directed at reducing the risk of rotavirus infection in their foals is available.