Diarrhea is a leading cause of disease in foals, affecting about 6% per year. In foals 7 days old and younger, it causes about 25% of disease. But diarrhea itself has many causes. One of the more worrisome causes to veterinarians is equine rotavirus (ERV) infection, which causes illness in nearly 100% of infected foals, and has high mortality rates in neonates and untreated cases.

Christian Leutenegger, DrVetMed, PhD, FVH, worldwide head of molecular diagnostics at IDEXX Laboratories, in West Sacramento, California, and Ron Vin, DVM, Dipl. ACVIM, also of IDEXX and the Myhre Equine Clinic, in Rochester, New Hampshire, discussed ERV during a Sunrise Session at the 2016 American Association of Equine Practitioners (AAEP) Convention, held Dec. 3-7 in Orlando, Florida.

Rotavirus can be found across the world, but within the United States veterinarians see higher numbers of cases in areas with large breeding populations, such as California, Florida, Kentucky, New York, and Texas.

Foals contract rotavirus via the fecal-oral route, meaning they ingest the pathogen in infected horses’ manure. Foals younger than 6 months of age are susceptible to ERV, with most clinical cases involving foals 3 months old or younger. Veterinarians have detected ERV in isolated cases of adult horse diarrhea, but they have not yet determined its clinical significance to this demographic.

The AAEP recommends that pregnant mares receive the commercially available ERV vaccine in a series of three inoculations at 8, 9, and 10 months of gestation. The foal gains passive immunity against the disease from the mare via her colostrum (antibody-rich first milk). There is currently no evidence that vaccinating a foal improves immunity to ERV. The only vaccine available for ERV protects against the one predominant ERV strain in the United States, but it likely does not protect against others. This is evidenced by ERV found in foals from vaccinated mares who received colostrum.

In a recent project, Vin and Leutenegger aimed to screen the possible pathogenicity (disease-causing ability) of different ERV strains in clinical cases of foal diarrhea in Kentucky. Because ERV is an annual problem that’s labor-intensive to manage, and sometimes it appears despite proper vaccination and management protocol, questions about vaccine protection and the presence of additional strains remain.

Vin and Leutenegger found that the currently available vaccine likely reduces the severity and incidence of clinical disease. As research continues into additional strains of the virus, Vin and Leutenegger suggested that vaccination, good biosecurity, and management are the best tools for prevention.

Leutenegger and Vin then turned their attention toward bacterial upper airway infections such as Streptococcus equi (which can cause strangles) and S. zooepidemicus. Identification and diagnosis is critical for proper treatment and recovery in these cases. The 2005 American College of Veterinary Internal Medicine consensus statement recommended culturing samples as the gold standard for diagnosis. Recent work at IDEXX showed that culture-only diagnostic techniques result in a high frequency of false negative and false positive results when diagnosing strangles and other streptococcal diseases in horses. To detect and eliminate disease and maximize biosecurity, veterinarians should therefore use a combination of correct sample collection site, polymerase chain reaction testing (PCR) testing, and culture.

In summary, the presenters strongly recommended vaccinating pregnant mares for ERV as research continues into the presence of additional strains of the virus. It might also be prudent to diagnose bacterial upper airway disease using both culture and PCR to reduce the risk of misdiagnosis.