Taking the temperatures of every horse in a barn every day is a time-consuming task. However, collecting and graphing those values can help caretakers and veterinarians quickly recognize deviations from normal and even impending disease outbreaks.

Nicola Pusterla, DVM, PhD, Dipl. ACVIM, a professor in the Department of Medicine and Epidemiology at the University of California, Davis, emphasized the importance of taking daily rectal temperatures as a routine part of equine management during a Sunrise Session at the 2021 American Association of Equine Practitioners (AAEP) Convention, held Dec. 4-8 in Nashville, Tennessee.

As an example, Pusterla recalled details of a March 2021 equine herpesvirus (EHV) outbreak in Pennsylvania.

“This actually started as a silent outbreak, as the only clinical abnormality appreciated at the start of the outbreak was an elevated rectal temperature of 102-104.4°F in a small number of horses … as the farm veterinarian assessed the health of horses scheduled for routine dental care.” he said.

Faced with multiple horses spiking fevers, farm veterinarians collected nasal secretions and blood and sent them via the biosurveillance program supported by Merck Animal Health to Pusterla’s laboratory.

“The … horses tested qPCR-positive for EHV-1 targeting the universal gB gene but negative for the D752 (neuropathogenic) and the N752 (non-neuropathogenic) genotype,” he said. “This was a very unusual molecular signature, which lead us to contact the two farm veterinarians in order to collect data and more samples.”

Of the 31 horses, primarily young Warmbloods, tested during the Pennsylvania outbreak, 26 (84%) became pyrexic within a week of the veterinarian recognizing the initial fevers. Notably, said Pusterla, the remaining five horses did not develop clinical signs of disease, including fever, even though they tested positive on qPCR.

The treating veterinarians instituted medical intervention immediately. All horses received the antiviral valacyclovir, while the horses with fevers also received a non-steroidal anti-inflammatory drug and heparin. All the horses had cleared the virus from their blood by Day 7, presumably due to the valacyclovir, said Pusterla.

“One of the main lessons we learned from this particular EHV-1 outbreak was that taking a rectal temperature when performing a routine physical exam is an invaluable tool in the day-to-day management of horses,” he said. “We do not need to rely on chemical parameters (bloodwork) to determine whether a horse is sick or not.”

Kevin Corley, BVM&S, PhD, Dipl. ACVIM, ACVECC, MRCVS, founder of the EquiTrace app, then jumped in, explaining that while most everyone in the equine industry recognizes the value of rectal temperatures, they’re time-consuming (about three minutes per horse, in his estimation) and potentially dangerous to take.

Corley said Bio-Thermo microchips placed in the splenius muscle by the horse’s nuchal ligament can measure a horse’s temperature instantly and accurately. In a 2020 study published in Animals, Kang et al. reported a strong positive correlation between temperatures measured by the Bio-Thermo chip in the splenius muscle and central venous temperature.

Another feature making the chip valuable to equine operations is that users can upload the data to the EquiTrace smartphone app and graph it. This visual display can show changes in an individual horse’s temperature more clearly than a chart of individual measurements. This could be particularly important at the start of a disease outbreak, because each horse’s baseline temperature is unique, and simply looking at whether it is in the general normal range for all horses might cause caretakers to miss increases in body temperatures in individual horses.

The EquiTrace app can also reveal a microchipped horse’s whereabouts, proving invaluable in an outbreak situation for contact tracing, Corley said.