How Accurate is Your Equine Thermometer?

Years ago, the only thermometers available for equine use were made of glass. They were quite a bit longer and thicker than thermometers used by humans, though both types were filled with mercury. The equine instruments had a glass loop on one end where users could attach a string with a clip that secured the thermometer to the horse’s tail hairs. After the instrument was lubricated and then inserted into the horse’s rectum, the user needed to wait a few minutes to allow the temperature reading to stabilize. The string and clip prevented the thermometer from getting lost inside the horse, a very real possibility because of the structure and flexibility of the horse’s rectal muscles.

Fast-forward to the present, where digital thermometers have all but replaced the glass models, at least among horse owners. Digital thermometers are relatively inexpensive, and they quickly give a reading that is easy to see. A long waiting period is not necessary, so a horse owner or veterinary technician simply holds the thermometer until it is ready to be removed from the horse.

Results of a recent study suggest that some digital thermometers may have substituted convenience for accuracy. A report in New Zealand Veterinary Journal indicated that some digital thermometers gave readings that were several degrees lower than the horse’s actual temperature, and there was evidence that the instruments giving the quickest readings might be the least accurate.

Should horse owners throw away their handy digital thermometers and go back to mercury-filled glass? That probably isn’t necessary if a simple step is taken. The horse’s exact temperature might not be as important as knowing that he is or is not showing a higher temperature than his own particular average, and any thermometer can indicate this difference.

To find your horse’s normal reading (not necessarily his exact body temperature) on a specific digital thermometer, record the thermometer’s numbers every day for a week or so when the instrument is used at the same time and under the same conditions. This should be when the horse is calm, has not been exercised, and is in the same place (preferably in a barn with good air circulation) for each reading. At a later time when you suspect your horse may have an elevated temperature, use the same thermometer in the same circumstances. If the reading is higher than what is standard for this horse, this indicates fever, even though you won’t necessarily know exactly what the reading is in degrees.