Rectal temperature is an important clinical parameter in all animals. With horses, rectal temperature is commonly used in disease outbreak situations such as herpes virus or strangles, to determine if exposed horses are developing an infection. However, you have to know the normal rectal temperature to determine if a specific horse has an increased temperature, and although there is a normal temperature range published for horses, that is not necessarily correct for all horses.
There are several parameters that can have an effect on rectal temperature and one is the placement of the thermometer and the type used. Long rectal mercury thermometers have been shown to have a difference of almost 2°F in one study compared to a short digital thermometer such as you would purchase for humans. It is important to use a similar type of thermometer every time, and insert the thermometer in the horse’s rectum the same distance every time. The recommendation is to insert the digital thermometer in the rectum up to the read-out area or about 2 ½ inches, and when you do so, make sure the thermometer is not stuck in a ball of feces but is angling so the tip is in contact with the rectal mucosa. Stress and exercise can also affect rectal temperature. A study in England found that horses at different stables can have different temperatures than horses at other stables. In this study, the horses at one stable had normal temperature ranges from 96.8°F to 100.4°F.
The important point is to take your horse’s temperature with the same thermometer in the same manner every time so you will know your horse’s normal temperature, rather than relying on a range for all horses.