Body temperature is usually maintained between 37,5 and 38°C in adult horses.
“Pyrogenic” substances are those that are capable of causing an increase in body temperature. They can be pathogens (bacteria, virus) or molecules produced by the organism (cytokines).
Infectious disease can cause fever as can some vaccines. Decreasing the temperature at all costs is not always beneficial as fever is a non-specific reaction that is part of the body’s defence mechanism.
A rectal thermometer is used in horses. It is always a good idea to get your horse’s temperature before calling your veterinarian.