Page 132 - February 2016 Speedhorse

FEVER IN HORSES
“We think the higher temperature increases the horse’s metabolism and thus the ability to fight off infections.”
by Heather Smith Thomas
The definition of fever is body temperature
higher than normal, caused by something
that triggers an increase in the body’s set point for normal. “Normal” for a horse, however, can vary from about 98 degrees to 101 degrees, depending on the individual animal, with 100.5 being average. A newborn foal has higher temperature than an adult horse - maybe up to 102 degrees. Individual horses, just like humans, tend to have an individual “normal” temperature. It’s wise to get an idea of what your horse’s normal temperature is by taking it daily for
a few days. If your horse’s normal temperature is 98 and it rises to 100.5, this might be considered a fever for that horse, but not necessarily for another horse.
Rose Nolen-Walston, DVM, DACVIM, Assistant Professor, Large Animal Internal Medicine, University of Pennsylvania New Bolton Center says that anything over 101 is worth looking at in case
it might be a fever. “There are several reasons why horses can have an increased body temperature that would not be a fever. So, the first question to ask when you take a horse’s rectal temperature and it is high is, ‘Is this a fever or not?’” she says.
The brain has a temperature control mechanism. “The hypothalamus at the base of the brain deter- mines the temperature set point. If there is some- thing wrong in the body, like an infection, the body produces chemicals that change that temperature set point and make it higher for a while, and this is a fever. In other situations the body simply becomes hotter, but the brain set point hasn’t changed,” says Nolen-Walston.
WHAT CREATES A FEVER?
In various situations the body’s immune system recognizes something foreign in the body, such as an invading pathogen, and decides to get rid of it. The body responds to a variety of stimuli by producing chemical messengers called cytokines. “These tell the immune system how to react. The specific cytokines involved in fever are called IL-1 and IL-6 (inter- luekins are cytokines that play a major role in regulat- ing immune and inflammatory responses), along with TNF-alpha (Tumor Necrosis Factor alpha) released by blood cells,” says Nolen-Walston.
“There are a number of pyrogens that prompt the body to have a fever, to fight the invader. Some of the most common pyrogens (fever-causing molecules) are infections, either bacterial or viral, or sometimes even a fungal infection,” she says.
When there’s an infection, the body recognizes the invading organism and the first-alert immune cells signal to other immune cells and tell them to start releasing cytokines. “Many of those cytokines go to the brain and increase the set point in the hypothalamus, and the body chooses to be at a higher temperature, to maintain a higher temperature while fighting the infection,” she explains.
“We think the higher temperature increases the horse’s metabolism and thus the ability to fight off infections,” she says. This would speed up all the body processes, including blood circulation, bringing more nutrients, oxygen and healing factors to wher- ever they are needed.
We see this response, a fever, with a lot of dif- ferent infections. Fever in itself doesn’t tell us what is wrong with the horse. “There are a number of infections that are generally associated with low to moderate fevers, and a few infections that tend to cause very high fevers. Whenever I see a horse with a
Rose Nolen-Walston, DVM, DACVIM, Assistant Professor, Large Animal Internal Medicine, University of Pennsylvania New Bolton Center
“If you ride your horse and work him hard on a hot day his temperature rises, but this is called hyperthermia rather
than a fever. The main causes of hyperthermia include exercise, extreme heat and humidity, and anhidrosis (a condition in some horses that interferes with their ability to sweat—and they can’t cool themselves). Most of the time, if a resting horse has an increased rectal temperature, it’s because it has a fever,” she says.
130 SPEEDHORSE, February 2016
EQUINE HEALTH