Each time a veterinarian pulls into a client’s driveway, the case he or she is about to see is unique—there’s no telling what lies ahead. But what’s one of the first things the practitioner does for nearly every case? He or she pulls out a thermometer and checks the horse’s rectal temperature.
Temperature is an important vital sign that can help guide practitioners’ next diagnostic steps. An elevated body temperature can also be a dangerous clinical sign that needs controlling to prevent potentially catastrophic consequences. Here’s a look at fever in horses and the tools veterinarians have to control them
Equine Fever 101
Body temperature control starts in the horse’s brain, where neuronal controls work to maintain a set point: the horse’s normal temperature, which ranges from 99 to 101.5°F. Sensors throughout the horse’s body recognize internal and ambient temperature changes, send signals to the brain, and trigger mechanisms (such as sweating or shivering) to keep the body temperature as close to that set point as possible.
A fever (also called pyrexia) develops when some inciting factor causes a change in the brain’s thermoregulatory set point and the horse’s body temperature elevates above normal. (Horses can also develop an elevated body temperature in response to things like hot weather, strenuous exercise, an inability to sweat, or reactions to drugs or toxins, but this is generally considered hyperthermia rather than a true fever.)
Fever is a common condition for veterinarians to encounter in horses, especially given its wide range of potential causes. It can result from issues ranging from respiratory (e.g., shipping fever) and other infections (e.g., Potomac horse fever) to vaccine reactions and organ failure … even tumors. Fevers can also have no identifiable cause (termed fevers of unknown origin).
Perhaps surprisingly, fever has several benefits.
“A fever is one of the body’s defense mechanisms against infections,” says Jennifer Davis, DVM, PhD, Dipl. ACVIM, ACVCP, an associate professor of clinical pharmacology at the Virginia-Maryland College of Veterinary Medicine, in Blacksburg, Virginia. “It is a signal that the immune system has been activated and helps speed delivery of immune cells to the site of infection. Additionally, most bacteria and viruses that infect a specific species grow optimally at that species’ normal body temperature. Altering that body temperature may slow growth of infectious organisms.”
Fever can also have harmful effects, largely dictated by its degree and duration, she adds.
Short-lived fevers typically present few to no problems for the horse but can still be disconcerting for owners, she says. Longer-lived fevers can cause decreased feed and water intake, leading to dehydration, gastric ulcer development, and other, more serious problems.
“Temperatures less than 102.5-103°F in horses are often not harmful in and of themselves,” Davis says. “Higher temperatures (greater than 104°F) often produce clinical signs, and, in horses, we begin to worry about their association with the development of laminitis. Very high temperatures (greater than 107°F) may be associated with the development of seizures, although these temperatures are rarely reached with typical infectious diseases.”
This means fever control at an appropriate time is an important part of keeping horses comfortable, warding off additional ailments, and aiding recovery.
Fever Control Options
Veterinarians use non-steroidal anti-inflammatory drugs (NSAIDs) for fever control, and they have several options to choose from. Dipyrone (marketed as Zimeta), for instance, is the only FDA-approved drug labeled for use in controlling pyrexia in horses. Several other common NSAIDs—phenylbutazone (bute), flunixin meglumine (Banamine), firocoxib (Equioxx), and ketoprofen (Ketofen)—are labeled for the control of inflammation and pain associated with musculoskeletal issues (and, in flunixin’s case, to alleviate visceral pain associated with colic) and are also effective in controlling fever.
Of course, each case is unique, so veterinarians must evaluate all factors before deciding which medication to use in a particular instance. They must consider, for example, how each drug behaves in the horse’s body.
“Many of the NSAIDs labeled for horses do not reach high concentrations in the central nervous system (CNS) because they are highly bound to plasma proteins and, therefore, remain in circulation,” Davis says. “Dipyrone and its metabolites reach higher concentrations in the CNS and can therefore act directly on that portion of the brain that causes the fever.”
Vets might also use different approaches if they’re treating a sport or show horse and must contend with medication regulations. Take a mild case of shipping fever, for instance. Considering Fédération Equestre Internationale (FEI) drug rules, NSAIDs approved for use in the United States, and administration routes, estimated detection times range from:
- Dipyrone at 72 hours (3 days) following intravenous (IV) administration;
- Ketoprofen at 96 hours (4 days) following IV administration;
- Flunixin at 144 hours (6 days) following IV administration;
- Phenylbutazone at 168 hours (7 days) following IV or oral administration; and
- Firocoxib at 336 hours (14 days) following oral administration.
Ultimately, veterinarians have a lot to consider when dealing with a fever and determining how and when to control it. Davis encourages owners to work with their veterinarians as they determine the fever’s underlying cause, whether it is harmful or beneficial to their horse, and which drug is the most appropriate choice for treatment based on those findings.