There are many potential causes of fever, from normal physiologic events such as exercise to infectious agents. Two big causes seen in our area in the fall, however, are Anaplasmosis and Potomac Horse Fever.
Anaplasmosis is caused by a gram negative bacteria, Anaplasma phagocytophilum. It is spread by tick bites, specifically by the deer tick (Ixodes scapulalris). Clinical signs take about 10 days to become apparent following exposure, and consist mainly of a high fever, sometimes up to 104 F or higher. Other clinical signs include: inappetance, lethargy, edema in the distal limbs, and, occasionally, neurologic signs. Severity can vary greatly and, if untreated, the disease is usually self-limiting over the course of approximately two week. Treated horses, however, recover much more rapidly and are often back to normal after three to five days.
Diagnosis of Anaplasmosis can be done through a variety of blood tests. The quickest diagnosis can be made by the detection of inclusion bodies within neutrophils (a type of white blood cell), which can be performed by your veterinarian that same day by making a blood smear in the laboratory. However, lack of inclusion bodies under the microscope does not indicate the absence of Anaplasma. The most sensitive test for detection of Anaplasmosis that is routinely run is a PCR (polymerase chain reaction) test which typically takes 3-4 days for results. Treatment for Anaplasmosis is intravenous Oxytetracyline (an antibiotic), with Banamine as needed to bring down the fever. Response time to treatment varies by case, but is usually limited to 3-5 days. If needed, supportive treatment with oral or intravenous fluids may be utilized. Anaplasmosis is not contagious from horse to horse. Overall, the prognosis for horses with Anaplasmosis is excellent.
The second common cause of fevers at this time of year in our area is Potomac Horse Fever (PHF), also known as Monocytic Ehrlichiosis. It is spread by inadvertant ingestion of freshwater snails and aquatic insects that carry the bacteria, Neorickettsia risticii. This bacteria causes damage by infecting monocytes (another type of white blood cell) which then travel to the digestive tract and cause inflammation within the wall of the intestine. This leads to diarrhea and signs of toxemia, often including laminitis. Fever, depression, and anorexia are, again, some of the first clinical signs noted, but with PHF mild colic signs, diarrhea, and laminitis are also commonly seen. PHF can be fatal, often due to the laminitis that occurs as a sequellae.
Potomac Horse Fever diagnosis can tentatively be made based off of clinical signs, region of the country, and season, but definitive diagnosis requires a PCR test to be done on blood and fecal samples. Though PHF is also treated with Oxytetracycline, supportive care is very important in preventing laminitis and keeping the horse adequately hydrated in the face of diarrhea. Horses usually respond quickly to aggressive treatment, which typically runs 3-5 days; however, more severe cases often require hospitalization to treat dehydration and toxemia. The best prevention from PHF is biannual vaccination and management changes, such as preventing access to ponds and streams and keeping feed sources covered and away from lights at night. The vaccination will not prevent the disease from occurring, but will help decrease the severity and limit the more dangerous clinical presentations, like laminitis. Similarly, Potomac Horse Fever is not contagious from horse to horse.
With both Anaplasma and Potomac Horse Fever, the best way to protect your horse is early detection and treatment. Monitoring your horse’s temperature once daily is an important management practice to aid in detecting not only these two diseases early, but many other infectious diseases as well.