Potomac Horse Fever

Potomac Horse Fever (Equine Monocytic Ehrlichiosis)

What is Ehrlichiosis?

     A category of diseases caused by a type of gram negative bacteria that infect white blood cells.  They are called "intracellular" bacteria because they live and are only able to reproduce inside the cells of a host animal.  Once inside the host animal's white blood cells they can form circular chains that can often be seen on blood smears and used to diagnose some forms of ehrlichiosis. The two commonly seen forms of ehrlichiosis in horses are Anaplasmosis (transmitted by ticks) and Potomac Horse Fever.


How is Potomac Horse Fever Transmitted?

     Potomac Horse Fever (Neorickettsia risticii) is believed be transmitted by horses accidentally ingesting infected small aquatic flies (May flies, Caddis flies) that are infected with Neorickettsia risticii by consuming the larvae of infected snails. Transmission is typically increased in the hot months of July and August, especially in times with lots of standing water for the snail – aquatic fly life cycle to occur. Horses infected with Potomac Horse Fever are not believed to be able to spread the disease to other horses, however other diseases that present with very similar signs can be extremely contagious so biosecurity measures should be put into place as soon as signs are seen or suspected.


What is the "Incubation Period"?

     An incubation period is the time during which a disease establishes itself in the body.  During this time the animal is infected with the organism (bacteria in this case), but does not show clinical signs of the disease.  The incubation period for Potomac Horse Fever is typically 3-9 days. Horses may start to show a fever and depression about on day 5 and diarrhea about day 14 post infection.  


What are the Clinical Signs of Potomac Horse Fever?

     Horses with typically present with fever, depression, anorexia, and diarrhea. The first sign that a veterinarian is called for may be colic or abdominal discomfort prior to the development of diarrhea. This is typically seen in adult horses, and presents very similarly to Salmenelosis. Laminitis is a very common sequelae to Potomac Horse Fever.


How is it Diagnosed?

     Most forms of ehrlichiosis can be diagnosed by viewing the characteristic circular chains of bacteria inside white blood cells.  Potomac Horse Fever, however is not able to always be diagnosed this way.  A polymerase chain reaction (PCR) test on blood samples can be used to identify the presence of the Neorickettsia risticii bacteria in the blood.


How is Potomac Horse Fever Treated?

      If diagnosed early a tetracycline antibiotic such as Oxytetracycline or Doxycycline is commonly used, along with supportive care to treat clinical signs such as colic, fever, laminitis, edema, or diarrhea.  Treatment can be successful in most cases if diagnosed early.  It can be life threatening when associated with colic and/or laminitis, and should be treated quickly and aggressively.


What can be done to Prevent Potomac Horse Fever?


     There is a vaccine for Potomac Horse Fever, however it is not protective against ALL of the disease causing strains of the bacteria. Horses that are affected by Potomac Horse Fever and recover have been found to be protected for up to 20 months from reinfection.