:: POTOMAC HORSE FEVER
Diagnosis, Treatment, and Prevention

Potomac Horse Fever (PHF), or equine monocytic ehrlichiosis, is an infectious disease of the horse's gastrointestinal tract. It was originally reported near the Potomac River in Maryland and Virginia, but today PHF can be found in many areas of the U.S., as well as Europe and Australia.

PHF is caused by Ehrlichia risticii, which is a rickettsial organism. The disease tends to occur in two different patterns. The first and most common pattem is where PHF occurs in individual horses isolated from other cases. The second pattern occurs on farms or tracks where horses are concentrated in large numbers and where outbreaks of PHF occur in clusters. In areas where PHF has occurred, recurrence is likely.

PHF occurs seasonally from late spring to early fall, with the summer months being most likely. Sudden hot weather seems to bring on outbreaks of disease. How PHF is spread is still unknown, but biting insects (mosquitoes, black flies) or arthropods (ticks) are suspected. It is known that the causative agent of PHF is shed in the infected horse's feces, so other horses may become infected by eating or drinking materials that are contaminated with feces that contain the organism.

CLINICAL SIGNS

The clinical signs of PHF vary widely among individual cases. Most horses show colic, depression, appetite loss, and reduced gut sounds. Others may develop a fever before other signs develop, but the fever may last only a few hours or may occur in cycles throughout the day. Thus, the fever is often missed by observers. Some horses develop diarrhea, which ranges from mild to severe. Horses with relatively mild signs usually recover spontaneously within a week or so. Horses with severe diarrhea  may become dehydrated which can ultimately result in death.

Laminitis may also develop secondary to severe diarrhea. Its severity ranges from mild to life-threatening. Laminitis may develop early in the disease or at anytime afterwards, including during recovery.

DIAGNOSIS

Diagnosis is difficult, because signs vary so much between individuals. However, if typical signs are seen in horses in an area where PHF is known to occur, PHF is usually suspected. Blood tests that reveal antibodies to the PHF organism are not 100% reliable because the tests may not be accurate. Even if the results are correct, the antibodies may be due to a previous infection. The best approach is to send two blood tests, one taken immediately after signs begin and one about a week later. If the second test shows higher levels of antibody, the horse is experiencing an active infection.

TREATMENT

Because the signs and severity of PHF vary widely, nursing and supportive care must be tailored to the specific signs present. Treatment against the actual organism includes antibiotics. Most horses must be given fluids to prevent dehydration.

PREVENTION

Vaccination against PHF is recommended in areas where PHF occurs and if the horse will be traveling to shows or meets. Frequency of vaccination may vary, depending upon the horse's particular situation. A veterinarian should be consulted to determine vaccination recommendations for a specific horse or area of the country.

Mystique^®
Ehrlichia risticii Bacterin
   For vaccination of healthy horses as an aid in the prevention of Potomac Horse Fever (equine monocytic ehrlichiosis) caused by E. risticii.

Mystique^® II
Rabies Vaccine, Ehrlichia risticii Bacterin
   For vaccination of healthy horses against disease caused by rabies virus and as an aid in the prevention of Potomac Horse Fever (equine monocytic ehrlichiosis) caused by E. risticii.

Administration and Dosage
   For primary immunization, aseptically inject 1 mL intramuscularly in healthy horses 3 months of age or older. Repeat the dose in 3 to 4 weeks. A 1 mL booster dose should be administered annually and at any time epidemic conditions exist or are reported and exposure is imminent.

This brochure was developed by the American Association of Equine Practicitioners through a grant from Bayer Corporation.

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