Potomac Horse fever is a warm-month disease, with horses at pasture at greatest risk. Although it’s curable, left undetected, it can be fatal. Fatality rates are between 5 to 30+%. Eventual euthanasia because of laminitis, a frequent complication, accounts for some of the deaths. Pregnant mares may abort.
Prevention And Treatment
A vaccine is available, but its efficacy is questionable (see sidebar). Your horse’s best line of defense is early administration of tetracycline when symptoms suggest you may be dealing with PHF. However, some cases progress so quickly that the horse dies before he is observed to be ill.
Horses develop a fever soon after they’re infected, but symptoms otherwise usually take about two weeks to appear. At that time, the horse may again have a fever. The blood may show elevated monocytes and depressed numbers of lymphocytes initially. Once damage to the colon is severe, clotting tests may be abnormal and white counts may drop.
The diagnosis may be made by indirect fluorescent antibody tests, or ELISA tests, but both false positives and false negatives occur, so the horse is often treated due to clinical signs.
When tetracycline is started early, the response is usually rapid and good. It’s also important to administer anti-inflammatory drugs and correct the dehydration and electrolyte abnormalities caused by colitis and diarrhea. The diarrhea can be severe enough to require as much as 100 liters of fluid in 24 hours.
How They Get It
This illness was dubbed Potomac Horse fever because the disease-producing organism was first isolated from horses living along the Potomac River. However, similar clustering of cases has been reported around other bodies of water, such as lakes.
The exact connection was unclear for years until Dr. John Madigan at the University of California, Davis, found that the E. risticii organism is present within segments from flukes parasitizing fresh water snails and aquatic insects. The disease has been produced in horses by feeding infected insects, but they may also pick it up by drinking water containing the larval flukes or by skin penetration.
PHF appears only in the warmer months because the organisms carrying the Ehrlichia only release infected fluke larvae when temperatures are at or above 70?°.
Technically called equine monocytic ehrlichiosis or equine ehrlichial colitis, PHF is a disease caused by infection with the E. risticii strain of Rickettsia. Rickettsia are organisms that share characteristics with both viruses and bacteria, but they’re classified with bacteria. The organism occurs throughout North and South America and Europe. It’s also been found in Korea and Japan.
Avoiding pasturing your horses in areas with natural water sources is a good preventative measure but not a guarantee. Snails and insects that may harbor PHF organisms can also be found in ditches, gullies — anywhere water may accumulate. Paradoxical outbreaks may even occur in dry years. The theory behind this occurrence is that when water is scarce horses are likely to only find green vegetation to eat in spots where water tends to accumulate.
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