Potomac Horse Fever
Potomac Horse Fever (PHF), a disease named for the river near Washington DC where it was first identified, is caused by Neoricketsia risticii. The technical name for PHF is equine monocytic ehrlichiosis (EME). This causative organism is normally found in a trematode parasite with a somewhat complex life cycle involving bats, snails and certain aquatic insects. It is therefore associated with swampy/marshy areas in certain regions. Horses are aberrant hosts of N. risticii, in which infections can cause a range of disease from very severe colitis and laminitis to mild non-specific illness. The organism is typically very sensitive to treatment with tetracyclines, but the disease can progress very quickly, so treatment early in the course of disease (if PHF is strongly suspected) is crucial.
In general, PHF is an uncommon disease in horses, but there are certain areas of Ontario where cases occur with some regularity. For example, cases have been reported in Eastern Ontario (Brighton to Kemptville), Keswick area and Ancaster. The incidence is usually highest in late summer. Transmission is normally associated with horses grazing pastures containing infected flies (caddisflies).
Although vaccines for PHF are available and appear to be safe, their efficacy is very questionable. At least in part, this may be due to the fact that multiple strains of N. risticii occur naturally, but only a single strain (from the 1980s) is included in the vaccines. Vaccination is not considered necessary for horses in areas where the disease has not been previously reported. Given the peak incidence of the disease in late summer and the nature of the vector, if someone wants to vaccinate, they should do so in late spring. Preventing access of horses to areas near swamps and other insect breeding grounds may be as (or more) effective than vaccination alone.
Photo Credit: Buck Cash, Great Falls, Potomac River, VA
