Potomac Horse Fever

By: Katie Jones, CVT

For many, once the snow begins to melt excitement for spring and summer riding sets in. And the bugs start. There are many diseases carried by the summer bugs, and in our area, Potomac Horse Fever (PHF) is one owners and riders worry about.

Potomac Horse FeverWhat is Potomac Horse Fever? The first documented outbreak of Potomac Horse Fever occurred in 1979 along the Potomac River banks in Maryland; it can now be found in 43 states. PHF cases are commonly found near creeks and rivers. PHF is caused by multiple strains of Neorickettsia risticii. Studies have found that parasites living in freshwater snails and aquatic insects are the reservoirs for this bacterium. The aquatic insects the bacterium can be found in are: adult and immature forms of caddisflies, mayflies, damselflies, dragonflies, and stoneflies. Horses ingest the infected insects from drinking water, grass from pasture, or from their hay source. Insects are attracted to barn lights during the nighttime hours; thus, they are attracted to areas where horses are kept. Potomac Horse Fever is not usually seen until the last half of summer (July-September) due to insects being the main source of transmission and the reason outbreaks are found seasonally. It is important to remember horse to horse transmission does not occur with PHF.

The first thing most owners observe when their horse has contracted Potomac Horse FlysFever is a decrease in their appetite. Other clinical signs are: fever (ranging from 102-107°F), colic episodes, depression, no manure, diarrhea, and laminitis. Commonly horses will develop an inflamed large intestine (colitis) which causes many of the symptoms associated with PHF. The development and severity of their clinical signs are dependent on individual cases. The clinical signs are very similar to many other diseases; therefore, it is important to isolate any horses showing signs to first verify it is PHF and not another contagious disease.

Potomac Horse Fever is diagnosed by performing a polymerase chain reaction (PCR) on a blood, tissue, or, in some instances, a fecal sample.   The treatment of choice is a course of Oxytetracycline injections over a several day period. The N. risticii organism hides in the horse’s macrophages, are part of the body’s natural immune system, thus it hides from the body’s immune system. Oxytetracycline, an antibiotic, hinders the organism’s control mechanisms, allowing the macrophage to remove N. risticii from the body. There are no environmental changes that can be made to protect horses against PHF, but horses living next to standing water are at a higher risk of contracting PHF.

The best way to prevent the development of PHF is to vaccinate every year after May 15th. The vaccine, while protecting the horse against one strain of PHF, does not completely prevent the disease, but may reduce the severity if a horse does contract the disease. When planning for the riding months, horse owners begin scheduling their annual veterinary visits to have their horse(s) ready for the year in the spring; typically this includes all needed vaccines.   Due to having this visit done earlier in the year, it is not uncommon to forget this vaccine. Horse owners should first consult a veterinarian to determine their horses’ risk level of contracting Potomac Horse Fever, before adding the yearly PHF vaccine to their schedule.