Potomac Horse Fever (PHF) is a sporadic cause of acute diarrhea in many parts of the country. The disease is caused by bacteria called Neoriketssia ristici, which is carried by aquatic insects that consume tiny parasites (cercariae; a fluke) of fresh water snails. A horse may be exposed by ingesting the insects while grazing; however, because of the unique life cycle of cercariae, the horse is not infected directly by the snails or the water that contains the parasites of the snails.
PHF is more prevalent in warmer months when large numbers of the parasites are released into bodies of water and aquatic insects are most active. Horses kept near ponds and creeks are more likely to be exposed to Neoriketssia ristici, but this is not prerequisite. Anywhere the carrier insects gather, a horse may be exposed.
Although the bacteria are shed in the infected horse’s stools, PHF is not considered directly contagious. Group outbreaks are usually associated with close confinement, where the horses were all exposed to insect contaminated feeds.
PHF occurs when cells that line the intestine become infected with N. ristici. The incubation period after ingesting the carrier insects is about 10 days to 2 weeks. The horse will become depressed and go off feed in most cases, accompanied by fever as high as 107 degrees. Moderate to severe diarrhea from cow-patty to watery consistency occurs in more than 60% of cases, and is often the indicative symptom. Horses are at risk of dehydration during this stage of the disease. Approximately 40% of cases will be complicated by laminitis. It can be severe and will most often affect all four feet. A few horses will develop edema in the skin over the limbs and the underside of the girth and flanks, and occasionally bruises (petechiae) will be noticed on the mucous membranes. Pregnant mares are at risk of spontaneous abortion even months after infection.
A presumptive diagnosis is based upon clinical symptoms and location within endemic areas. A positive blood and fecal DNA test, where available, for N. ristici is definitive, but antibody titers are unreliable. In any case, horses suspected of PHF should be treated aggressively to prevent dehydration and laminitis. Pretreatment x-rays of the feet should be taken as a reference should laminitis occur.
Treatment will consist of IV fluid and electrolyte replacement, antibiotics, frog pads, and anti-inflammatory drugs. Although vaccination is of little value to an infected horse, it may help reduce symptoms in other horses in case of an outbreak. The vaccine has limited duration of protection and will not prevent the infection, only reduce the severity of disease.
Consult your veterinarian about whether vaccinating for PHF is appropriate in your area.