Increased Incidence of Potomac Horse Fever in Area 2010

Lindsey L. Peck, DVM and Stacy M. Kenyon, DVM

The relatively wet conditions this summer have provided ideal conditions for Potomac

Horse Fever to be present as evidenced from several documented cases in Lewis, Oneida, Jefferson

and St. Lawrence counties during the past two months. We are highly recommending fall

boosters of Potomac Horse Fever to those that have already been vaccinated and continue

to recommend vaccinating horses that have not been vaccinated previously.

Clinical symptoms of Potomac Horse Fever can be vague and at times difficult to identify

specifically as often horses in the very early stages of the disease may only appear dull and have

loss of appetite. This generally develops into signs of colic and they continue to get significantly

worse. Severe diarrhea is one of the more consistent symptoms that will develop as the infection

progresses and laminitis is a serious complication of the disease in some horses. Early treatment

and intense supportive care including intravenous antibiotic and fluid therapy provide the best

prognosis for affected horses.

Potomac Horse Fever is a disease caused by infection with Neorickettsia risticii and

symptoms include fever, colic, diarrhea and subsequent laminitis. This organism has a complicated

life cycle that continues to be the topic of intense research. N. risticii is found in certain species

of freshwater snails and aquatic insects (caddisflies and mayflies) that populate areas along

rivers. It is believed that horses become infected through ingestion of snails and insects that live

in grasses along streams and ditches and in areas of standing water. The adult insects may also

contaminate feed and water of nearby stables. Aquatic insects probably provide the means by

which this organism moves from one geographic area to another through weather patterns that

can literally blow swarms of flies to a new location.

Strategies to prevent infection include vaccination and reduction of snails/insects in area

bodies of water. As with most rickettisial diseases, vaccination is not fully protective against infection

but the severity of disease following infection is greatly reduced in vaccinated horses. It will

continue to be our recommendation that horse owners consider including PHF vaccination as part

of their preventative healthcare program. The initial series of PHF vaccination requires 2 doses

approximately 4 weeks apart and it is recommended that the vaccination be given approximately

every 6 months to provide optimal antibody titers during the peak seasonal incidences of disease

vectors (snails/insects).

Why consider vaccination for your horse? The vaccine costs approximately $18 per dose

(2-3 doses required over course of year depending on whether it is the first year of vaccination).

The average treatment costs, as determined from recent cases, averaged $1000-$1500. Although

most horses survive infection the physical, emotional, and financial toll on patient and owner is tremendous.

Please consider vaccination today!