Potomac Horse Fever

There have been at least 2 confirmed cases of Potomac Horse Fever (PHF) this summer, one in Norfolk County and one in Essex County.  It can therefore be said definitively that the causative organism, which is Neorickettsia risticii, is present in the environment of our practice area.

Horses are exposed to the causative organism by oral consumption of aquatic insects (ex: dragonflies, mayflies) that have picked up flukes (flatworms) that develop in snails in fresh water, ponds, streams, and low lying pastures.  Horses can then ingest the insects by either drinking from these bodies of water, or in their own water buckets and tubs that have been contaminated by the insects.

Signs can include: fever, diarrhea, lethargy, and mild colic.  The onset can be slow in affected animals, and there is a significant mortality rate in untreated horses.  There is an incidence of laminitis in more than 20% of recovered cases.

Preventative management measures include: prevention of consumption of water from streams, ponds, and water pools in turnout areas.  Buckets and water tubs should be evacuated on a daily basis to reduce the concentration of insects in the water.  Turning off lights at night in stables may reduce the attraction of insects to the water sources in the barn.  There is an available vaccine for the disease that is administered either alone or in combination with rabies vaccinations.  The vaccine has mixed scientific reviews for efficacy but it has been considered helpful in areas of the country where Potomac Horse Fever is more endemic.  The vaccine protocol involves 2 vaccines administered 3-4 weeks apart initially, followed semiannual or annual boosters.  In our practice area annual boosters in the spring are adequate since the disease only appears in the warm weather of late spring to early fall. 

Given the current situation we recommend vaccination for horses whose stabling and turnout situations expose them to the aquatic insects which may carry the disease.

Here is a link to the AAEP website for additional information: