Potomac Horse Fever

What are the symptoms?
Look for one or more of the following signs: high fever (up to 107 F), depression, anorexia (off-feed), equine dehydration, diarrhea, colic and laminitis. Horses with PHF show signs of colitis (inflammation of the colon) in varying degrees.

Where does it come from?
Potomac horse fever has been linked to the bacteria Ehrlichia risticii (recently renamed Neoriketssia ristici). This bacteria is associated with a parasite of freshwater snails. The bacteria also infects the larvae of mayflies and caddis flies which live in fresh water.

How is it diagnosed?
Currently, the most convenient way to diagnose PHF is by the IFA (indirect fluorescent antibody) test, a serologic measurement of a horse’s antibody titer to Neorickettsia risticii. Most veterinarians opt to begin treatment right away, because the disease can progress rapidly. If treated early, outcomes are good.

How is it treated?
Treatment of horses with PHF includes antibiotics, fluid and electrolyte replacement therapy for animals with diarrhea, and nonsteroidal anti-inflammatory drugs for the relief of pain in cases of colic and/or equine founder.

How is it prevented?
Vaccines are available for protection against PHF, but protection may be short-lived and incomplete. In many cases, vaccination has been shown to reduce clinical signs rather than provide complete protection.

To reduce your horse's exposure to the bacteria, try these techniques:

• Turn off the lights inside and outside your barn
• Move water troughs away from light sources
• Keep horses away from ponds, lakes and creeks during mayfly season
• Keep food covered when not in use

Horses that recover from PHF may be resistant to infection for at least 20 months due to natural immunity.

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