Potomac Horse Fever in Horses

Potomac Horse Fever (PHF; also called Equine Monocytic Ehrlichiosis) is caused by a bacteria called Neorickettsia risticii (formally known as Ehrlichia risticii). The bacteria are accidentally ingested/taken in orally by a horse, which can then result in infection and clinical disease.  

Symptoms of PHF can include:

decreased appetite

depression

fever

*diarrhea- varies from none to mild to severe, watery, protracted diarrhea

colic

dehydration

signs of toxemia/endotoxemia

can be fatal in severe cases (5-30% mortality)

laminitis (founder)

The signs can be very mild (only off feed, fever) to severe and even life-threatening. The horse may recover from the initial signs of Potomac Horse Fever, but if laminitis occurs as a complication, this can result in an ongoing problem. If severe, the laminitis may require long-term management and can even result in the horse needing to be euthanized. Duration of signs: 2-10 days.

The life-cycle of the PHF bacteria in the environment has taken many years of research and study to ascertain and is fairly complicated. The N. risticii bacteria live in a fluke (flatworm parasite) that infects freshwater snails. This fluke also can infect aquatic insects such as:                                                              caddisflies

mayflies

damselflies

dragonflies

stoneflies

Birds and bats can also become infected with this fluke. A horse may become infected with Potomac Horse Fever by consuming the fluke that carries the N. risticii bacteria in water (pond, stream or water trough), on the pasture, or in hay/feed. This can occur from ingesting: fluke eggs shed in bat or bird feces, free flukes present in pond/stream water, fluke-infested snails or aquatic insects. PHF is not thought to be directly contagious from horse to horse.

Highest risk for PHF:  July & August (range June-October) and near freshwater streams and ponds (where the bacteria cycles in snails, water & aquatic insects).

Treatment: IV antibiotic and other supportive treatment as recommended by your veterinarian

Prevention:

·  vaccination: recommended in “endemic” areas/where cases have occurred

                          -not 100% preventative, but thought to decrease severity of signs

·  minimize exposure to the bacteria (in the fluke)

-     minimize contact & ingestion of aquatic insects

-     keep horses away from rivers and ponds during the late summer

-     clean water troughs & feed buckets- keep free of aquatic insects