Conditions or ailments that are the cause of a problem that you see - your observation.

Your vet may diagnose

Potomac Horse Fever, Neorickettsiosis

Synonyms: Neorickettsia Risticii Infection, Ditch Fever, Equine Ehrlichial Colitis, Equine Granulocytic or Monocytic Ehrlichiosis, Shasta River Crud


Potomac Horse Fever is a potentially fatal bacterial (Rickettsial) disease that is believed to be transmitted by inadvertent ingestion of aquatic insects (mayflies and caddis flies) carrying the organism. These are thought to fly into barns or feeding areas, die, and end up in feed.

The life cycle of the causative Neorickettsia risticii is complex and involves stages within a freshwater fluke and snail.

PHF occurs more frequently in wet warm climates, and particularly affects horses pastured along rivers, creeks and ditches. It is much more common in the Northeastern and Northern states, but has been diagnosed in most states within the U.S.

Initial signs of this disease are subtle and include mild colic, fever, inappetence, and diarrhea. Typically it progresses to severe colitis and diarrhea. Abortion in pregnant mares may occur due to fetal infection. Laminitis is a common consequence of infection.

Diagnosis is difficult, and often arrived at after other diseases are ruled out. The appearance of this disease is very similar to other causes of colitis and so laboratory work is necessary to definitively diagnose. In some cases, the diagnosis is presumed if a horse improves when given oxytetracycline (an antibiotic that is effective against the protozoa) Although PHF is not contagious from horse to horse, precautions should be taken immediately to prevent contact with other horses until other contagious diseases are ruled out.

Treatment is supportive nursing care (usually hospitalization) including IV fluids, plasma, colloids, and laminitis prevention measures. The only antibiotic class typically used against the organism is the tetracyclines.

In areas where the disease is common, oxytetracycline is often given at the earliest signs of disease, even when a definitive diagnosis has not been made. This likely reduces the severity of infection.

my vet's role



Other conditions or ailments that might also need to be ruled out by a vet.

Very Common
Less Common
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The prognosis is good with early diagnosis and treatment with the appropriate antibiotic. If disease becomes severe, horses will die without appropriate treatment. In severely affected horses, laminitis commonly develops later.

my role


I might observe

You might make these observations when a horse has this condition.

Very Common
Less Common
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Questions To Ask Your Vet:
  • How did my horse get infected?
  • What diagnostics are you using to rule out this condition?
  • What can be done to minimize the likelihood of laminitis?
  • Should my horses be vaccinated against this disease?

If possible, reduce access to rivers, creeks, or ditches during the warm wet summer season.

Vaccination may reduce severity of infection, but is not completely protective. Vaccination is recommended in regions in which the risk of this condition is greater.

Keep lights at stables off at night to reduce attraction of aquatic insects.

Monitor feed and water containers, and clean frequently.

Although PHF is not contagious from horse to horse, precautions should be taken to prevent contact with other horses until other contagious diseases are ruled out.

further reading & resources

Author: Doug Thal DVM Dipl. ABVP