Bit & Bridle

Equine Potomac Horse Fever

Other Names

  • Equine Monocytic Ehrlichiosis
  • Ditch Fever
  • Shasta River Crud
  • Equine Ehrlichial Colitis

What is Potomac Horse Fever?

Potomac Horse Fever (PHF) is a non-contagious acute illness caused by the rickettsial bacterium Neorickettsia risticii. The disease primarily affects the gastrointestinal tract of horses, causing a severe enterocolitis (inflammation of the intestinal lining) that can rapidly progress to life-threatening laminitis and endotoxaemia if not treated promptly.

First identified in horses along the Potomac River in Maryland, USA in the 1970s, the disease has since been documented across North America, Europe, and South America. In the UK it remains relatively uncommon but is present, and horse owners near rivers and wetland areas should be aware of the seasonal risk. The disease is not transmitted directly between horses — its lifecycle involves freshwater snails, aquatic insects, and trematode (fluke) parasites that act as intermediate hosts.

Causes

PHF is caused by Neorickettsia risticii, an obligate intracellular bacterium that infects monocytes (a type of white blood cell) in the horse’s bloodstream. The transmission route is indirect:

  • Infected trematodes (flukes) residing in aquatic snails carry N. risticii
  • Aquatic insects (mayflies, caddisflies, stoneflies, damselflies) become infected as larvae when they feed on infected snails
  • Horses inadvertently ingest infected adult insects — often attracted to stable lights at night — or drink infected water
  • The bacterium then disseminates throughout the gastrointestinal tract

Disease occurs most commonly in late summer and autumn (July to September in the northern hemisphere), coinciding with peak aquatic insect activity near rivers, streams, and irrigation ditches. Horses grazing near or drinking from natural water sources are at highest risk.

Symptoms and Signs

Clinical signs typically appear 10-18 days after exposure and can range from mild to severe:

  • Fever: Often the first sign, with temperatures of 38.5–41°C (101.3–105.8°F) in a biphasic pattern (two separate peaks)
  • Depression and anorexia: The horse appears dull and loses interest in feed
  • Mild colic: Gut cramping and discomfort are common
  • Diarrhoea: Profuse watery diarrhoea typically develops 24-48 hours after the initial fever. Not all horses develop diarrhoea — some present with fever alone.
  • Dehydration and electrolyte imbalances: A consequence of fluid loss through diarrhoea
  • Laminitis: Occurs in 20-40% of affected horses and can appear simultaneously with or after the gastrointestinal signs. Can be severe.
  • Oedema of the distal limbs: Ventral oedema may be present
  • Abortion: PHF is a recognised cause of late-term abortion in pregnant mares

Diagnosis

Early diagnosis is important because treatment is most effective when initiated promptly:

  • Clinical signs and history: Seasonal presentation, proximity to water, biphasic fever, and diarrhoea are highly suggestive
  • PCR (Polymerase Chain Reaction): Blood PCR testing for N. risticii DNA is the most reliable diagnostic test in the acute phase. Faecal PCR testing is also available.
  • Serology (IFA testing): Indirect fluorescent antibody tests detect rising antibody titres. A fourfold rise in paired samples two to three weeks apart confirms exposure, but results are retrospective and less useful for acute management.
  • Blood count: Leucopaenia (low white cell count) is a common early finding that supports the diagnosis
  • Faecal tests: To rule out other causes of diarrhoea including Salmonellosis and Clostridial colitis

Treatment

Treatment should begin on the basis of clinical suspicion — do not wait for laboratory confirmation in a sick horse:

  • Oxytetracycline: The antibiotic of choice. Administered intravenously at 6.6 mg/kg twice daily. Clinical improvement is often dramatic within 12-24 hours, which itself supports the diagnosis.
  • Intravenous fluids: Essential to correct dehydration and electrolyte imbalances caused by diarrhoea
  • NSAIDs: Flunixin meglumine (Banamine) to reduce fever, pain, and counteract endotoxin effects
  • Laminitis prevention and management: Deep sand or shavings bedding, restricted movement, and close monitoring of digital pulses. Frog support may be used in severe cases.
  • Probiotics and biosponge: May help restore gut flora after antibiotic therapy
  • Supportive nutrition: Offering palatable feed in small, frequent amounts to maintain gut motility

Prevention

  • Vaccination: A killed whole-cell vaccine for PHF is available in North America. Efficacy is considered moderate — vaccinated horses can still contract the disease but often have milder signs. Discuss with your vet whether vaccination is warranted given your geographical risk.
  • Reduce insect exposure: Turn off stable lights at night during late summer to reduce insect attraction. Use insect screens where possible.
  • Water sources: Avoid horses drinking from stagnant or slow-moving natural water bodies during high-risk months
  • Early treatment: At the first sign of fever during the risk season in endemic areas, contact your vet. Early oxytetracycline treatment is highly effective.

Prognosis

With prompt and appropriate treatment, the prognosis for PHF is good. Horses treated early with oxytetracycline typically recover within days. The greatest risk to long-term prognosis is the development of severe laminitis, which can be debilitating and in some cases career-ending or life-limiting. Mares that abort due to PHF usually recover fully themselves. Untreated or late-presenting cases carry a higher mortality risk due to endotoxaemia and laminitis complications. See the BEVA website for updated guidance on exotic equine infections in the UK.

When to Call the Vet

  • Your horse has a fever above 38.5°C (101.3°F) that does not resolve within 24 hours
  • A horse near a river, stream, or wetland area develops diarrhoea in late summer or autumn
  • You observe digital pulses becoming strong and bounding in a horse with gastrointestinal signs — early warning of laminitis
  • A pregnant mare shows signs of fever or gastrointestinal illness at any stage of pregnancy
  • Multiple horses on the same yard develop similar signs in a seasonal pattern

Frequently Asked Questions

Can my horse catch PHF directly from another horse?

No. Potomac Horse Fever is not directly contagious between horses. The infection requires ingestion of an infected aquatic insect or contaminated water. If multiple horses on a property are affected simultaneously, they have likely been exposed to the same environmental source.

Does PHF occur in the UK?

PHF is more prevalent in North America, but cases have been documented in Europe including the UK. Horses near rivers and wetland habitats in the warmer months carry some level of risk. Awareness and prompt veterinary attention at the first sign of fever during the late summer period is the most practical preventive approach for UK owners.

What is the connection between PHF and laminitis?

The severe gastrointestinal inflammation associated with PHF allows toxins from gut bacteria (endotoxins) to pass into the bloodstream. These endotoxins trigger a systemic inflammatory cascade that affects the blood supply to the sensitive laminae within the hooves, causing laminitis. This is the same pathway seen with other causes of severe colitis and diarrhoea in horses. Rapid treatment reduces the duration of endotoxin exposure and therefore the risk.

How is PHF different from Salmonellosis?

Both conditions can cause fever and diarrhoea in horses, but they differ in their causative organisms, treatment, and transmission. Salmonellosis is caused by Salmonella bacteria, is potentially contagious to humans, and requires biosecurity measures. PHF is caused by a rickettsial bacterium acquired from the environment and is not directly contagious. Distinguishing them requires specific diagnostic testing, as clinical signs can overlap significantly.