Potomac Horse Fever

Courtnee Morton, DVM
By Courtnee Morton, DVM on Oct. 17, 2022
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What is Potomac Horse Fever?

Potomac Horse Fever (PHF), also referred to as Shasta River Crud or Equine Monocyte Ehrlichiosis, is a disease that leads to acute, sometimes fatal, enterocolitis (inflammation of the lining of the digestive system) in horses. If you notice your horse has become lethargic with a decreased appetite and fever along with diarrhea, you should call your veterinarian straight away.

PHF is caused by the bacteria Neorickettsia risticii and is named after the Potomac River where it was first discovered in the 1980s.  It is most commonly observed in warmer months in geographical areas near a creek or river in the majority of the United States and parts of Canada. Any breed or age of horse may be affected, although it is not typically seen in horses younger than 12 months of age.

Symptoms of Potomac Horse Fever

Signs your horse may be suffering from PHF include:

This bacteria-induced colitis is more likely to create secondary laminitis than some other organisms. If a horse with PHF has a very high fever, becomes toxic, or is dehydrated, they are at a higher risk for developing laminitis.

Causes of Potomac Horse Fever

Horses acquire PHF by ingesting Neorickettsia risticii-carrying freshwater insects when drinking such as:

  • Dragonflies

  • Mayflies

  • Caddisflies

  • Stoneflies

  • Damselflies

Freshwater snails often carry small parasites in their GI tracts called trematodes or flukes. Juvenile trematodes harbor this bacterium; when they are released into the water they are consumed by the flies. Horses drinking from these freshwater sources are more likely to accidentally ingest an aquatic insect or trematode carrying this intracellular bacterium.

The fluke that develops in the aquatic flies is resistant to the acid in the horse’s stomach, so it doesn’t die after being eaten. N. risticii then infiltrates the white blood cells, particularly in the lining of the gastrointestinal tract where it causes inflammation and damages tissues. Toxins are created from the tissue damage and are then absorbed directly into the bloodstream.  

How Veterinarians Diagnose Potomac Horse Fever

If your horse has become depressed with a decreased appetite, fever, and has a history of being near fresh water within the past few weeks, PHF may be a possible cause. Other causes of diarrhea such as salmonella and clostridium should be ruled out if diarrhea is present. A fecal and blood real-time PCR test (PCR tests check for presence of genetic material from a specific organism or organisms) can check for responsible organisms; this test is rapid enough to help determine treatment protocols. Because the organism doesn’t stay long in the bloodstream, even a negative PCR test does not rule out PHF as a diagnosis; however, a positive PCR does confirm infection.

Cultures of the blood or feces can also be performed to help rule out other diseases and aid diagnosis, although they take longer while waiting for the bacteria to grow on special media. If possible, these tests should be performed before antibiotic therapy is started to help prevent falsely negative results.

Treatment of Potomac Horse Fever

Many cases of PHF are mild or subclinical, only showing signs of lethargy and decreased appetite. If a horse becomes ill quickly (acutely) or the horse becomes toxic, however, the condition can become life-threatening and require intensive treatment.

Potomac Horse Fever is typically treated with the antibiotic Oxytetracycline for 5-7 days. Most horses will start to show significant improvement within 12-24 hours of starting treatment. However, if colitis, diarrhea, dehydration, or laminitis occur, your horse may need IV fluids, anti-inflammatories, and hospitalization for close management. If PHF is diagnosed, many veterinarians will recommend ice boots to prevent the early onset of laminitis. Laminitis is difficult to treat in PHF cases and is sadly the leading cause of elected euthanasia.

Recovery and Management of Potomac Horse Fever

If your horse has a fever of unknown origin, they should be isolated from others until a diagnosis is made, as many causes of fever and diarrhea can be contagious. If a diagnosis of PHF is made promptly and treatment is instituted, fever, lethargy, and appetite usually start to show significant improvement quickly. Antibiotic therapy is usually given for five days; if signs recur after finishing the first round, a second may be implemented. Appetite and energy levels may take several days to return to normal.

Your horse should be kept in a quiet, calm environment during treatment, maintaining hydration and food intake. If your horse becomes sore and reluctant to walk, it is important to have your horse re-evaluated so the treatment regimen can be adjusted as this can be an indicator of laminitis.

Mares infected during gestation can also abort their fetus up to several months after infection, as the foal can be infected in utero.

Prevention of Potomac Horse Fever

Several measures can be taken to help prevent instance of PHF, including the following:

  • Do not allow your horse to drink or graze near fresh water such as creeks or rivers in the summer months

  • Clean water buckets and troughs frequently

  • Prevent bug attraction at night with light control in and around your barn

PHF vaccines are available, which may lessen the severity of clinical signs should a horse become infected. The PHF vaccination is usually given as a combination with the annual rabies vaccine. If you live in an endemic area, vaccination is typically recommended yearly in the spring to boost protection.

Potomac Horse Fever FAQs

Can a horse recover from PHF?

If the signs are recognized early and treatment is instituted quickly, most horses will recover from PHF.

Is PHF contagious?

This disease is non-contagious, non-reportable, and not zoonotic, meaning people can’t acquire it from horses. However, multiple horses in an area can become infected if drinking from the same water source or if multiple dead aquatic insects end up in one area.

Is PHF fatal?

If a horse becomes severely dehydrated, septic, goes into shock, or develops uncontrollable laminitis, PHF can be fatal. The mortality rate ranges from 5-30%, mostly inflated by laminitis that is not responsive to treatment.


Lenher, Eleanor. Potomac Horse Fever (PHF). AAEP.

Featured Image: iStock.com/Yuliya Apanasenka


Courtnee Morton, DVM


Courtnee Morton, DVM


Dr. Courtnee Morton is a 2017 Ross University School of Veterinary Medicine graduate. Since graduation, she completed an equine internship...

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