Potomac Horse Fever (PHF) is a bacterial infection that can result in severe colic, diarrhea, inflammation, depression and laminitis. In serious cases, it can be fatal to the horse.

PHF is caused by infection with Neorickettsia risticii and typically affects horses grazing in pastures that border rivers or creeks. This bacteria may be found in bodies of water or in aquatic insects.

PHF occurs seasonally with most cases reported between late spring and early fall. Cases are concentrated in regions of the United States and Canada including Ontario, Pennsylvania, Michigan, Illinois, Kentucky and California.

If you live in an area with high prevalence of Potomac Horse Fever, your veterinarian may recommend vaccination. However, vaccines may not fully protect against infection or symptoms. Limiting exposure to affected waterways is the best way to prevent infection.

What is Potomac Horse Fever?

Potomac Horse Fever (also known as Equine Neorickettsiosis, Equine Monocytic Ehrlichiosis, Shasta River Crud, and Equine Ehrlichial Colitis) is an acute illness that causes a fever, inflammation and gastrointestinal issues.

The predominant clinical sign of PHF is inflammation that affects the small and large intestine (enterocolitis) and leads to diarrhea.

Additional symptoms of the disease include colic and laminitis. [1][2] PHF can also cause the abortion of fetuses in pregnant mares that are infected. [3]

Symptoms of this syndrome were first documented by Dr. Frank W. Schofield, a veterinarian at the Ontario Veterinary College now located in Guelph, Ontario.

In 1924, he described a condition that was referred to by local horse owners as “horse cholera”, “dysentery” or “abdominal typhoid.” Dr. Schofield suspected this condition had a bacterial origin, but the cause remained unknown until many years later. [12]

PHF was first officially recognized in Maryland in 1979 as an illness that affected horses living near the Potomac River in the eastern United States. The disease has since been discovered in most American states, Canada, South America (Brazil and Uruguay), Europe (France and The Netherlands), and India.

In 1984, the bacterial organism that causes PHF was finally identified as Neorickettsia risticii. [14][15]

Neorickettsia risticii

This illness is caused by an infection of the intestinal tract with the bacterium Neorickettsia risticii (N. risticii). [1][2] N. risticii was previously named Ehrlichia risticii and is an obligate intracellular gram-negative bacteria.

PHF is a vector-borne disease transmitted by organisms that host a type of parasitic flatworm which N. risticii infects. N. risticii is capable of living in multiple hosts, primarily aquatic insects, that spread the disease.

PHF occurs when horses ingest parasitic flatworms or organisms carrying parasitic flatworms infected with N. risticii. They may become exposed to the bacteria through contaminated water or food sources.

The disease is not contagious and cannot be transmitted between horses via casual contact or from horses to humans. However, infections typically occur in clusters as horses drinking from the same water source or eating the same feed may all become infected.

Prevalence and Epidemiology

PHF affects horses of all ages, although foals have a lower risk of attracting the disease.

As a seasonal disease, PHF typically occurs in the spring, summer, and early fall. For example, 46 PHF cases were diagnosed between 2015 and 2019 in Ontario during the months of late June and early September. [4]

PHF most often occurs in temperate areas and affects animals living near creeks or rivers. If the disease has affected animals on a particular property or geographic region, the disease is likely to occur again in that same location in the future.

Aside from horses, N. risticii can also infect dogs and cats. Cattle are believed to be resistant to infection by the bacterium. [5]

PHF can be fatal if not treated promptly. The estimated mortality rate from the disease is between 17% and 36%. [6]

Symptoms of Potomac Fever

The initial clinical indications of PHF include mild depression and weight loss which usually occur in advance of a fever ranging from 38.9°C- 41.7°C (102°F- 107°F). These symptoms are typically followed by reduced intestinal sounds and mild colic. [6]

Within 24 to 48 hours of noticeable signs of PHF, most horses develop diarrhea and abdominal discomfort. Some horses become severely dehydrated due to fluid loss.

Laminitis is a common occurrence in horses with PHF and is often severe and difficult to treat if it develops due to the infection. The condition occurs in 20% to 30% of horses with PHF and often affects all four feet. [6]

In a study of 44 horses with PHF, the most common clinical signs included: [2]

  • Fever (66%)
  • Anorexia (50%)
  • Depression (45%)
  • Colic (39%)
  • Lameness (18%)

Pregnant mares infected with PHF are at risk of aborting their fetuses several months after becoming infected. Research demonstrates that mares infected between 100 to 160 days of gestation abort between 190 to 250 days of gestation. [5]

Horses that ingest the bacterium that causes PHF typically exhibit signs of the disease within one to three weeks of being infected.

Other infectious diseases such as Salmonella can cause enterocolitis and produce similar clinical signs to PHF.

Causes of PHF

N. risticii infects a type of parasitic flatworm (Acanthatrium and Lecithodendrium species) that has a complex life cycle comprised of several different stages. [7]

While in its various stages of life, this flatworm lives in a range of hosts. If the parasite is infected with the N. risticii bacterium, it will transmit the bacterium to the host organism. [7]

Flatworms that harbor N. risticii can infect snails and other aquatic organisms or be free-floating in water. N. risticii has been found in free-swimming flatworms, freshwater snails, birds, bats, and a range of insects including caddisflies, mayflies, damselflies, dragonflies, and stoneflies. [8] [3]

Snail secretions and insects that harbor an encysted larval form of flatworms infected with N. risticii are believed to pose the greatest risk for causing PHF in horses. [3][9] Upon ingestion, the bacterium infects the horse.

The bacterium can also be found in the feces of horses with PHF, but not in large amounts. Exposure to feces that contains N. risticii from other horses is not a significant risk factor for contracting PHF.

Skin penetration by the larval form of the parasitic flatworm that carries N. risticii has also been proposed as a possible transmission route for causing PHF in equids. [9]

Pathology

Once ingested, the encysted larval form of the flatworm containing N. risticii can survive the acidic environment of the equine stomach and reach the intestinal tract intact.

When in the intestines, the bacteria infiltrate the cells of the bowel wall and promote inflammation.

In horses with PHF that develop enterocolitis, N. risticii is consistently present in the wall of the large colon. [1]

The bacterium propagates in the cells that line the wall of the colon and initiates exfoliation of the structure’s protective cellular layer (mucous membrane).

How is PHF Diagnosed?

The clinical symptoms of PHF are similar to those of other diseases which can make diagnosis more difficult. Specific testing is required to determine if a horse has Potomac Horse Fever.

If your horse shows signs of this illness, the season and location are important factors to consider. When evaluating the likelihood of a horse having PHF, being located in a region with a high prevalence of this disease is a strong sign that testing is warranted.

Testing for PHF

PHF is definitively diagnosed by isolating the N. risticii bacterium from blood or feces samples taken from a horse. A Polymerase Chain Reaction (PCR) test can be used to detect the DNA of N. risticii in blood and feces samples. [10]

Both blood and feces samples should be collected from horses with suspected PHF and tested using the PCR test. The bacterium may be present in one or both types of samples.

N. risticii is likely to be detected earlier via blood sample rather than via feces sample. In experimental infection, positive test results confirming the presence of the bacterium can be detected in blood samples at approximately 7 to 21 days post-infection and in fecal samples at approximately 11 to 16 days post-infection. [2]

Cell culture tests can also be performed to isolate the bacterium. Unfortunately, this type of testing is not convenient to complete and therefore not offered by many diagnostic laboratories.

Assessing antibody levels via blood serum testing to determine previous exposure to the bacterium that causes PHF is considered unreliable since false-positive results are common. [10]

Bloodwork

An analysis of blood retrieved from horses with PHF may show a range of abnormalities including high blood nitrogen levels (indicating kidney disfunction), elevated packed cell volume (indicating dehydration), and low sodium, chloride, and potassium levels due to enterocolitis. [2][5]

It is also common for horses with PHF to have an elevated white blood cell count within a few days of the onset of the illness. [5]

Because the mucous membrane of the equine colon is responsible for reabsorbing sodium and chloride, this process is inhibited when the structure is damaged by infection with N. risticii. Testing the blood concentrations of electrolytes like sodium and chloride is useful for indirectly determining the severity of enterocolitis in horses with PHF. [2]

Ruling Out Other Causes

When considering a diagnosis of PHF in the presence of enterocolitis, other conditions must be ruled out.

Enterocolitis can occur in association with salmonellosis (caused by salmonella bacteria), clostridial colitis (caused by Clostridium difficile or Clostridium perfringens bacteria), small strongyles (cyathostomes) infestation, and antibiotic use.

Treatment for PHF

The most effective treatment for horses with PHF is oxytetracycline, an antibiotic that is delivered intravenously over a period of several days. Treatment with the antibiotic can reduce the risk of death if provided during the early onset phase of the disease. [2]

Even before a definitive diagnosis of PHF can be made, administration of oxytetracycline is recommended for horses living in endemic areas with clinical signs of the disease given that early drug treatment can increase survival rates. [2]

If antibiotic treatment with oxytetracycline is successful in horses with PHF, improvements in appetite, intestinal sounds, and attitude are typically evident within three days. The antibiotic treatment also works to resolve fever.

An additional course of IV oxytetracycline is recommended for horses that experience a relapse of PHF upon having their initial IV treatment discontinued. [11]

Aside from antibiotic treatment, horses with PHF induced enterocolitis typically require IV fluid replacement therapy. These horses also require the administration of fluids and nonsteroidal anti-inflammatory drugs (NSAIDs) to reduce pain and inflammation. [11]

To help prevent the development of laminitis in horses with PHF, icing the hooves is recommended. A veterinarian can provide instructions on how to check the horse’s feet for signs of laminitis including heat in the area and a strong pulse behind the fetlock.

Vaccination

A commercial stand-alone vaccine and a preparation in combination with the rabies vaccine are available to aid in the prevention of PHF. The stand-alone vaccine for PHF is more effective than the combination vaccine. [12]

In horses experimentally infected with PHF, vaccines were demonstrated to be 78% effective. [6] However, in the field, they have proven only marginally protective.

The rate of PHF vaccination failure in the field has been reported as high as 89%. [2] While the vaccine may not prevent infection, according to anecdotal reports it may help to reduce the clinical symptoms of the disease.

There are multiple strains of bacteria that can cause PHF and vaccines do not provide protection against each strain. Vaccines may fail to stimulate adequate antibody production in some cases. [2]

PHF vaccines also do not prevent pregnant mares infected with PHF from aborting their fetuses. Extra caution is recommended to prevent pregnant mares from being exposed to this bacteria.

Vaccines for PHF should be admin