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 contracting the disease because they are nursing.

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]

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Symptoms of Potomac Fever

The initial clinical indications of PHF include mild depression and anorexia (loss of appetite) 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. In the worst infections, horses may die before diarrhea even develops.

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 the free cercaria in water or metacercaria in insects are ingested, 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, as well as mast cells and macrophages, 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. It causes damage to the microvilli at the tips of the cells, impairing the ability to absorb electrolytes and water which is a key function of the colon. [16]

It is thought free cercaria, the water stage, may also enter the horse through the skin. [17]

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 and 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 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 in eliciting a significant change in antibody titer than the combination vaccine but only one third of the horses had a significant rise and that was gone by three months post vaccination. [12]

In horses experimentally infected with PHF, vaccines were demonstrated to be 78% effective when challenged with the same strain as in the vaccine. [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 only target one strain. Vaccines may also fail to stimulate adequate antibody production to the vaccine strain in some cases. [2][12]

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 administered prior to the most active period for the disease, typically during the summer and fall seasons.

In unvaccinated horses, a two-dose series of the vaccine is recommended. Peak immunity against the disease is reached three to four weeks after receiving the second dose.

Previously vaccinated horses should receive an annual booster for PHF. In endemic areas, boosters are recommended every three to four months to provide optimal protection.

Pregnant mares should receive the booster vaccine for PHF four to six weeks before they are due to foal.

Prevention Strategies for PHF

While vaccination may provide some protection against PHF, there are other steps you can take to minimize the risk of your horse contracting this disease.

It is especially critical to take additional precautions if you live in an area known to have high rates of Potomac Horse Fever. Measures should be implemented to block access to affected water sources and to reduce exposure to organisms that may harbour the N. risticii bacteria.

Important preventive measures against PHF include the following:

1) Observe horses for health and behavioral changes

Horse owners should watch for potential signs of PHF that include depression, anorexia, diarrhea, colic, fever, and laminitis.

Signs may be noticeable 2 to 18 days after infection and can worsen if left untreated. Early detection is crucial to limiting the negative effects.

2) Turn off lights in the stable at night

Bright lights left on during the night invite insects to swarm the area.

When these insects die, they may fall into water buckets and hay feeders where they are consumed by horses. Keeping lights off near your horses will avoid attracting flying insects that may carry the bacterium that causes PHF.

3) Install window screens in the stable

If stable lighting is left on after dark, use screens at the entrances and on windows to prevent insects from entering the barn.

4) Monitor your horses’ hay

Check hay for insects and ensure any that are found are removed. Hay grown near natural water sources is more likely to contain insects than hay grown away from water.

5) Protect feed and hay from insects

Keep feed stored in a secure container and hay covered with a tarp to prevent insects from getting into it.

6) Keep horses away from natural water sources

This will help to reduce their risk of ingesting aquatic insects and other organisms that may carry N. risticii bacteria.

7) Consider fencing off water sources

In grazing areas, prevent your horses from accessing the water during the seasons when PHF is most prevalent. Only provide access to fresh water that is changed regularly.

8) Limit the spread of aquatic insects.

Encourage aquatic organisms to stay near a water source by allowing tall grasses and other plants to grow around it.

9) Provide access to clean water sources

Water should be changed frequently, and water buckets should be cleaned often.

Always place outdoor water troughs away from overhead lights that are on during nighttime to help reduce the number of bugs that fall into them.

Remove dead insects from water troughs at least twice daily.

10) Stay informed

Local agricultural extension agents may be able to provide information regarding impending insect hatches in specific areas.

Knowing when the risk for PHF is highest allows horse owners to take extra precautions in advance of insect hatches.

If you suspect your horse has Potomac Horse Fever, consult with your veterinarian to determine an appropriate treatment protocol.

Dietary changes may be recommended during the recovery period. Submit your horse’s diet online and our nutritionists can help you design a gut-friendly feeding plan to help minimize gastrointestinal upset following PHF infection.

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References

  1. Rikihisa, Y. et al. Ultrastructural study of ehrlichial organisms in the large colons of ponies infected with Potomac horse fever. Infect Immun. 2021. View Summary
  2. Bertin, F.R. et al. Clinical and Clinicopathological Factors Associated with Survival in 44 Horses with Equine Neorickettsiosis (Potomac Horse Fever). J Vet Intern Med. 2013.View Summary
  3. Dawson, J.E. et al. Isolation of Ehrlichia risticii the causative agent of Potomac horse fever, from the fetus of an experimentally infected mare. Sessions Veterinary Record. 1987. View Summary
  4. Arroyo, L.G. et al. Potomac horse fever in Ontario: Clinical, geographic, and diagnostic aspects. Can Vet J. 2021. View Summary
  5. Madigan, J.E. Potomac Horse Fever.. Merck Veterinary Journal. 2013.
  6. Xiong, Q. et al. An Ecotype of Neorickettsia risticii Causing Potomac Horse Fever in Canada. Appl Environ Microbiol. 2016. View Summary
  7. Pusterla, N. et al. Digenetic trematodes, Acanthatrium sp. and Lecithodendrium sp., as vectors of Neorickettsia risticii, the agent of Potomac horse fever. J Helminthol. 2003. View Summary
  8. Chae, J. et al. Infection of Aquatic Insects with Trematode Metacercariae Carrying Ehrlichia risticii, the Cause of Potomac Horse Fever. JMed Entomol. 2000. View Summary
  9. Madigan, J.E. et al. Transmission of Ehrlichia risticii, the agent of Potomac horse fever, using naturally infected aquatic insects and helminth vectors: preliminary report. Equine Vet J. 2000.
  10. Teymournejad, O. et al. Isolation and Molecular Analysis of a Novel Neorickettsia Species That Causes Potomac Horse Fever. mBio. 2020. View Summary
  11. Lenher, E. Potomac Horse Fever (PHF). American Association of Equine Practitioners. 2020.
  12. McKenzie, H.C. et al. Immunogenicity of Potomac horse fever vaccine when simultaneously co-administered with rabies vaccine in a multivalent vaccine or as two monovalent vaccines at separate sites. Equine Vet J. 2019. View Summary
  13. Baird, J.D. et al. Historical aspects of Potomac horse fever in Ontario (1924–2010). Can Vet J. 2013. View Summary
  14. Holland, C.J. et al. Isolation, experimental transmission, and characterization of causative agent of Potomac horse fever. Science. 1985. View Summary
  15. Mott, J. et al. Molecular analysis of Neorickettsia risticii in adult aquatic insects in Pennsylvania, in horses infected by ingestion of insects, and isolated in cell culture. J Clin Microbiol. 2002. View Summary
  16. Rikihisa, Y. et al. Loss of absorptive capacity for sodium and chloride in the colon causes diarrhoea in Potomac horse fever. Research in Veterinary Science. 1992.View Summary
  17. Pusterla, N. and Madigan, J.E.Neorickettsia risticii. IN: Equine Infectious Diseases. Saunders. 2007.