top of page

Potomac Horse Fever Diagnosed in Pierce County

Horse drinking water

Last month a horse in Graham was confirmed to have Potomac Horse Fever and was successfully treated at Pilchuck Veterinary Hospital in Snohomish. While there have been confirmed cases in Southwest Washington, specifically the Chehalis/Centralia area, this is the first we have heard of a confirmed case being in Pierce county in many years.

Due to this recent development, we thought it might be helpful to talk about a disease that many people may not be very familiar with. Research is still being done to learn more about this somewhat confounding disease and to develop better vaccine protection.

What is Potomac Horse Fever and how does a horse get it?

Also called Equine Monocytic Ehrlichiosis, Potomac Horse Fever (PHF) is an infection with a somewhat complicated path to the horse. The causative organism of PHF, Neorickettsia risticii, is an intracellular bacteria, meaning that it can only live inside a cell, not freely in the environment. The bacteria is carried by a trematode, a small parasitic worm sometimes referred to as a fluke, that infect freshwater snails. The bacteria carrying flukes multiply in the infected snails and then are released back into the water. The flukes then infect aquatic insects, including caddisflies, mayflies, damselflies, dragonflies and stoneflies. It is believed that horses become infected by drinking from rivers or streams containing infected fluke larvae or by inadvertent ingestion of the bacteria-infected aquatic insects. This is a seasonal disease and is typically seen anywhere from late spring to the fall.

PHF Life Cycle

What happens when a horse is infected?

The incubation period, or the amount of time it takes for a horse to become ill following ingestion, is about 10 to 18 days. PHF can show many different clinical signs, but most commonly a horse will show depression and loss of appetite, followed by the development of a fever. Decreased abdominal sounds and mild colic can also occur. Within 24 to 48 hours, the hallmark symptom of PHF will appear: moderate to severe diarrhea. Occurring in about 60% of the cases, the diarrhea can range from loose “cow pie” to pipe stream liquid and frequently is accompanied by abdominal discomfort. Horses can become severely dehydrated and develop sepsis. In about 20 to 30% of the cases, horses can develop laminitis.

How do we find out if a horse has PHF?

Diagnosis of Potomac Horse Fever can be challenging, as it’s symptoms can be extremely similar to other infections, like Salmonella, that cause intestinal inflammation, also called enterocolitis. The only way to definitively diagnose the infection is by identifying the bacteria, Neorickettsia risticii, via a laboratory test called a real-time PCR from the suspected horse’s blood and feces.

What do we do now that a horse has been diagnosed?

Treatment is very effective if started early in the disease. Treatment typically includes intravenous (IV) antibiotics and, for horses showing signs of enterocolitis, IV fluids and NSAIDs (e.g. Bute). Response to treatment is usually seen within 12 hours and can include a decrease in fever and an increase in appetite and abdominal sounds. Unfortunately, if the horse develops laminitis it is usually quite severe and frequently unresponsive to treatment.

Can a horse die from PHF?

Unfortunately, yes. Anywhere from 5 to 30% of PHF cases eventually die, a portion of these due to laminitis.

How can we protect horses from this disease?

Potomavac Vaccine

PHF can be a challenge to prevent. The vaccinations available have a somewhat questionable protection rate, in part this is likely due to the fact that the infection occurs after oral ingestion, and not from a sting, bite, or wound through the skin, which is what makes intramuscular vaccines so effective against diseases such as West Nile Virus and Tetanus. There are also multiple strains of Neorickettsia risticii that cause the disease and the vaccine is only developed from one strain.

It is important to note that while the vaccine may not prevent a horse from becoming infected, it is generally thought that the severity of the clinical signs are reduced, which can lead to a better survival rate. Proper vaccination is important to minimize the lack of effectiveness as well. Horses need to receive an initial primary series 3 to 4 weeks apart, preferably in the late spring or early summer, and then annual boosters.

Due to the route of transmission, minimizing insect activity around your horse is one of the best ways to decrease potential exposure. This can include limiting use of bug-attracting lights at night. Limiting access to water sources such as streams and ponds, and to areas with known aquatic insect activity can also be helpful.

PHF is uncommon on the West Coast but has occurred. It’s important to remember that a horse can be at risk if and when it is near an area with water sources, even just a short trail ride could expose them to risk! Horses could also just as easily become infected after ingesting an infected aquatic insect that they pick up while grazing a pasture, eating their hay or grain, or drinking from their water trough.

Incidences of PHF cases can depend a lot of factors, particularly climate conditions. Mild fall weather can lead to a longer breeding and hatching season of aquatic insects. Fortunately PHF is not transmissible from horse to horse so herd risk is minimal once a horse has been diagnosed.


Madigan, John E., DVM, MS. "Potomac Horse Fever." Merck Veterinary Manual. N.d. Potomac Horse Fever: Intestinal Diseases in Horses and Foals: Merck Veterinary Manual. Merck Sharp & Dohme Corp, Sept. 2013. Web. 02 Dec. 2016. <>.

1,098 views0 comments

Recent Posts

See All
bottom of page