Potomac Horse Fever: A Review

By Nathan M. Slovis DVM, Dipl. ACVIM, CHT, director, McGee Medical Center, and reprinted with permission from the Kentucky Equine Education Project newsletter

Potomac horse fever (PHF) is a bacterial disease that can affect horses of any age. PHF made headlines in the 1980s, when an outbreak of diarrhea in the Potomac River area of Maryland drew attention to the disease. The causative agent, a type of bacteria named Ehrlichia risticii (recently named Neoriketssia ristici) has been linked to parasites (juvenile "flukes") of fresh water snails. These juvenile parasites are called cercariae.

During hot weather the bacteria infested cercariae are released into the water and consumed by aquatic fly larvae such as mayflies, caddis flies, dragonflies and 14 other aquatic insects (commonly noted in Kentucky from July through early September). These aquatic insects will now be infected with PHF. The cercariae will mature into metacercariae after being consumed by the aquatic insects. These metacercariae are RESISTANT to the acid pH of the horse's stomach and therefore will not be destroyed. Horses grazing or eating feedstuffs can inadvertently consume these infected aquatic insects. Horses kept near fresh-water streams or ponds are more likely to be at risk for getting the disease, because of the close proximity of the aquatic insects (horses cannot get the disease from drinking infested cercariae water or eating the snails because the cercariae are easily digested in the stomach and are NOT RESISTANT to the acid pH of the stomach). A horse does not have to be near water to contract PHF.

Several horses diagnosed with PHF at our hospital were stabled and did not have access to a stream or pond. Aquatic insects carrying the bacteria can travel and easily congregate around a stall light that has been left on. These insects would eventually die and fall into the horse's water bucket, feed bucket and/or hay where they can be consumed.

The bacteria does its damage when it gets into the horse's white blood cells and migrates towards the bowel, where it enters the colonocytes and enterocytes lining the mucosa of the bowel.

Symptoms of PHF include depression, anorexia, colic, diarrhea, fevers and signs suggestive of laminitis (inflammation of the feet). All of the horses diagnosed with PHF at our hospital had a history of fevers and when routine blood work was performed, an extremely low white blood cell count can be noted. If the symptoms are caught early, the horse can be treated with intravenous tetracycline. If caught early, most horses will respond to treatment in 24 hours and have a dramatic recovery.

Diagnosis of PHF consists of a thorough physical examination, complete white blood cell count and PCR blood testing (polymerase chain reaction: DNA testing looking for the bacteria in the white blood cells). This type of test looks at the DNA of the bacteria for identification, and can be completed in 24 hours. Testing the horse's serum for an antibody response can be unrewarding as some sick horses will have low titers.

In Kentucky, we tend to notice an increased case load during hot dry summers. One theory is that the hot weather and drought-like conditions produce dried up regions where low-level water was once present. This will leave areas of greener grass where horses can now have access for grazing. In these areas, there may be a large number of dead mayflies and caddis flies. The second theory is that some of the aquatic insects are traveling from the areas that have dried up in search of an aquatic environment. During their travels, the horse inadvertently ingests the insects when they die out in the pasture.

There is no guarantee that vaccination will prevent the disease, therefore here are some quick easy ways to help reduce the risk of your horses being infected with PHF:

  1. Keep stable lights off in the barn at night.
  2. Do not place your pasture water troughs or feed troughs near a light source. Insects will congregate around the light and then may inadvertently fall into the troughs.
  3. Restrict your horses grazing access from low-lying water sources (creeks, etc.) during July through September.

About the Author

Nathan M. Slovis, DVM, Dipl. ACVIM, CHT

Nathan M. Slovis DVM, Dipl. ACVIM, CHT, received his DVM from Purdue University. He is board certified in large animal internal medicine and he is currently the Director of the McGee Medical Center at Hagyard Equine Medical Institute in Lexington, Ky. His special interests are in neonatology, infectious diseases, and hyperbaric medicine (in which he is certfied as a hyperbaric technologist).

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