What is Potomac Horse Fever?
This illness starts with a microscopic organism found in tiny parasites (flukes) that infect snails. These parasites have offspring that are released into water sources where caddis flies or mayflies ingest them. Within a few days, the flies die and their bodies contaminate your horse’s bedding, food, and water. Once your horse eats one of the dead flies, they multiply and fill the gastrointestinal tract, which creates the digestive symptoms such as nausea, vomiting, fever, and diarrhea. Getting treatment early is the best chance for the survival of your horse so if you suspect PHF, see a veterinarian immediately.
Potomac horse fever is a serious digestive system illness that is caused by Neorickettsia risticii (Ehrlichia risticii). This condition can bring on a sudden bout of severe diarrhea and vomiting that can progress to dehydration. This disease is usually prevalent in the summer or early fall. Because of the possibility of high fever, dehydration, and laminitis, this is a dangerous and sometimes fatal disease. In areas where the disease has been found, you should get your horse the vaccine annually to prevent PHF from being contracted.
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Symptoms of Potomac Horse Fever in Horses
PHF is comparable to Rocky Mountain fever or Lyme disease in that it spreads through a similar organism. Even though they share a few symptoms such as fever, depression, and diarrhea, many of the signs are different. The most common include:
- Diarrhea (this is the first sign of PHF in more than 60% of horses)
- Colic (abdominal pain)
- Loss of appetite
- Nausea and vomiting
- Increase in body temperature (up to 107 degrees Fahrenheit)
- Dehydration (dry skin, increased thirst, lack of urination, malaise)
- Bloated stomach
- Rolling on ground
- Pawing at the ground
- Laminitis (20% to 30% of horses with PHF get laminitis)
- Death (20% to 25% of horses with PHF die)
Causes of Potomac Horse Fever in Horses
Hot and wet conditions are prime for PHF, but there are other risk factors such as:
- Keeping your horses in areas with standing water (streams, ponds, lakes)
- Irrigated pastures
- Food and water dispensers kept in areas where flies inhabit
- Lighted barns (attracts flies)
Diagnosis of Potomac Horse Fever in Horses
A complete physical examination and medical history are needed for diagnosis. This includes a thorough whole horse examination to find and record your horse’s weight, height, reflexes, capillary reflex test (CRT), body condition score, behavior, balance, pulse, breath sounds, respirations, conformation, and body temperature. The veterinarian will also request that you walk and trot your horse to see how well your horse’s muscles are working in motion.
A careful assessment of body posture, attitude, and lameness will all be taken into consideration and recorded. The veterinarian may also test your horse’s feet with a hoof tester to determine if there is any degree of lamination since this is a major complication of PHF that can become serious enough to warrant euthanasia if not treated. A stool and urine sample will be acquired at this time to do microscopic tests for bacterial and fungal infections.
Other laboratory testing to confirm diagnosis are a complete blood count (CBC) for measuring antibody titers and chemistry analysis to check for decreased white blood cells and protein. A PCR test can find DNA from the organism in your horse’s white blood cells and an IFA (indirect immunofluorescence test) can confirm with samples that are paired. Radiographs (x-rays) are necessary to check for signs of laminitis. In fact, a bone scan of the entire body with CT scans or MRI may be warranted in some cases.
Treatment of Potomac Horse Fever in Horses
Your horse’s treatment for PHF may vary depending on the severity and strain of the disease. Some of the strains are worse than others and the severity increases every day that treatment is not started. Intravenous antibiotics, fluid and oxygen therapy, anti-inflammatories, and electrolytes are all commonly used in treating PHF in horses.
The most essential part of treating PHF is the antibiotics, which are usually given intravenously (IV). The veterinarian will likely keep your horse in the hospital because it is easier to treat in a controlled and sterile environment. The antibiotics that work well for Neorickettsia risticii are tetracyclines such as oxytetracycline. For very mild cases of PHF, the veterinarian may decide that oral doxycycline taken at home can be prescribed.
Fluid and Oxygen Therapy
Fluids and electrolytes will be given via the IV to prevent dehydration and help to decrease body temperature. Anti-inflammatory drugs such as corticosteroids or NSAIDS are used to decrease inflammation and fever.
Many veterinarians prescribe flunixin meglumine (Banamine) to treat the abdominal pain, endotoxemia, intestinal inflammation, and laminitis associated with PHF. In addition, flunixin meglumine is a good choice for analgesic and antipyretic (to reduce fever).
Recovery of Potomac Horse Fever in Horses
If your horse gets treatment within 24-36 hours of symptoms, the prognosis is good, providing laminitis is not involved. However, with laminitis or if your horse does not respond well to treatment, chances of a full recovery are decreased. Be sure to finish all medication and call the veterinarian if you have any questions.