Potomac Horse Fever Surfaces in Virginia

Sept. 9, 2013 -- Several cases of Potomac horse fever (PHF) have been reported in central Virginia. Although the disease is not contagious, it can be fatal.

Sept. 9, 2013 — This summer, several cases of Potomac horse fever (PHF) were reported in central Virginia.1 Although the disease is not contagious, it can be fatal, and the Virginia Department of Agriculture and Consumer Services made horse owners aware of the threat. 

Megan Green, DVM, manager, Merial’s Large Animal Veterinary Services, advises horse owners, especially those in the geographic area, to consult with their veterinarians. “He or she will have the latest information about the cases in the surrounding community and help assess the situation. But since there have been cases reported, it’s very likely he or she will recommend vaccinating. It’s the only way to help protect horses against Potomac horse fever.” 

PHF is most commonly found near creeks and rivers2 and likely caused when horses ingest infected aquatic insects such as damselflies, caddisflies and mayflies.3 Although named Potomac horse fever because the initial 1979 outbreak occurred near the Potomac River in Maryland,4 the disease has since been identified in 43 states, three Canadian provinces, parts of South America, the Netherlands and France.5

In determining a horse’s possible exposure to PHF, owners should consider the horse’s immediate surroundings and the local projections for this year’s aquatic insect population. In the case of central Virginia, Dr. Michael Erskine, Interim Director, Virginia-Maryland Regional College of Veterinary Medicine, says the recent heavy rainfall may have contributed to the increase in the number of disease carriers.1

PHF can have serious complications, so the decision about whether or not to vaccinate is an important one. The fatality rate in untreated cases can be up to 30%.6 Another devastating effect of PHF is the possible development of laminitis, which occurs in up to 40% of affected horses.2

Green cautions horse owners to be vigilant as the disease is difficult to diagnose and has clinical signs that are subtle and mimic other diseases.2 “Early detection is key to potential recovery. Signs at the disease’s onset include fever ranging from 102- 07°F, 2 decreased intestinal sounds and diarrhea.2 As the disease progresses, some horses suffer from toxemia and dehydration.”2

Horse owners in the area threatened by PHF can rely on Merial’s POTOMAVACTM vaccine, which has been proven to help protect horses against the most serious side effects of PHF, including death.7 It is also proven safe for horses as young as three months of age.8

Besides assessing risk in the immediate area, horse owners should consider the conditions in any areas they might be traveling to during the peak PHF season, which is July, August and September.4Outbreak Alert, a free program created by Merial, helps horse owners protect the health of their horses, not just in their own backyard, but around the country.

As reports of influenza, WNV, PHF, rabies, EEE/WEE and EHV-1 are identified and confirmed, they are posted on the website map. Some diseases, such as WNV, EEE and rabies, are considered “reportable” diseases and veterinarians are required to notify the proper authorities as to their occurrence. In the case of PHF, however, reporting is not required. If horse owners have been made aware of additional PHF cases and can provide details such as county, ZIP code, date of report and veterinarian contact information, they can report it for inclusion on the map by emailing

Besides the map featuring all the confirmed disease locations, the Outbreak Alert program offers horse owners and veterinarians the ability to sign up for a free notification service when an equine disease has been confirmed in their area or an area they may be traveling to. Sign up for the service by visiting

Megan Green, DVM, is a member of the Merial Veterinary Professional Services Team. Green is certified in equine acupuncture and has a strong interest in equine internal medicine. She has expertise in ambulatory medicine working with broodmares, foals, performance horses and racehorses. Green has practiced in Alabama and Kentucky, and she continues to practice in Georgia and Tennessee when time allows. Green earned her Doctor of Veterinary Medicine degree from Tuskegee University School of Veterinary Medicine.


1. Leesburg Today. “State issues Potomac Horse Fever Alert.” Available at: Accessed July 15, 2013.

2. Kahn, D.M. Merck Veterinary Manual. 10th ed. 2010:558-559.

3. Wilson, J.H.; Pusterla, N.; et al. Incrimination of mayflies as a vector of Potomac horse fever in an outbreak in Minnesota. AAEP Proceedings 2006;52:324-328.

4. American Association of Equine Practitioners. Potomac horse fever. Available at: Accessed May 3, 2012.

5. Madigan, J.E.; and Pusterla, N. Life cycle of Potomac horse fever—implications for diagnosis, treatment and control: a review. AAEP Proceedings 2005;51:158-162.

6. Palmer, J.E. Potomac horse fever. University of Pennsylvania. Lecture notes and slides. 2008. Available at: Accessed May 22, 2012.

7. Merial trial ER 8-88-2.

8. POTOMAVAC product label.






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