From the Maryland Department of Agriculture – The Maryland Department of Agriculture received confirmation of a case of Potomac Horse Fever in a pony stabled in Frederick County from the University of Kentucky’s Equine Diagnostic Laboratory on August 2. The pony fell ill on July 26 and did not respond to treatment. The gelding died on July 30. A necropsy of the pony was performed at the Frederick Animal Health Laboratory on July 31.

The Maryland Department of Agriculture is urging horse owners — especially those with horses that graze near rivers, streams and creeks — to watch their horses closely for signs of this disease. Clinical signs include mild to severe fever, diarrhea, dehydration, loss of appetite, laminitis and mild colic. Potomac Horse Fever is most commonly contracted by horses that ingest infected aquatic insects such as caddisflies, mayflies and dragonflies.

“Potomac Horse Fever surfaces in Maryland every few years,” said Maryland Department of Agriculture’s State Veterinarian Dr. Michael Radebaugh. “With this summer’s heavy rains, pastures and meadows where equines graze are more likely to flood, increasing the chances that a horse could ingest these infected aquatic insects.”

Equine owners are encouraged to keep horses off of flooded pastures, and to turn stable and barn lights off at night since the aquatic insects that carry this disease are attracted to bright light.

Potomac Horse Fever has a mortality rate of 5 to 30 percent in equines. Since there are 14 different strains of Neorickettsia risticii, the organism that causes Potomac Horse Fever, the vaccine is not always effective, but may lessen the severity of the disease. Horse owners are advised to follow the recommendations of their private practitioner concerning vaccination protocols.

The incubation period for the disease is one to three weeks. The department encourages horse owners to contact their veterinarian as soon as possible if their equines are showing symptoms of this infection, even if the horse has been vaccinated. Most horses infected with Potomac Horse Fever will respond to treatment with antibiotics.

Potomac Horse Fever cannot be transmitted from horse to horse, and humans are not at risk; however, veterinarians who diagnose this disease must report it to the Maryland State Veterinarian Dr. Radebaugh at 410-841-5787 or

For more information about Potomac Horse Fever, please visit:

The Equiery will continue to follow this story and post updates as they occur.