The Maryland Department of Agriculture (MDA) reported that test results confirmed a case of Potomac horse fever in a Maryland pony.
The MDA said samples sent to the University of Kentucky’s Equine Diagnostic Laboratory on Aug. 2 returned positive for Potomac horse fever (PHF). The pony from Frederick County fell ill on July 26 and did not respond to treatment. The gelding died on July 30, and staff at the Frederick Animal Health Laboratory performed a necropsy on July 31.
Neorickettsia risticii, an organism found in some wormlike parasites that infect aquatic snails and insects (such as caddisflies and mayflies) causes PHF. Horses are exposed by inadvertently ingesting aquatic insects infected with flukes carrying the bacteria and by drinking flukes directly from rivers or streams
“Potomac Horse Fever surfaces in Maryland every few years,” said MDA State Veterinarian Michael Radebaugh, VMD. “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.”
What to Watch For
The MDA encouraged owners—especially those with horses grazing near rivers, streams, and creeks—to watch horses closely for clinical signs, including:
- Mild to severe fever;
- Loss of appetite;
- Laminitis; and
Potomac horse fever’s incubation period is one to three weeks and its mortality rate is 5 to 30%; many horses with PHF respond to treatment. Vaccines against PHF are not always effective, but could lessen disease severity. For that reason, horse owners should consult their veterinarians concerning vaccination protocols.
Equine owners are encouraged to keep horses off of flooded pastures to reduce exposure potential. While horses most commonly contract PHF when they ingest infected insects from water bodies, even those residing far from water could be at risk. Barn and stall lights can attract vectors, which could end up in horses’ feed or water sources. As such, owners should keep barn and stall lights off at night.
The department encouraged owners to contact their veterinarian as soon as possible if horses exhibit clinical signs, even if the horse is vaccinated.