Q: There have been reported cases of Potomac Horse Fever (PHF) in my area (Isanti, MN). A local horse owner told me that mayflies are the vectors and that dead mayflies can still carry the disease. Is this true? Any information about PHF would be helpful.
A: Potomac horse fever (PHF), caused by Neorickettsia risticii, has been frequently diagnosed in case clusters in horses near waterways during the summer. The disease has been association with several vectors (hosts) including flukes living in water snails and aquatic insects including caddisflies, mayflies, damselflies, dragonflies, and stoneflies. The disease also appears to be carried in bats, birds and amphibians.
Clinical signs observed in horses with PHF include fever, anorexia (not eating), colic, depression, ileus (non-motile gastrointestinal tract), diarrhea, and laminitis (founder). Clinical signs and severity vary, but common to all cases is the manifestation of colitis (inflammation of the bowel). Clinical signs of the disease have been experimentally produced in horses 10-14 days after ingesting caddisflies.
In 2005, an outbreak of PHF was confirmed in Winona, MN and was linked to numerous dead mayflies that were positive for PHF. Horses are thought to accidentally ingest infected flukes from snails or infected insects while drinking and grazing, or from foraging or living in areas with high amounts of dead insect hosts. PHF cannot be spread horse to horse.
There is a killed vaccine for PHF, which may reduce the severity of illness but will not fully prevent infection. A veterinarian can perform tests on feces or blood to confirm PHF and most cases will respond to specific antibiotics and fluid therapy, however, some horses have died or have been euthanizes due to severe laminitis. The best treatment is prevention: reduce the contact your horse has with insect vectors by turning off lights in the evening that attract insects, turning off lights over outside water tanks, and removing dead insects from the barn. In MN, PHF is most often seen in isolated cases, but case clusters of PHF can occur given the wide distribution of rivers, lakes and ponds. Horse owners should ask their veterinarian about the frequency of PHF in their area to determine if the PHF vaccine is warranted. If so, the vaccine should be given in the late spring, and in high risk areas, boostered in late summer.
By Julia Wilson, DVM, University of Minnesota