Potomac Horse Fever

July, 2015: We would like horse clients to be aware that there have been a number of Potomac Horse Fever (PHF) cases recently diagnosed in our practice area. Horse owners and caretakers should be on the lookout for horses that are initially depressed and go off feed, they may then develop an elevated temperature, and diarrhea. In severe cases, horses may develop laminitis (founder) and, unfortunately, die.

Keeping water troughs clean of water-born insects aids in the prevention of Potomac Horse Fever

Keeping water troughs clean of water-born insects aids in the prevention of Potomac Horse Fever.

Potomac Horse Fever is caused by a bacterium, Neorickettsia risticii, found in freshwater snails, caddisflies, mayflies, stoneflies, damselflies, and dragonflies. The disease is most commonly seen in mid to late summer. Potomac Horse Fever is not contagious (not spread directly from horse to horse); horses acquire the illness by accidental ingestion of infectious insects. Named for the Potomac River Basin where the disease was first recognized, Potomac Horse Fever is thought to be most prevalent near bodies of water. It has, however, been diagnosed in arid areas such as northern Wyoming.Reducing the risk of PHF infection includes both vaccination and premise management. Several strains of Neorickettsia risticii have been identified, not all of which are included in the current PHF vaccine. Although not all of the strains of bacteria that cause PHF are included in the vaccine, it is believed that it aids in reducing the severity of the signs if a horse does contract the disease. The vaccine should be administered in late spring to afford the best protection. In the event of an outbreak, it may be advisable to booster the spring PHF vaccination.

In addition to vaccination, limiting the horse’s exposure to vectors will reduce the risk of disease. Although night lights offer some comfort and security to horse owners, the light attracts the very insects that may carry the disease. It’s best to turn off the night lights as well as keep lights off of feedstuffs so that the insects do not die and fall into the hay or grain. Keep grain and hay (if possible) covered to keep the insects out of it. Water troughs and buckets can be cleaned with a strainer to remove any water-born vectors.

Early diagnosis and treatment greatly increases the chance for a positive outcome. Treatment consists of tetracycline antibiotics, and, depending on the severity of the signs, intravenous fluids and electrolytes to counteract the diarrhea as well as non-steroidal anti-inflammatories for laminitis.

Do not wait to contact your veterinarian if you suspect Potomac Horse Fever. The signs may be subtle at first, but the horse can quickly deteriorate. Call immediately if you see the following signs:

  • Depression
  • Anorexia
  • Diarrhea
  • Colic
  • Fever
  • Laminitis