Causative Agent: This disease is caused by the parasite Neorickettsia risticii, formerly known as Ehrlichia risticii.
Clinical Signs: At first, the horse may have a mild fever that is difficult to detect. Around day 10-14, after the initial infection begins, the fever will increase, the horse may go off feed, and show signs of colic. In many cases there is mild to profuse, watery diarrhea. Occasionally, the parasite can cause abortions in mares. Laminitis is a common problem associated with this infection.
Disease Transmission: Recent research has shown that Potomac horse fever is spread by aquatic insects such as immature and adult forms of caddisflies, damselflies, dragonflies, mayflies, and stoneflies. A horse can develop this disease when it ingests one of these insects that is infected with N. risticii. Studies show that this happens most often when the horse is grazing on pasture or drinking contaminated water. It can also happen when aquatic insects that are attracted to lights at night die in large numbers and then fall into the horses feed, water or bedding.
Diagnosis: A blood sample from a suspect animal can be sent in for Indirect fluorescent antibody (IFA) or for an Enzyme-linked immunosorbent assay (ELISA). It is also beneficial to test blood samples to determine if there are titers to E. risticii.
Treatment: Symptoms of the disease should be treated on an individual basis, focusing on hydration. Many times intravenous fluids (IV fluids) are required to maintain proper hydration. Treatment with antibiotics like oxytetracycline (300 mg per 100 lbs., diluted out with saline) or erythromycin/rifampin seem to be successful in most cases. Often significant improvement is noted within 3-5 days of treatment. Laminitis treatment and prevention is also recommended. See page E465 for details on laminitis.
Prevention: In areas where the disease is prevalent, vaccination is recommended. Studies indicate that the vaccine does not stimulate a high level of protection in the horse. On the other hand, the immunity stimulated when a horse gets infected naturally can last for several years.
See page A905 and C538 for specifics on vaccination schedules and common vaccines.