Potomac horse fever (PHF) is an acute enterocolitis syndrome
producing mild colic, fever, and diarrhea in horses of all ages,
as well as abortion in pregnant mares. The causative agent is
Neorickettsia risticii. The infection of enterocytes of the
small and large intestine results in acute colitis, which is one
of the principal clinical signs of PHF. The disease is seen in
spring, summer, and early fall and is associated with pastures
bordering creeks or rivers. Recently, the epidemiology of PHF
has been shown to involve a trematode vector. Sporadic disease
caused by N risticii has been reported in dogs and cats; cattle
appear to be resistant to infection. PHF has been reported in
many areas of the USA and Canada using an indirect fluorescent
antibody test as evidence of exposure; however, recent studies
indicate a high rate of false-positive titers with this test,
and the true geographic range of distribution is not known.
Isolation or detection of the causative agent from clinical
cases of PHF using convent