Potomac Horse Fever (PHF, Equine Monocytic Ehrlichiosis, Acute
Equine Diarrheal Syndrome, Equine Ehrlichial Colitis) is a
seasonal viral disease that is caused by Ehrlichia risticii.
Small animals may serve as reservoir hosts for this sporadic,
summertime disease. Biting insects that allow the virus to live
and reproduce are the suspected transmission vectors. PHF
parasitizes the monocyte white blood cell in the horse. After
nine to 12 days of incubation, the symptoms first appear as
depression and transient high fever. This episode may go
unnoticed for seven to 10 days, until a loss of appetite, colic,
"pipestream" diarrhea and laminitis occur.
In half of the cases, the characteristic stool is watery, profuse
and nonfetid, causing dehydration. Edema of the legs, abdomen and
head indicates poor circulatory and protein balance. The virus
crosses the placental barrier and abortions are seen in late
gestation. Antibiotics are used in treatment.
A vaccine is available for control, with an initial series of two
injections. Boosters should be given at least every six months in
* Editor's Note:
Potomac horse fever which is caused by Neorickettsia risticii
(formerly Ehrlicia risticii) is not a virus as was stated on
your web page, but is a gram-negative obligate intracellular
bacteria. There is much more pertinent info. on this disease
which was not noted on your site.
I came upon your page on a google search and thought I would
be remiss in not clarifying this error.
Ebony Gilbreath, DVM