Abstract
A newly recognized disease of horses, called Potomac Horse Fever
(PHF), emerged during the late 1970's in a region close to the Potomac
River near Washington, D.C., United States of America. The studies
which were carried out by a research group to define the epidemiology
and establish the cause of the disease are described. Further studies
examined the possible route of transmission of the causative organism,
and the role of farm animals and wild rodents in the epidemiology of
the disease.
Results of a case-control study conducted in 1983 characterised the
disease as a non-contagious, infectious seasonal disease of horses of all
age, sex and horse-use categories. The study found a lack of association
with most of the variables studied, and assisted in discounting some of
the numerous aetiologies proposed at the time. The results showed
positive associations with a few variables, which included the previous
presence of the disease in a barn, the presence of other livestock and
the presence of habitats favourable for arthropod breeding and development.
The disease was established experimentally in ponies following blood
transfusion from natural cases of PHF, an Ehrlichia was isolated from
the white blood cells of the experimentally infected ponies and this
organism subsequently reproduced the disease on inoculation into
susceptible ponies. The experimental disease was consistent with that
seen in field cases. Pathological studies on the disease were carried
out, and the causative Ehrlichia was identified on the wall of the
large intestine of affected animals.
Experiments established that the intradermal route was effective
in transmitting the disease to ponies. A serological survey of farm animals
and wild rodents from affected farms showed no indication of previous
exposure to the causative Ehrlichia in 98% of samples tested. None of
the mammalian species studied appears to serve as a reservoir of the
infection.
