Rickettsial Diseases - EHRLICHIOSIS

(Tick-borne fever)

AGENT:

An intraleukocytic rickettsia, E. canis (many species of Ehrlichia exist. Previously only E. sennetsu was known to infect man). Occurs intracytoplasmically, singly or in compact clusters (morulae) in circulating leukocytes.

RESERVOIR AND INCIDENCE

First recognized in dogs in 1935. Epizootic occurred in military working dogs in Vietnam 1968-1970. Now known to have worldwide distribution. 11 to 58% of dogs in U.S. are serologically positive. First reported case of E. canis in man in 1987. Several cases since then.

TRANSMISSION:

Tick vector, Rhipicephalus sanguineus, Brown Dog Tick. It is presumably transmitted to humans by tick bite.

DISEASE IN DOGS:

Incubation period 10 to 14 days. Fever, lymphadenopathy, edema of legs and scrotum, epistaxis. Acute disease followed by a subclinical carrier stage.

DISEASE IN MAN:

Similar to Rocky Mountain Spotted Fever, but no rash. 12 to 14 day incubation period and prodrome consisting of malaise, back pain and nausea, the patient develops sudden fever, bradycardia, and headache. Leukopenia and absolute lymphopenia as well as thrombocytopenia occur frequently.

DIAGNOSIS:

Not easy to identify in peripheral blood smears but can attempt to identify organisms in leukocytes. An IFA assay that may be used to diagnose infection is available thru CDC and requires acute and convalescent sera.

TREATMENT:

Tetracycline

PREVENTION/CONTROL:

Control ticks.