Ehrlichiosis

Definition:

An infectious disease transmitted by the bite of a tick.

Causes, incidence, and risk factors:

Ehrlichiosis is a recently-described disease caused by Ehrlichia organisms. It was first described in 1990. The two specific organisms which have been implicated are Ehrlichia chaffeensis and granulocytic Ehrlichia. The Ehrlichia bacteria belong to the family Rickettsiaceae. Rickettsial organisms are responsible for a number of serious diseases with world-wide distribution such as Rocky Mountain Spotted Fever, Typhus, Murine Typhus, Scrub typhus, Queensland tick typhus, Boutonneuse fever, North Asian tick-borne rickettsiosis, Rickettsialpox, and others. All of these diseases require an insect vector (tick, flea, mite) to transmit them to humans or other animals.

The infectious organism of Ehrlichiosis is transmitted to humans by the bite of ticks. Rickettsiae are often associated with a very specific ticks. Ehrlichia appears to inhabit the Lone Star tick (Amblyomma americanum), the American dog tick (Dermacentor variabilis) and the deer ticks (Ixodes damni and Ixodes scapularis). (NOTE: the deer tick is also the carrier of Lyme disease.)

The incubation period for Ehrlichiosis is approximately 9 days from the time of tick bite. Ehrlichiosis begins with fever, chills, headache, muscle pain (myalgia), and nausea. A maculopapular or petechial rash appears in over half the cases and the disease may be mistaken for Rocky Mountain spotted fever. The symptoms are often quite general but the infected individual is usually sick enough to seek medical attention. Ehrlichiosis can be fatal. Recovery takes three weeks. The organism is sensitive to tetracycline and treatment results in improvement in 24 to 48 hours.