Ehrlichiosis

Ehrlichiosis is an infectious disease transmitted by the bite of a tick, first described in 1990; the two specific organisms that have been implicated are Ehrlichia chaffeensis and granulocytic Ehrlichia.

Causes and Development; Contributing Risk Factors

The Ehrlichia bacteria belong to the family Rickettsiae.  Rickettsial organisms are responsible for a number of serious diseases with worldwide distribution, such as Rocky Mountain spotted fever, typhus, murine typhus, scrub typhus, Queensland tick typhus, Boutonneuse fever, North Asian tick-borne rickettsiosis, rickettsial pox, 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 specific tick: Ehrlichia inhabit the Lone Star tick (Amblyomma americanum), the American dog tick (Dermacentor variabilis), and the deer ticks (Ixodes dammini and Ixodes scapularis).  Note that the deer tick is also the carrier of Lyme disease.  The main geographic areas where ehrlichiosis is found in the United States are the southern central states and southern Atlantic states.  It has recently been reported in the upper midwestern region.

Risk factors for ehrlichiosis include living in an area with a lot of ticks, passing through high grasses, and owning a pet that may bring ticks home.

Signs and Symptoms

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 sometimes sick enough to seek medical attention.  Surveys show that the majority of people exposed do not seek treatment and likely never knew they were infected.

Symptoms include:

• fever
• chills
• headache
• nausea
• muscle aches (myalgia)
• malaise
• rash, maculopapular
• rash, petechial

Diagnosis and Tests

A CBC shows decreased white blood cells (leukopenia), decreased platelets (thrombocytopenia); a granulocyte stain shows clumps of bacteria inside white blood cells; the platelet count shows decreased platelets (thrombocytopenia); liver enzymes show elevated transaminase; a fluorescent antibody test may turn positive for E. chaffeensis or granulocytic Ehrlichia.

Treatment and Prevention

Preventing tick bites will prevent Ehrlichiosis and other tick-borne diseases.  Common measures include the use of insect repellent, appropriate clothing, sticking to trails and avoiding dense brush when hiking, and not standing under overhanging foliage.  After hiking or outings in the woods it is a good idea to examine yourself for ticks and remove any that may be found.  Studies suggest that a tick must be attached for at least 24 hours in order to cause disease, so early removal can prevent infection.  The date and time of a tick bite should be recorded because the incubation period of some tick-borne diseases is long enough (the incubation period for Lyme disease is up to 1 month) that the event could be forgotten.

Prognosis

Some patients may recover on their own, but Ehrlichiosis can be fatal.