Illinois Natural History Survey - University of Illinois

Human Monocytic Ehrlichiosis

Human monocytic ehrlichiosis, first recognized in 1987, is a disease characterized by fever, headache, malaise, myalgia, and nausea or vomiting. Most cases of this sometimes fatal disease have occurred in the Southeast and the Midwest usually in spring and early summer. The vector of the causative agent of the disease is thought to be the lone star tick, Amblyomma americanum.

The disease was first thought to be caused by infection with Ehrlichia canis, a pathogenic organism found in dogs, but in 1991 the causative agent was found to be an unrecognized species of Ehrlichia. This organism was named Ehrlichia chaffeenis late in l991. Five cases of human monocytic ehrlichiosis have been reported in Illinois. In 1994 a case was reported in Jackson County. To date in 1995, four cases have been reported, all in southern Illinois: one case in Alexander County, one case in Johnson County, and two cases in Perry County. The Johnson County case was fatal. The suspected vector, the lone star tick, is abundant in the southern third of Illinois, and it is of widespread but sporadic occurrence in the northern two-thirds of the state. Lone star tick populations are dependent upon white-tailed deer populations.

The lone star tick, Amblyomma americanum.
(Drawing courtesy U.S. Department of Agiculture.)

In spring 1992, microbiologist Jacqueline Dawson of the Centers for Disease Control and Prevention, Atlanta, Georgia; physician Jeffrey Nelson, an INHS affiliate from Rush Presbyterian St. Luke's Medical Center, Chicago; INHS mammalogist Edward Heske; and INHS entomologist John Bouseman initiated collaborative field studies of human ehrlichiosis in Illinois. Investigations were conducted in Monroe, Clark, Lee, and Winnebago counties. As a result of these studies the first seropositive (positive for Ehrlichiosis antibodies in blood serum) wildlife (raccoons) in Illinois were discovered in Lee County in 1992, and the first seropositive deer were discovered in Monroe County in 1992. Seropositive deer also were found in Winnebago County in 1993. The deer findings were reported in the Journal of Wildlife Diseases, vol. 30, no. 2 (April 1994), pp. 162-168.

Jacqueline Dawson and Jeffrey Nelson conducting field studies of human
ehrlichiosis in Illinois. (Photo by John Bouseman, INHS Center for Economic Entomology.)

It appears that white-tailed deer serve as a reservoir for ticks infected with human ehrlichiosis and that they play an important role in the natural history of the disease in this country. Deer carrying Ehrlichia antibodies seem to be more prevalent in southerly latitudes and at low elevations that have milder climates. Deer may serve as sensitive markers of Ehrlichia's distribution. Further research will be needed to clarify this relationship.

John K. Bouseman, Center for Economic Entomology, and Jeffrey A. Nelson, M.D., Rush Presbyterian St. Luke's Medical Center, Chicago, and an affiliate of the INHS Center for Economic Entomology.

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