Lyme Disease and Related Tick-Borne Infections - Introduction

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Cycle of Infection in the Northwest. In the Northwest, the infecting insect is the Western blacklegged tick, Ixodes Pacificus. Here, the frequency of Lyme disease is much lower than in the other two regions because the animal carrier of the infection is the dusky-footed wood rat. This animal is bitten and infected by the Ixodes neotomae tick, which does not bite humans. The actual tick that spreads B. burgdorferi to people is Ixodes pacificus, which must feed first on an already infected wood rat.

Other Infections Carried by the Ixodes Tick

The two other important infections carried by the Ixodes scapularis tick are human granulocytic anaplasmosis (HGA) and babesiosis. Although they are both borne by the same tick as Lyme disease, all three of these infections are entirely different diseases.

Risk for Coinfection. Because Lyme disease, HGA, and babesiosis can all be carried by the same tick, there is some risk for co-infection with two or more of these organisms. The risk, however, is not wholly known. Studies have reported that between 2 - 25% of ticks in several high-tick locations carry both HGA and Lyme. In a 2002 study of patients located in high-risk areas in New England, 39% had more than one of these infections transmitted by the Ixodes tick. There is no evidence that co-infection with one or more of these infections causes a more severe condition than either infection separately.

Review Date: 01/18/2007
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital