Babesiosis; Human granulocytic anaplasmosis (HGA)
Lyme disease is the most commonly reported vector-borne disease in the United States. Vector-borne infections are transmitted by insects.
The Lyme disease infection in the U.S. is caused by a spirochete called Borrelia (B.) burgdorferi. A spirochete is a bacteria-like organism with a cylinder-like shape surrounded by an outer membrane.
Lyme researchers have completed the DNA encoding of B. burgdorferi. Researchers learned that certain proteins coat its outer surface. These proteins, collectively called Osp, are responsible for attaching the spirochete to cells in humans and other mammals.
The vector that carries B. burgdorferi in the U.S. Northeast and North Central states is the Ixodes scapularis tick. The Ixodes scapularis tick goes through three stages over the course of about 2 years:
Cycle of Infection in the Northeast and North Central U.S. For Lyme disease to exist in these regions, three factors must come into close contact:
The following describes the most common cycle in the Northeast and North Central U.S. by which the Lyme disease infection eventually reaches a person:
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.
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 the same tick can carry Lyme disease, HGA, and babesiosis, 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 2 - 25% of ticks in several high-tick locations carry both HGA and Lyme. In one 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.
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