Introduction:

Lyme disease is the most commonly reported vector-borne disease in the United States. Vector-borne infections are transmitted by insects.

Borrelia Burgdorferi

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.

Ixodes Ticks

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:

• It is born from eggs as a larva.
• It develops into the nymph stage.
• It develops into the adult stage.

The Cycle of Infection

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 Borrelia (B.) burgdorferi spirochete
• The spirochete's host, the Ixodes scapularis tick
• The mammal for the tick to bite

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:

• The cycle of infection is related to the tick's life cycle, which requires 2 years to complete. The tick typically first picks up the spirochete during its larva stage, when it needs a blood meal to mature further.
• The tick's initial meal is typically blood from the white-footed mouse, which is commonly infected with Borrelia burgdorferi. After it dines on the infected blood, the tick then becomes a carrier of this spirochete.
• Borrelia burgdorferi lodges in the tick throughout one of both of its following life stages, nymph and adult. It is during these stages that the infection is passed on to other animals, including humans. Nymph ticks emerge around mid-June and can be about the size of poppy seeds. They are very difficult to spot and are estimated to be responsible for 90% of all Lyme disease cases. Adult ticks can be as large as a raisin after feeding, and easy to spot, but they usually prefer their dinner on the white-tailed deer.
• The infected nymph or adult tick crawls (it does not fly or jump) onto another animal, which can be mice or larger animals, such as deer, birds, or humans. If the tick bites these animals, it may then infect them with the B. burgdorferi spirochete. (Infected humans cannot pass the spirochete on to other humans by any means, including infected blood or urine or sexual contact.)
• A tick can feed for several days while being imbedded in the skin, after which it falls off. The tick's bite is painless, however, so only about half of people with Lyme disease recall being bitten.

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 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.