Lyme Disease and Related Tick-Borne Infections - Introduction

Introduction

Lyme disease is the most commonly reported vector-borne disease in the United States. Vector-borne infections are those that are transmitted by insects. Reports of U.S. Lyme disease cases peaked in 2002, when nearly 24,000 cases were reported. In general, about 20,000 cases of Lyme disease are reported in the U.S. each year.

Borrelia Burgdorferi

The Lyme disease infection in the US 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.

One of the most exciting advances in Lyme research was the completion of 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.

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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 two 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. In order 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 two 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 such animals then it may then infect them with the B. Burgdorferi spirochete. (It should be noted that 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.