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 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.

Ixodes Ticks

The vector that carries B. burgdorferi in the U.S. Northeast and North Central states is the blacklegged tick (also called deer tick) Ixodes scapularis. In the U.S. Northwest states, it is the blacklegged tick Ixodes pacificus. The tick goes through three stages over the course of about 2 years:

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

The Cycle of Infection

Lyme disease depends on three factors coming into close contact:

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

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 (or in the Northwest, dusky-footed wood rat), 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.

Other Infections Carried by the Ixodes Tick

The two other important infections carried by the Ixodes scapularis tick (deer 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. Deer ticks can also transmit deer tick virus. In very rare cases, deer tick virus may cause serious brain infection (encephalitis).

Review Date: 01/13/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

A.D.A.M., Inc. is accredited by URAC, also known as the American Accreditation HealthCare Commission (www.urac.org).