Transmission of Lyme Disease

Lyme disease is spread by the bite of ticks of the genus Ixodes that are infected with Borrelia burgdorferi. Ixodes, commonly known as the deer tick (or bear tick), normally feeds on the white-footed mouse, the white-tailed deer, and certain other mammals. It is responsible for transmitting the spirochetes to humans in the northeastern and north-central United States. On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick, and in the southeastern states by the related black-legged tick.

Distribution of Ixodes ticks that transmit Lyme disease in the U.S. CDC.

Ixodes ticks are much smaller than common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Adult ticks are slightly larger. The tick nymphs, which are most likely to feed on a person and are rarely noticed because of their small size (less than 2 mm), are usually involved in the transmission of the disease.

Ixodes ticks: larva, nymph, adult male, adult female. CDC.

Spirochete prevalence in adult Ixodes ticks is highly variable depending on geographic location. It was shown to be present in approximately 35% of ticks in the Baraboo Hills northwest of Madison, Wisconsin, while in regions of California, 2% prevalence has been reported, and in regions of New York, 50% has been reported.

For Lyme disease to exist in an area, at least three closely interrelated elements must be present in nature: the Lyme disease bacteria, Borrelia burgdorferi, ticks that can transmit them, and mammals (such as mice and deer) to provide food for the ticks in their various life stages.

The tick life cycle consists of three distinctive stages: larvae, nymphs, and adults. A blood meal is required for ticks to molt from the larvae stage to the nymph stage and from the nymph stage to the adult stage. The tick larvae and nymphs typically become infected with Borrelia burgdorferi when they feed on infected small animals, particularly the white-footed mouse. The bacteria remain in the tick as it changes from larva to nymph or from nymph to adult. Infected nymphs and adult ticks then bite and transmit the bacteria to other small rodents, other animals, and humans, all in the course of their normal feeding behavior. Adult ticks preferentially feed on the white-tailed deer, which thereby becomes an important reservoir in regions of infestation. The tick life cycle takes two years to complete (see diagram below).