Vector Ecology

Lyme disease is spread by the bite of ticks of the genus Ixodes that are infected with Borrelia burgdorferi. For Lyme disease to exist in an area, at least three closely interrelated elements must be present in nature: (1) the Lyme disease bacteria B. burgdorferi (2) Ixodes ticks that can transmit the bacteria, and (3) mammals such as mice and deer to provide a blood meal for the ticks through their various life stages.

Picture of the type of forested area that could be a habitat for ticks.

Tick Habitat: In the United States, ticks of the genus Ixodes serve as the competent vectors for transmitting the Lyme disease bacteria, B. burgdorferi to humans. Ixodes ticks can be found in temperate regions with high relative humidity at ground level. Known as the deer tick or black-legged tick, Ixodes scapularis is responsible for transmitting bacteria to humans in the northeastern and north-central United States. In eastern states, ticks are associated with deciduous forest and habitat containing leaf litter. Leaf litter provides a moist cover from wind, snow, and other elements. Importantly, research demonstrates that tick populations are reduced 72-100% when leaf litter is removed. In the north-central states, I. scapularis is generally found in heavily wooded areas often surrounded by broad tracts of land cleared for agriculture. On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick (Ixodes pacificus) and habitats are more diverse. Here, ticks have been collected in habitats with forest, north coastal scrub, high brush, and open grasslands. Coastal tick populations thrive in areas of high rainfall, but ticks are also found at inland locations.

Life cycle of the deer tick: Knowing the complex life cycle of Ixodes ticks is important in understanding the risk of acquiring Lyme disease and in finding ways to reduce this risk. For this example, we will describe the two year life cycle of an I. scapularis deer tick located in a northeastern state. Life cycles may vary slightly for other ticks located in different regions of North America.

Image presented by authority from the American Lyme Disease Foundation.

The life cycle requires 2 years to complete. Adult female ticks lay eggs on the ground in early spring. By summer, eggs hatch into larvae. Larvae feed on mice, other small mammals, deer, and birds in the late summer and early fall, molt into nymphs, and then are dormant (inactive) until the next spring. Nymphs feed on rodents, small mammals, birds and humans in the late spring and summer and molt into adults in the fall. In the fall and early spring, adult ticks feed and mate on large mammals (especially deer) and bite humans. The adult female ticks then drop off these animals and lay eggs in spring, completing a 2-year life cycle.

Picture of some natural sources of blood meals for Lyme disease ticks (e.g., deer, dogs, birds, mice).

Natural reservoirs for Borrelia burgdorferi: Ticks, small rodents, and other non-human vertebrate animals all serve as natural reservoirs for B. burgdorferi. This means that the Lyme disease bacteria can live and grow within these hosts without causing them to die. Larvae and nymph ticks typically become infected with the Lyme disease spirochete, B. burgdorferi, when they feed on small animals that carry the bacteria in spring and summer. The bacteria remain in a tick as it changes from larva to nymph or from nymph to adult in late summer or early fall. Infected nymphs bite and transmit B. burgdorferi bacteria to other small rodents, mammals, and humans, all in the course of their normal feeding behavior.

From left to right: The deer tick (Ixodes scapularis) adult female, adult male, nymph, and larva on a centimeter scale. (View enlarged image.)

Transmission to Humans: Research in the eastern United States has indicated that, for the most part, nymphal ticks transmit Lyme disease bacteria to humans from May to July. Feeding nymphs are rarely noticed because of their small size, and thus have ample time to feed on humans. Tick-to-human transmission of the Lyme disease bacteria usually occurs after approximately 2 or more days of feeding. Although tick larvae are smaller than nymphs, they rarely, if ever, carry B. burgdorferi at the time of feeding and are not important in the transmission of Lyme disease to humans. Adult ticks can transmit the disease, but since they are larger and more likely to be removed from a person's body within a few hours, they are less likely than nymphs to have sufficient time to transmit the infection. As expected, few cases of Lyme disease are reported in the cooler months of the year, when adult Ixodes ticks are most active.

National Lyme disease risk map with four categories of risk. (View enlarged image.)

Ticks search for host animals from the tips of grasses, shrubs, and leaf litter and transfer to animals or persons that brush against vegetation. They only crawl, not fly or jump. Ticks can attach to any part of the human body but often crawl to the more hidden areas to feed such as the groin or armpit and often where clothing is tight. They feed on blood by inserting their mouths into the skin of a host animal. Their bodies slowly enlarge with blood as they feed over several days.

Picture of people enjoying the outdoors in a spot where risk of exposure to ticks may be greatest.

Campers, hikers, outdoor workers, and others who frequent wooded, brushy, and grassy places are commonly exposed to ticks, and this may be important in the transmission of Lyme disease in some areas. Because new homes are often built in wooded areas, transmission of Lyme disease near homes has become an important problem in some areas of the United States. The risk of exposure to ticks is greatest in the woods and garden fringe areas of properties, but ticks may also be carried into lawns and gardens by animals.

There is no evidence that a person can get Lyme disease from the air, food or water, from sexual contact, or by handling wild or domestic animals. There is no convincing evidence that Lyme disease can be transmitted by insects such as mosquitoes, flies, or fleas. Lyme disease infection from a blood transfusion or other contact with infected blood or urine has never been documented.

United States map of reported cases: Lyme disease cases have been reported by 49 states and the District of Columbia. Cases remain concentrated in the northeastern, north-central, and Pacific coastal regions. Another approach to understanding the distribution of Lyme disease is to examine where the tick vectors are present. Not surprisingly, areas with black-legged ticks report the highest incidence of Lyme cases, and studying the tick distribution can also highlight areas of potential risk.
The map at right is based on a comprehensive review of the literature in the United States regarding established and reported distribution of the Lyme disease vectors Ixodes scapularis and Ixodes pacificus. Data compiled from the literature review are presented by county. "Established" means at least six ticks or two life stages (larvae, nymphs, adults) were identified in the county. Note that the principal vectors are Ixodes scapularis in the northeast and upper-Midwest states and Ixodes pacificus along the West Coast of the United States. Although I. scapularis is widely distributed in the southern U.S., it is not an established vector of Lyme disease to humans in that area.
This map is useful in helping the public and health authorities assess the relative importance of Lyme disease in their area.

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This page last reviewed November 18, 2003

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