The black-legged tick, or deer tick, is known to transmit Lyme disease to humans and other animals. Being aware of the tick's ability to transmit the disease and taking precautions to prevent infection is a good strategy for preventing Lyme disease. Other species of ticks have not been shown to transmit the Lyme disease bacterium.
In the northeastern and northcentral United States, the black-legged tick (or deer tick, Ixodes scapularis) transmits Lyme disease to humans and other animals. This transmission occurs when an infected tick bites a human or other animal and passes on the Borrelia burgdorferi bacterium. This bacterium is responsible for Lyme disease and normally lives in mice, squirrels, and other small animals.
Deer ticks live for two years and have three feeding stages: larvae, nymph, and adult. When a young deer tick feeds on an infected animal, the tick takes the bacterium into its body along with the blood it consumes. The bacterium then lives in the tick's gut. If the insect feeds again, it can transmit the bacterium to its new host. Usually, the new host is another small rodent, but sometimes the new host is a human.
Most cases of human illness occur in the late spring and summer when the tiny deer tick nymphs are most active and human outdoor activity is greatest.
Although adult deer ticks often feed on deer, these animals do not become infected with Lyme disease. However, deer still play an important role in transporting ticks and maintaining tick populations.
In the Pacific coastal United States, Lyme disease is spread by the western black-legged tick (Ixodes pacificus). Other tick species found in the United States have not been shown to transmit Borrelia burgdorferi.