I. scapularis: Green areas are the range of this tick/CDC
I. pacificus: Green areas of the map are the habitat of the tick/CDC
Lyme disease is the most common tickborne disease in North America. It is caused by Borrelia burgdorferi, a spirochetal bacterium. Spirochetes are extracellular, spiral organisms found in the plasma of blood. The Centers for Disease Control and Prevention (CDC) estimates about 300,000 people are diagnosed each year, based on testing data, although many fewer cases are reported to the CDC. It is ranked fifth among all nationally notifiable conditions and second among infectious diseases. Cases are reported from almost all states, as evident by the dots on the map which represent the patient's county of residence. 96% of cases in the United States occur in just 14 states.
The tick vectors are the so called black-legged ticks: Ixodes scapularis on the East coast and Ixodes pacificus on the West coast. The maps to the right show the range of the two ticks (ticks live in areas where the map is green).
The life cycle of the ixodid tick vectors ranges over two years. Typically, the life cycle is between the white footed mouse (Peromyscus leucopus)), white-tailed deer, and the ixodid tick. Nymph stage ticks are most likely to infect humans. This is because they are active when people are outside in their habitat and they are very small and may go unnoticed. Between 15-25% of ticks may be infected in areas of New England and the Mid-Atlantic states. Cases in the northern states are seasonal -- April to September -- when the ticks are active and people are outside in the tick environment. Cases on the West coast may occur year round where the climate is constantly mild.