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. Based on testing data, the Centers for Disease Control and Prevention (CDC) estimates that approximately 476,000 people get Lyme disease each year. This number includes patients who are most likely treated based on clinical symptoms but do not actually have Lyme disease. Over 63,000 cases are reported to the CDC by state health departments annually. 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 below, which represent the patient's county of residence. 96% of cases in the United States occur in just 14 states.
Figure 5. Reported cases of Lyme disease from 2019–2022. Cases are reported by county of residence, not by site of occurrence.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.
Figure 6. Ixodes scapularis lifecycle
The life cycle of the ixodid tick vector, as shown above, 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, 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, from 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.
