Lyme Disease and Related Tick-Borne Infections - Risk Factors

Risk Factors

The U.S. Centers for Diseases Control (CDC) reports that more than 145,000 Lyme disease cases have been diagnosed since 1982. Reports of Lyme disease cases in the U.S. peaked in 2002, when nearly 24,000 cases were reported. Lyme disease is now the most common vector-borne disease in America. (Vector-borne infections are those that are transmitted by insects.) In general, about 20,000 cases of Lyme disease are reported in the U.S. each year.

General Risk Factors

Anyone exposed to ticks is at risk for Lyme disease and other tick-borne diseases. Pets are also at risk. Naturally, anyone who is regularly outside in areas where tick rates are high has a greater than average risk for becoming infected.

Age. The highest reported incidence of Lyme disease occurs among children 5 - 9 years old and adults 50 - 59 years old.

Gender. The reported incidence is higher for females than for males. The gender discrepancy may be attributable to increases in:

• Exposure to infected ticks
• Less use of personal protective measures
• Men may tend to report symptoms less often than women.

The Risk for Lyme Disease After a Tick Bite

In general, the actual risk for developing Lyme disease after a tick bite is only between 1 - 3%. The risk varies depending on different factors:

• The longer the tick has fed, the greater the risk. In fact, in one study, no individuals developed Lyme disease after being bitten by a nymph tick for less than 72 hours. In those on whom the tick had been feeding for longer than that, the risk was 25%.
• Nymph ticks carry a greater risk than adult ticks, probably because they are often too small to be detected (about the size of a pinhead). In addition, only nymph ticks that are at least partially swollen when removed pose any significant risk. (This suggests that they have feeding for a prolonged period.)

Geographic Locations

Locations in the U.S. Lyme disease has been reported in nearly all U.S. states. However, nearly all of Lyme disease cases are concentrated in three regions:

• In northeastern states from Maine to Maryland
• Two north central states (Minnesota and Wisconsin)
• In the northwest (California and Oregon)