Risk Factors

The 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 that year. Lyme disease is now the most common vector-borne disease in America. (Vector-borne infections are those that are transmitted by insects.)

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. In the U.S., the highest reported incidence of Lyme disease occurs among children 5 to 9 years old and adults 50 to 59 years old.

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

  • 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. In 2002, Lyme disease was reported in all states except Hawaii, Montana, and Oklahoma. However, nearly all of Lyme disease cases are concentrated in three regions:

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

In such areas, between 1 - 3% of people who live there become infected at some time. New York has had the highest number of total cases to date, followed by Connecticut, which actually has the highest number of infected patients per total population. Pennsylvania and New Jersey have the next highest infection rates.

One interesting report found that the blood of the Western fence lizard contains a chemical that destroys B. burgdorferi, the Lyme disease spirochete. The presence of this lizard may help explain the low incidence of Lyme disease in western states. Experts differ on whether Lyme disease exists with any significance in the southern United States. A Lyme-like disease has been reported in the south (Mississippi, South Carolina, Georgia, Florida, and Texas), which apparently responds to antibiotics (as Lyme does). Researchers have also identified spirochetes on Lone Star ticks that appear to belong to the Borrelia family. Studies are underway to determine the exact nature and extent of this condition.

Worldwide Locations. Pockets of Lyme disease exist around the world. The disease is common in Europe, particularly in forested areas of middle Europe and Scandinavia. The Borrelia family is also responsible for tick infections in Europe, but different subspecies (B. garinii and B. afzelii) may be more common there and cause slightly different symptoms. The infection has also been reported in Russia, China, and Japan.

High-Risk Landscapes

One analysis suggested the risk for Lyme disease exposure is highest in wet, green areas, and lower in areas with lawns or low vegetation. Hiking and camping in the Northeastern woods carries a significant risk for tick bites and Lyme disease.

The disease is not limited to rural settings. It is becoming increasingly common in suburban areas. Beaches are also home to ticks.

Time of Year

The time of year of highest risk depends on the geographical region.

Northeast and North Central U.S. In the northeastern and north central U.S., most cases of Lyme disease are reported from May through August, with July being the peak month.

Exceptions may occur in years of warm winters and wet springs, which can bring the deer tick nymphs out about 2 weeks earlier. This same climate also creates the wet, green environment that increases the prevalence of ticks.

Noncoastal Western States. In the noncoastal western U.S., the disease most often occurs between January and May.

West Coast. The risk is higher on the west coast between November and April.