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Where Ticks Come From

Medical Authors: Melissa Conrad Stöppler, MD
Medical Editor: William C. Shiel Jr., MD, FACP, FACR

Ticks carry the bacteria that cause Lyme disease. People contract Lyme disease by being bitten by these ticks. Prevention of tick bites is therefore the best way to prevent developing Lyme disease or other tick-borne illnesses such as Rocky Mountain spotted fever.

Studies have investigated whether specific human behaviors increase the risk of tick exposure. These have shown that sitting on a log carried the greatest risk of picking up a tick. If you sit on a log for only five minutes, you have a 30% chance of getting a tick on you. The three actions found to be riskiest for acquiring the western black-legged tick were leaning against a tree, carrying wood, and sitting on a log. When sitting on a log, it didn't matter whether the log was bare or covered with moss in terms of tick exposure.

The western black-legged tick, found primarily in the far western United States as well as in British Columbia, is the primary carrier of the corkscrew-shaped spirochete Borrelia burgdorferi, a bacterium named after its discoverer, Dr. Willy Burgdorfer. B. burgdorferi is responsible for Lyme disease, which can lead to debilitating symptoms in humans. Most cases of Lyme disease in northwestern California appear to be transmitted by young nymphal ticks, which are notoriously difficult to detect because they are as small as poppy seeds.

Why is the strongest risk when sitting on a log? The clue may be in an important animal host for the larvae and nymphs of the western black-legged tick. The western fence lizard is an important host for the ticks, and the lizards often use logs in sunlit areas as basking sites. Ticks that are seeking hosts to feed upon may be going to the place where there is a good chance of finding a lizard. DNA tests revealed that 3% to 4% of the ticks the researchers found on their bodies tested positive for B. burgdorferi and another, less prevalent disease-causing bacterium Anaplasma phagocytophilum. Tick infection rates normally are significantly higher in the northeastern and upper midwestern United States, where most Lyme disease occurs. Even if you're in a wooded area, you can take precautions to prevent tickborne diseases.

  • Avoid prolonged contact with wood as well as with leaf-litter areas.
  • Inspect yourself carefully after spending time in tick-infested areas.
  • Continue checking your skin for two to three days after any potential exposure. (Nymphal ticks are so hard to see in the beginning; probably less than one in three people bitten by nymphs ever discovers the tick that bit them. But they become easier to detect once they start swelling up after they've had a blood meal.)
  • Insect repellents containing 20%-30% DEET can be used on the skin, and products containing 0.5% permethrin can be used on clothing.
  • Bathing or showering after outdoor activity can help you more easily locate ticks.
  • Examine clothes, pets, and gear as well as your skin after outdoor activity in a wooded area. Tumble clothes in a dryer on high heat for an hour to kill remaining ticks.
  • Be most careful in warmer months, when ticks are most active.

Animal studies suggest that it usually takes longer than one day after the tick becomes attached for the bacteria to be transmitted to the host. So, the sooner the tick is found and removed, the smaller the chance of acquiring an infection from the tick.

REFERENCES:

UC Berkeley News Release, Apr. 8, 2004.

United States. Centers for Disease Control and Prevention. "Lyme Disease." June 23, 2014. <http://www.cdc.gov/lyme/>.


Last Editorial Review: 6/27/2014




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