Antibiotic Treatment of Tick Bites Prevents Only 20 Percent of Lyme Disease Cases

Yale School of Medicine
Thursday, 14 June 2001

Even if every patient who noticed they had a tick bite received prompt antibiotic treatment and if it were 100 percent effective in preventing Lyme disease, it would only prevent 20 percent of the total Lyme disease cases, Yale researcher Durland Fish said.

Fish, an author on the New England Journal of Medicine's (NEJM) newly released article on the antibiotic doxycycline's preventative effect on Lyme disease, said the study was based only on patients who had seen and removed a deer tick. "Most patients contracting Lyme disease never see the tick that caused infection," Fish said. "The nymphal stage of the deer tick is about the size of a poppy seed and even though it takes three to four days for it to feed before removing itself, most people will not notice it. This is because the tick also secretes an anesthetic while it is feeding, as well as infectious bacteria."

One published study showed that only about 20 percent of patients with Lyme disease recalled a tick bite. "This is an amazing, but true, fact of tick biology," said Fish, associate professor in the Department of Epidemiology and Public Health at Yale School of Medicine. "The key difference in this issue is between tick bites that are recognized by the patients and those which are not. Most are not."

Unrecognized tick bites in humans cannot be studied, but animal studies done by Fish, show that more than 80 percent of infected ticks will cause Lyme disease in mice after 72 hours of feeding, the maximum feeding time for nymphs. This rate, coupled with a 25 percent natural rate of infection of deer ticks in the Northeast, yields a 20 percent chance of infection from a nymphal deer tick, proving that Lyme disease is easy to catch.

"People who live in Lyme disease areas should take the risk of tick bites seriously and do all they can to prevent them, keeping in mind the 20 percent chance of infection if they miss a tick, instead of the 3 percent chance if they find one," Fish said. "Bites from deer ticks are extremely common in the Northeast."

One Centers for Disease Control study co-authored by Fish estimates that one in five Westchester Country residents are bitten by deer ticks each year, an annual average of 179,000 tick bites.

The NEJM study also reported that over 18 percent of the tick-bite patients received a second tick bite within the 90-day study period and over 6 percent of the patients had multiple ticks attached when they were initially observed. "With such a high frequency of contact between ticks and people, prevention methods other than treating tick-bites with antibiotics are badly needed," Fish said.

For more information, or to contact Yale School of Medicine, see their website at: info.med.yale.edu/ysm/

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