Early summer brings Lyme disease concern

May 2007

As peak tick season approaches, experts warn that Lyme disease is not as common as the abundance of myths surrounding it.

Physicians should expect case numbers similar to last year as no definitive studies have linked climate changes to altered life cycles of the ticks associated with Lyme disease. About 23,000 cases of Lyme disease were reported in the United States in 2005.

Early summer is the peak time for Lyme disease infection because ticks are in the nymphal stage and are so small, about the size of a head of a pin, that they are not noticed by the host.

“The fact is it’s the tick you don’t see that is most likely to give you Lyme disease,” Eugene Shapiro, MD, professor of pediatrics, epidemiology and investigative medicine at Yale School of Medicine, said in an interview with Infectious Diseases in Children.

“There’s this myth about how hard it is to treat Lyme disease, but usually it can be easily treated.”

More than 95% of people who are treated for Lyme disease are cured with a single conventional course of treatment, according to Shapiro.

Transmission

The nymphal tick is most prevalent in early summer, when most people are inclined to visit tick-infested areas.

The tick has to feed for at least 48 to 72 hours before the risk of transmission becomes substantial. About 25% to 30% of the nymphal ticks are infected in the most high-risk areas.

The adult stage tick is active at the end of summer and into fall. A higher proportion of adult ticks are infected with Lyme, and these ticks are typically easier to notice because of their larger size than those active in the beginning of the summer.

The bacteria that causes Lyme diseases lives in the gut of the tick. The tick has to become engorged for the bacteria to activate and migrate to the salivary glands (from which it is passed to the host), which takes time.

Tick bites are painless because the insects produce substances that break down mediators of inflammation.

“It’s not like a mosquito bite. It takes hours for a tick to fully implant and begin to feed,” Shapiro said.

Diagnosis

Most people infected with Lyme disease have a characteristic red rash, erythema migrans, at the tick bite site. Contrary to popular belief, a bull’s-eye rash does not always develop more often the rash is uniformly red. Less than one-third of erythema migrans rashes look like a bull’s-eye.

The erythema migrans rash enlarges and can persist for weeks without treatment. About 90% of Lyme disease cases present with erythema migrans.

“It is usually quite characteristic and easy to diagnose,” Shapiro said.

Prevention and treatment

In children, ticks often implant on the head and neck. Ticks thrive in warm areas, so the armpit and groin areas also should be checked after time spent in tick-infested environments. Tick repellents, including DEET (applied to the skin) and permethrin (which is applied only to clothing) are effective when used as directed. Parents should be advised not to use DEET on the face or hands of young children.

Fine tip tweezers should be used to remove ticks. One should get as close to the head of the tick as possible and pull up. If a small piece of the tick is left, it can remain. It may increase risk for bacterial infection, but not for Lyme disease.

“Just leave it, don’t amputate the limb. People cause more damage just trying to take out a little piece of the head,” Shapiro said.

Removed ticks do not have to be brought to a physician for identification. In studies of suspected ticks brought to doctors, about a third were not ticks but were instead pieces of dirt, crab lice or other unassociated objects or parasites.

“The point is you need training to correctly identify these things,” Shapiro said.

For most people older than 8 years old, one dose of 200 mg doxycycline is effective for preventing Lyme disease when given within 72 hours of a tick bite, according to Shapiro. However, because of the low risk of Lyme disease after a bite, administering prophylaxis is not recommended routinely – only in high-risk areas for persons known to have been bitten by an engorged, nymphal stage tick. It is not recommended for children younger than 8 years of age.

“Patients should be reassured that Lyme disease is a very treatable illness and recognize that there is a lot of misinformation about Lyme disease,” Shapiro said.

Prevalence

The ticks that carry Lyme disease are found only in certain parts of the United States. They are most prevalent in Southern New England, the Eastern Mid-Atlantic states, the upper Midwest and in a small part of Northern California.

For more information:

• The Infectious Disease Society of America, Guidelines for Lyme Disease Assessment and Treatment.
• www.journals.uchicago.edu/CID/journal/issues/v43n9/40897/40897.html