Saturday, May 31, 2008

A Twist of Lyme

My dear husband, Smintheus, came home earlier this week looking less than his usual robust self. He soon spiked a fever. Since he had been camping and morel hunting in a Lyme endemic area the two previous weekends, I took him to the clinic for some doxycyline. Although he didn't have a documented tick bite, several in his party did, and the ticks are so small at that stage, they would be easily missed. The doctor agreed that it would be prudent to treat the disease as Lyme, since the blood test is often inconclusive and early treatment can prevent serious long-term consequences.

"Here she goes again," you say. "Another off-topic post."

Stay with me.

Lyme (not Lyme's) disease was named for the town in Connecticut where it was first identified. It is caused by the bacterium Borrelia burgdorferi, transmitted to humans by the black-legged tick, also known as the deer tick. This tick (Ixodes scapularis) has a complex two year life cycle. From the common name of the tick, you would expect to find them associated with deer, and they are.

But they also feed on mice, and it is the mice that are the source of the infection.

The ticks only take a blood meal two or three times in their lives. The first, which is usually from a mouse, allows it to mature from a larva to a nymph. It is the nymphal stage that is most likely to bite humans, especially in the late spring and summer. The second blood meal (which can be from a variety of animals, including humans) allows the nymph to mature into an adult tick. The female takes a third blood meal from a deer so that she can lay eggs.

Deer are important for the life cycle of the ticks and for carrying them around and spreading them through the environment, but they aren't infected by the Lyme bacterium.

It's our friend the mouse, specifically the white footed deer mouse (Peromyscus leucopus), that is the major reservoir. Up to 90% of mice in some areas are infected with the Lyme bacterium.

Here is a fascinating discussion of the ecology of Lyme disease and how the prevalence and transmission are affected by deer, mice, weather, acorns, and human behavior. Here is a paper (and a more user-friendly press release) about mouse vaccination as a way to interfere with the transmission of Lyme disease.