Lizards and Lyme

I took a long walk the other day with my friend the Google Doctor, and we watched a Sceloporus occidentalis guard his territory on a sunny rock.

Commonly known as the "Bluebelly" lizard, or the Western Fence Swift, the sighting led to a discussion of a remarkable protein in the blood of the lizard, and an interesting ecological relationship between the lizard, the western black legged tick Ixodes pacificus, and the spirochete Borrelia burgdorferi, the cause of Lyme disease.

The lizard is immune to the disease. Although the tick feeds on the lizards, the protein in the lizard blood kills the spirochete.

But the effect does not stop there. The protein in the lizard blood kills the spirochetes in the tick that feeds on the lizard blood. So that tick can no longer infect other animals (deer, mice, or humans) with the disease.

In the ecology of the disease, this makes a huge difference. By cleansing the ticks of the disease, the lizards cause a dilution in the "vector space" of disease transmission, and protect a larger population of animals from infection.

The protein in the lizard blood is part of the alternative complement pathway, one of three mechanisms in the innate imune system, the fast-acting part of the immune system that can recognize thousands of pathogen molecules without being trained first by prior exposure.

An unrelated lizard, Elgaria multicarinata, the Southern Alligator lizard, also has the ability to recognize and destroy this spirochete, using the alternative complement pathway. Whether this is parallel evolution, or an indication that the disease has been around long enough to have infected their common ancestor 65 million years ago is not known.