Biodiversity Reduces Lyme Disease, Study Says

This is the first study showing that biodiversity may reduce the risk of disease in people.

"Our work addresses underlying ecological interactions that may cause large-scale variation in disease risk," says Richard Ostfeld of the Institute of Ecosystem Studies in Millbrook, New York, who collaborated with Felicia Keesing of Siena College in Loudonville, New York.

Caused by a spirochete bacterium, Lyme disease is transmitted by ixodid ticks. The disease can cause a characteristic skin rash, flu-like symptoms, arthritis and neurological damage. As many as 17,000 cases are reported to the CDC each year, making it the most common vector-borne disease in the U.S. Lyme disease is concentrated in the northeast, mid-Atlantic and upper midwest.

In part because the disease can be difficult to diagnose and a recently-approved vaccine is not completely effective, the best safeguard against Lyme disease is to avoid ticks. The ticks are born without the Lyme disease bacteria and can become infected during any of three stages (larva, nymph and adult), each of which takes a single blood meal from a host. The nymphs are the most dangerous to people both because they are most active during the summer when people are most likely to be outdoors and because they are hard to detect -- nymphs look like small freckles and are almost impossible to feel when they are crawling around on people's skin. Up to 40 percent of nymphs in a given area are infected with Lyme disease.

Many ticks never become infected because some hosts transmit the bacteria inefficiently. In North America, the host that transmits the bacteria most efficiently is the white-footed mouse.

Ostfeld and Keesing hypothesized that an area with more biodiversity would have a greater number of inefficient host species, which would mean fewer infected tick nymphs and thus fewer cases of Lyme disease in people. In a companion paper in press in the journal Ecology, Ostfeld and a colleague used a computer model that varied the diversity of hosts and confirmed that as host diversity rose from one to 20 species, the nymph infection rate fell by about 75 percent.

These results are supported by the finding that the incidence of Lyme disease falls in East Coast areas with a greater diversity of small mammals.

The question then is how to decrease the relative abundance of white-footed mice, which are typically the most abundant vertebrate in forests in the eastern and central U.S. The researchers point out that white-footed mouse densities are often up to 50 percent in fragmented woodlots surrounded by agriculture or urban areas, presumably due to the absence of predators such as barred owls and bobcats and of competitors such as eastern chipmunks and fox squirrels. Ostfeld and Keesing suggest that decreasing the forests' fragmentation might also decrease the local incidence of Lyme disease.

Increased biodiversity could also reduce the risk of the many other vector-borne diseases, which include plague, West Nile encephalitis and leishmaniasis.