The Lyme Disease Debate

Host Biodiversity and Human Disease Risk

Sharon Levy

Disclosures

Environ Health Perspect. 2013;121(4):a120-a125. 

In This Article

Lyme Hosts

Blame for the emergence of both black-legged ticks and Lyme disease has typically focused on deer, which have abundantly repopulated the northeastern and midwestern United States over the last few decades. Yet deer turn out to be immune to infection with Bb;[16] even though they're an important host for ticks, especially in the adult life phase, they don't transmit Lyme disease.

Early research tested the assumption that reducing deer populations would lower the risk of human infection by reducing numbers of infected nymphal ticks searching for a host. The results were mixed. Some studies showed a strong relationship between deer abundance and tick density.[17,18,19] Others, however, reported that tick density was tightly linked with numbers of white-footed mice[20] or small mammalian predators,[21] not deer. Experiments in the Italian Alps reported an increased density of questing nymphs in habitat patches where deer had been fenced out.[22]

In assessing such findings, it is essential to take into account the time scale, says Randolph. "We all know that tick abundance will increase at first in the absence of hosts; they accumulate on the vegetation with no hosts to attach to," she explains. "But later the abundance declines fast as the ticks die and are not replaced through natural reproduction—no hosts to feed adult ticks, no eggs."

A number of studies in Europe and the United States have shown that while some species are competent reservoir hosts for Bb (that is, they're likely to pass Bb along to the ticks that bite them), others are not.[23,24,25,26,27,28,29] In 1990 Durland Fish, an epidemiologist at Yale School of Public Health, coauthored a study in which wild raccoons, striped skunks, opossums, and white-footed mice were held in cages over water pans that collected all the engorged larval ticks that dropped off. In the laboratory, the larval ticks were incubated, and the researchers tracked the numbers that developed successfully into nymphs. They then tallied the percentage of nymphs that carried Bb. Forty percent of the nymphal ticks that had fed on white-footed mice as larvae were infected. The figures for ticks that had fed on raccoons and skunks were much lower. (In the jargon of zoonoses, such animals may be "dilution hosts," meaning they tend to make infection less prevalent in the tick population.) None of the nymphs from larvae that had fed on opossums survived long enough to be tested.[29]

Figure.

In 2011, the latest year for which statistics are available, 96% of Lyme disease cases were reported in 13 states: Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Verrmont, Virginia,and Wisconsin.

The relationship between forest fragmentation, biodiversity, and human risk of Lyme disease is still under debate—at least two research groups have shown that the density of Bb-infected ticks increased as habitat size shrank, yet one study showed that human infection rates went down at the same time. The role of urban-adapted "dilution hosts" such as raccoons and opossums remains unclear.

In later work Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies in Millbrook, New York, and his colleagues conducted a similar experiment with larval ticks collected from white-footed mice, chipmunks, deer, and four species of songbirds. They found white-footed mice to be much more competent reservoir hosts than the other species tested.[30] In separate work they also reported shrews to be highly competent reservoir hosts for Bb.[31]

In a study led by Felicia Keesing, a biologist at Bard College, the team captured six species of forest animals, held them in the laboratory long enough for any ticks they'd picked up in the wild to detach, and then reinfested them with tick larvae. They found that while almost half the larval ticks placed on white-footed mice fed to repletion, only 3.5% of those on opossums fed successfully.[32] Veeries, catbirds, chipmunks, and squirrels fell between these two extremes as tick hosts. The authors noted that confinement of the animals to the laboratory could have affected their grooming behavior, potentially biasing the results. They also acknowledged they could not control for prior exposure to ticks, which can result in an immune response that affects the survival of later feeders.

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