While some research has linked the spread of Lyme disease to climate change, the details of that connection are complex. A new study suggests that a warming world may help tamp down the disease at the southern edge of the Northeastern coastal region where it is most prevalent.
The research, published last week in the journal PLOS ONE, suggests that a warmer climate in the Southeastern United States has led to the evolution of deer ticks that are less likely to latch onto people, at least in some parts of the ticks' range. Other research has shown that climate change appears to be expanding the ticks' overall range, and that global warming may help spread many dangerous mosquito-borne diseases such as malaria and Zika into new areas.
Scientists have known for years that Lyme is less common in the South, despite widespread prevalence of the parasite that causes it. A couple of years ago, some of the same researchers involved in the new work confirmed that southern ticks tend to bed down below the leaf litter, while their northern counterparts prefer climbing up stalks of grass or brush, primed to hitch a ride on human legs.
The findings were revealing but baffling, said Howard Ginsberg, a research ecologist with the United States Geological Survey and the lead author of the new study. "No one understood why."
To answer the question, Ginsberg and colleagues took larvae from two populations of ticks—one from Rhode Island and one a cross-breed of northern and southern groups—and exposed them to different climatic conditions in a lab. The researchers discovered that when humidity dropped below about 85 percent, ticks tended to die quickly under hotter, southern temperatures. If humidity remained higher, however, as it generally does deep under a blanket of leaves, ticks were able to survive the heat. With consistent exposure to high temperatures, the authors surmise, the southern populations have evolved with the instinct to lay low.
Ginsberg says the findings suggest that as temperatures warm, ticks in some regions of the Mid-Atlantic, like the Chesapeake Bay, may evolve to exhibit similar behaviors, perhaps leading to lower incidence of Lyme there.
Sam Telford, an epidemiologist at Tufts University, said the work was very good and serves to confirm what decades of observations suggested about differing behaviors between northern and southern deer ticks. But he suspects that other genetic differences between the populations, which scientists are just beginning to explore, may also contribute to the disparate rates of Lyme infections.
The disease has become the most common illness spread by pests in Europe and North America. Each year, an estimated 300,000 Americans contract Lyme—which can cause rashes, swollen joints and neurological problems—and the number of cases reported has risen steadily over the past 25 years.
That has invited lots of attention from scientists and the public, including extensive discussion in last year's National Climate and Health Assessment. In a synthesis of existing research, the assessment stressed the enormous challenges to untangling the relationship between Lyme and climate change. Warmer weather can affect every aspect of the disease's cycle, from the abundance of ticks and host animals to the behavior of the bacterium itself, Borrelia burgdorferi. It noted that studies have established an influence of climate on the distribution of ticks, but that a direct link between temperature and incidence of Lyme has proven more elusive.
Ginsberg is quick to stress that their findings remain theoretical. There are other possible explanations for the lower incidence of Lyme in the South, including the different mix of vertebrate hosts. Skinks and other lizards, which are common in the South, are relatively poor vessels for the disease compared with mice. Ginsberg and his colleagues have been testing this hypothesis as well and expect to publish their results in coming years.
Maria Diuk-Wasser, who studies the effects of environmental change on tick-borne illnesses at Columbia University, said in an email that the study provides "a significant contribution to the field," because it links tick mortality to specific measurements. But she stressed that our understanding of the evolution of tick behavior "is still in its infancy, so it would be hard to predict how the ticks will evolve to changes in climate."