Lyme disease was first discovered in the 1970s in communities of southern New England and New York. Scientists discovered a brand new bacterium (a spirochete called Borrelia burgdorferi) that causes the disease and thought they had discovered a brand new species of tick that transmits the bacteria. They gave this tick the common name “deer tick” (scientific name Ixodes dammini).
The scientists then set out to describe the ecology of this “new” tick. They found the ticks predominantly in coastal areas but not further inland, and they found many of the ticks feeding on white-tailed deer, hence the common name. They concluded from these early studies that the ticks, and thus Lyme disease, were limited to benign climates near the coast. At the same time it was discovered that it was easy to kill ticks by subjecting them in the lab to extreme temperatures (low or high) and to low humidity. Scientists began to suspect that global warming might spread the warm, coastal conditions to inland and higher elevation areas, causing Lyme disease to spread as well. Ecological and climatic models also supported spread.
Three decades later, it’s evident that Lyme disease has, and is continuing to, spread dramatically in eastern North America. But the evidence now indicates that the models were too simple and sometimes flat-out wrong. Why were they wrong and why is Lyme disease spreading?
First, it turns out that there is no such thing as a "deer tick." The tick discovered in the areas where Lyme disease emerged was not, in fact, a new species that required a new name, but rather was a northern population of a species that had been known to science for 150 years. This species is the blacklegged tick (Ixodes scapularis). Blacklegged ticks range from Georgia and the Carolinas to Texas and Oklahoma, up to Minnesota and Wisconsin, and over into the mid Atlantic and New England states. Clearly, they can live in a broad range of climatic conditions.
Second, although it’s true that Lyme disease has moved inland, northward, and to higher altitudes, as would be expected under climate warming, Lyme disease has also spread southward from coastal New England into Maryland and Virginia.
And third, blacklegged ticks are sophisticated little creatures that are able to seek protection from extreme climatic conditions in the soil or under leaf litter. So they’re fairly well buffered against cold winters or dry summers. We know very little about which climate extremes are important causes of mortality.
So, what is the role of climate in the spread of Lyme disease? The short answer is, we don’t yet know.
It’s highly likely that the geographic range of the blacklegged tick is limited at high latitudes and high elevations by extreme cold. And climate change is pushing northward and upward the zone of extreme cold. So, we should expect the geographic boundaries of Lyme disease to move northward and upslope in the coming decades.
But the spread over the past 30 years seems to have been caused more by the movements of host animals – especially larger mammals (raccoons, skunks, deer) and migratory birds – which can move ticks many miles in just a few days.
So, the emerging picture is that highly mobile hosts are constantly introducing ticks into new environments as they disperse; that whether these new tick populations persist or die out depends in part on climatic conditions. Climate change is making their survival in northern, higher elevation areas more and more likely. Much more research in this area is needed, though, before we can confidently make predictions.
Richard S. Ostfeld, Ph.D., is a Senior Scientist at the Cary Institute of Ecosystem Studies, Millbrook, NY. For more information on climate and Lyme disease see the author’s new book, Lyme Disease: The Ecology of a Complex System. Oxford University Press. 2011.