Climate Change and Lyme Disease

White-tailed deer are a main host for deer tick, the vector of Lyme disease. Jeffrey Phelps/Getty Image News

Lyme disease is an illness due to an infection by a species of Spirochete bacteria (Borrelia burgdorferi), with complex and often serious health consequences. The bacteria are transmitted to humans by the bite of ticks of the genus Ixodes, mostly blacklegged or deer ticks (Ixodes scapularis) in the United States and Canada. The deer tick completes its life cycle by getting blood meals from various host mammals (and sometimes birds and lizards), most notably white-tailed deer and white-footed mice.

Lyme disease are a worsening public health issue in many states. For example, in the year 2016 alone there were over 12,000 diagnoses of Lyme disease in Pennsylvania. How will global climate change affect the occurrence of deer tick, and at the same time, the rate of Lyme disease infections?

Lyme Disease Expansion

With the global warming we have experienced in the last few decades, deer tick populations have already been expanding their geographical range to the north and west into Ontario and the upper Great Lakes states. With the predicted rise of minimum temperatures, deer ticks will survive winter conditions in ever greater numbers. Recent modeling efforts predicted a continued expansion into Canada, with the ticks taking the Lyme disease-causing bacteria with them. The rate of expansion is not dependent on how fast the ticks can move, but rather on how fast their hosts can cover ground.

Deer and humans being important hosts, dispersal into newly suitable areas is rapid.

And Some Lyme Disease Contraction

At the same time, some parts of the deer ticks range will become hotter, with daytime high temperatures in the summer expected to rise above what the deer ticks can tolerate. Consequently, deer ticks are expected to eventually leave behind large parts of Texas, Mississippi, and Florida.

These areas have dense human populations, so it means that the overall incidence rate of Lyme disease may decrease. The rest of the current geographical range is expected to continue offering favorable conditions for deer ticks. In reality, the rates of Lyme disease infection within the current range are dependent on a variety of factors. The use of pesticides against ticks, better protective measures, and vigilance by outdoor enthusiasts may protect more people.

Lyme Disease and Wildlife Populations

Changes in white-footed mouse and white-tailed deer populations will also affect Lyme disease infection rates. Incidentally, white-tailed deer populations have exploded in the last century. Down to a few thousands in the whole eastern United States, deer are now overabundant in many locations. They are edge specialists who have taken advantage of the intermingling of agriculture lands, woodlots, and suburban areas. In a series of complex interactions, deer have benefited from the recent abandonment of agricultural lands, followed by the emergence of early succession forests rich in food and cover. Deer have now become so abundant that they significantly hold back young tree growth, changing forest structure and even affecting bird populations.

It is likely that deer ticks have benefited from the white-tailed deer’s expansion, but the tick’s life cycle is not entirely dependent on the deer: they can use alternate hosts and rely solely on white-footed mice and other small mammals. Unlike deer, these small mammals are carriers of Lyme disease and can take a lot of the blame for spreading it. Promoting land use that reduces habitat fragmentation and favors mature forest cover might reduce deer population, but more importantly it may support healthier populations of mouse predators, putting much needed pressure on the Borrelia carriers.

What You Can Do

  • Learn the basic techniques to prevent tick bites, including appropriate clothing, tick repellants, and careful post-outing skin examination.
  • Support your local predator population and help them keep white-footed mice scarce. Specially designed nest boxes can attract American kestrels, barn owls, and eastern screech-owls.


    Ogden et al. 2006. Risk maps for range expansion of the Lyme disease vector, Ixodes scapularis, in Canada now and with climate change.

    Ostfeld et al. 2006. Climate, Deer, Rodents, and Acorns as Determinants of Variations in Lyme Disease Risk