Climate Change: Influence on the Spread of Lyme Disease

By Roberta Attanasio

Blacklegged ticks feed on blood — they attach to the skin of humans and many animals and, slowly, suck for several days. To get there, they patiently wait on the tip of grasses and shrubs using their lower legs to hold on, until a human or an animal passes by. Ticks can’t jump or fly, so they keep their upper pair of legs outstretched, ready to climb aboard. Once there, they insert their feeding tube into the skin and start to suck the blood — this is how they become infected by or transmit Borrelia burgdorferi, the micro-organism that causes Lyme disease.

Lyme disease is one of the most common infectious diseases in the United States, and it also occurs in many regions of Europe and Asia. It’s at the center of ongoing disputes on how to cure it and on whether or not a chronic form of the disease exists.  Now, something else may add to the controversy — the influence of climate change on its spread.

During their life cycle, ticks undergo profound transformations that require moisture in the air — eggs develop into larvae, which then become nymphs and, finally, adults. In addition, the ticks’ development from egg to larva depends on warm temperatures. Climate change, then, may make the ticks’ life harder or easier — if the climate becomes warmer, ticks will reproduce and spread more, thus increasing the risk of Lyme disease.

The blacklegged tick (also dubbed deer tick) Ixodes scapularis spreads Lyme disease in the northeastern, mid-Atlantic, and north-central United States, whereas the western blacklegged tick Ixodes pacificus spreads the disease on the Pacific Coast.

In the 1970s, populations of Ixodes scapularis expanded in the northeastern United States, thus leading to the diffusion of Lyme disease in this region. Now, Ixodes scapularisthe is expanding northwards, leading to the increased occurrence of Lyme disease in Canada.

Results from a study published on March 14, 2014, in the journal Environmental Health Perspectives (Estimated effects of projected climate change on the basic reproductive number of the Lyme disease vector Ixodes scapularis) show that, as temperatures increased northward between 1971 and 2010, so did the sprawl of Ixodes scapularis. Thus, the authors of the study suggest that climate warming may be responsible for the emergence of Lyme disease in northeastern North America. On the basis of climate projections to 2070, the authors conclude that, in the future, higher temperatures may drive Lyme disease into new geographic regions while, at the same time, increasing the risk of diseases carried by ticks in climates currently suitable for their spread.