Why has the geographic range of ticks expanded?
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The geographic range of ticks in the United States is expanding, which may place more people than ever at risk for tick-borne diseases, according to the CDC.
For an explanation, we asked Richard S. Ostfeld, PhD, distinguished senior scientist at the Cary Institute of Ecosystem Studies in Millbrook, New York, who studies the ecology of infectious diseases, including tick-borne diseases.
[Editor’s note: Click here to read our recent feature on the promise of a new Lyme disease vaccine.]
Ostfeld: Let’s focus on the blacklegged tick (Ixodes scapularis) because it’s by far the most common human-biting tick in the U.S. and the one responsible for transmitting Lyme disease, anaplasmosis and babesiosis, the three most common tick-borne diseases.
This tick is now common in areas of the eastern and central U.S. where it appears never to have been common before. But understanding what is causing this expansion is tricky. Northward expansion is consistent with well-documented climate change. Here, the most likely mechanism is that longer warm seasons — earlier springs and later winters — provide ticks a longer period in which to find a vertebrate host, which they require for blood meals. Interestingly there is no good evidence for winter cold snaps killing ticks, which are dormant in winter.
But these ticks have also expanded southward from New York and New England and from the upper Midwest. These areas should have been plenty warm enough for ticks decades ago, so why are they being invaded only now? We don’t have a good answer for that, but other anthropogenic changes may play a role. Fragmentation of forests can help populations of white-footed mice and deer thrive. The mice are responsible for infecting ticks with many pathogens, and both host species can promote tick populations.
Unfortunately, suburbanization not only helps ticks expand, it also tends to plunk people down in these newly created danger zones. In our ongoing battle against tick-borne disease, ticks have the upper hand, thanks to us.
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