Every year, somewhere between 30,000 and 300,000 people in the United States contract Lyme disease from the prick of a tiny bloodsucking bug. In Europe, the World Health Organization estimates that each year 85,000 people contract the disease. And in both places the number of cases each year has grown steadily over the last few decades, Gwynn Guilford reports for Quartz. And that increase is largely due to human activity.
Guildford argues that there are two main reasons Lymes disease is becoming more common: a growing population of mammals for ticks (carriers of the Borrelia burgdorferi bacteria that causes Lyme disease) to bite, and climate change. Both compounding factors have aided the spread of tick carriers — the blacklegged deer tick (Ixodes scapularis) in the U.S. and the castor bean tick (I. ricinus) in Europe.
Ticks live short life cycles, perhaps three years at most. At each stage of their lives, they need a belly full of blood to survive. Without blood, they die. To contract and spread the disease, ticks need to make one key transition from larvae to adolescent nymphs. While deer might be the preferred meal for adults, they’re a bit big for larvae to munch on, and thus, the least of our worries, notes Guilford. The real problem? White footed mice are petite and ill groomed, making them the perfect game for larvae. Plus, they’re also great at hosting and transmitting Lyme bacteria. The clearing of forests by humans, Guilford writes, has eliminated predators to hunt the mice and allowed their population to blossom.
In additon to having plenty to eat, ticks have nicer places to live. Balmier temperatures linked to climate change may have also allowed ticks to invade traditionally colder, northern locales and become serious problems in some states. Warmer autumn temperatures make it easier for young ticks to catch the disease, explains Guilford:
If larvae happen to feed at the same time as newly emerged nymphs—what biologists call “synchronous feeding”—they’re less likely to pick up the infection, since the bacteria won’t have had enough time to reproduce inside the mammal hosts.
But if newly hatched larval ticks feed much later than the nymphs (“asynchronous feeding”) they’ll be more likely to pick it up—and to transmit it the next season as nymphs.
When it stays warm longer, larval ticks feed later. That’s the theory, and some scientists think it’s what’s made Lyme disease such a problem in New England recently. If climate change continues as projected, colder areas of the country could share a similar fate.