Levin: Biodiversity And Lyme Disease
The name deer tick - although perpetuated by the media – hasn’t been in vogue among arachnid cognoscenti since 1993, when biologists determined that the vector for Lyme disease was not two separate species - northern and southern - but a single widespread species. And since the southern form had been classified first as the blacklegged tick, the rules of scientific nomenclature determined that as the name for the entire species. Of course, taxonomy quickly devolves to a footnote if you’re afflicted with Lyme disease.
Now, let me clear up another common misconception. The number of deer is not a reliable predictor of blacklegged ticks or the projected incidence of Lyme disease – even though adult blacklegged ticks do feed on deer. In fact, ticks mate on deer - and a female, having locked herself in place to feed, is an easy target for an aroused male. But research shows that the number of female ticks in an area remain fairly constant whether there’s an abundance of deer or a moderate number. And mother ticks - even if infected - do not pass Lyme disease to the egg. Tick larvae, which hatch in midsummer, are healthy. So deer do not cause Lyme disease.
Lyme disease is perpetuated because blacklegged ticks live for two years and have a three-stage lifecycle. Each stage requires a separate blood meal. Larvae feed on small and mid-sized mammals, as well as ground-nesting and ground-feeding birds. Collectively, they serve as the reservoir host for the spirochete bacteria that causes Lyme disease. Here, white-footed mice, short-tailed shrews, chipmunks, and robins are the most competent reservoirs. Shrews carry the most ticks, but white-footed mice are the most competent host of Lyme-causing bacteria. Mice are poor groomers and have ineffective immune systems that don’t clear the infection, so they pass the spirochete to more than 90% of the ticks that feed on them.
After they feed, larvae metamorphose to nymphs and hibernate for the winter, until the following May or June, when they revive, feed, and transmit the infection, sometimes to us.
Then a new generation of blacklegged tick larvae hatches in midsummer and likely encounters a fresh crop of infected small mammals and birds to feed on.
So the underlying reason that Lyme disease has become the most common vector-borne disease in the United States may be loss of biodiversity as we continue to simplify ecosystems. Our woodlots and suburbs favor white-footed mice and shrews over dilution hosts, the better groomers like grouse, and the mid-sized predators that eat small mammals – predators like weasels, foxes, hawks, owls, and snakes.
A 2012 article in the Proceedings of the National Academy of Science, suggests that as coyotes increase in the Northeast and red foxes decrease, mice increase - and so does the incidence of Lyme disease.
Milk snakes are mighty predators of mice. So the moral of this ecological parable might be that if a milk snake curls up in your nearby stonewall - count yourself lucky.