Many tick species are expanding their ranges, swelling in number and picking up new pathogens that can deliver disease to people should a tick latch on and bite.
On a warm spring day, disease ecologist Daniel Salkeld is hiking the hills of coastal scrub and chaparral of Marin County, north of San Francisco. It’s his favorite spot to collect ticks.
That’s reason to worry, because ticks are prodigious vectors—they bring more types of pathogens over to people from animals than any other creature. And they’re on the march. In the United States, the annual number of cases of six tick-borne diseases has roughly doubled since 2004, with most of the increase dominated by Lyme disease cases.
Ticks are expert spreaders of pathogens Ticks come in two main varieties: hard ticks, which have visible mouthparts and a hard plate on their back, and soft ticks, which lack the hard plate and have mouthparts hidden on their undersides. Though both types of arachnid can carry disease, hard ticks—the family Ixodidae—are by far the more serious vectors.
Not every host species can nurture pathogens in their blood, however. And this helps to explain certain geographic patterns. In the southeastern US, for example, black-legged ticks favor lizards for their blood meals. But lizards are poor hosts for Lyme disease bacteria—the blood of one species even kills the pathogen—so the reptiles rarely pass it on to ticks. As a result, Lyme disease is rare in the southern US.
Increasing temperatures allow their life cycles to speed up, potentially shortening a three-year life cycle to a two-year one, Gilbert says. And warmer winters increase the likelihood that ticks can survive in habitats that once would have killed them. The most studied case is that of black-legged ticks and Lyme disease—and here, research suggests that land use changes have been critical.
So it’s not surprising that some research has found that the wider the range of species in a tick habitat—especially if a lot of those species are bad Lyme reservoirs—the less likely ticks are to pick up the pathogen. Having a variety of potential hosts dilutes the chance that a tick will feed on an infected animal which, in turn, dilutes the chance that a tick bite will transmit disease.
Ostfeld has evidence for this from his research in maple forestland in southeastern New York. Collecting ticks from patches of forest of different sizes, his team found that in the smallest forest fragments they studied , the density of young, nymph-stage ticks was more than three times higher than it was in larger forest patches. And the nymphs were infected with Borrelia burgdorferi 70 percent of the time, compared to 48 percent of the time in larger patches.
But nothing, of course, is straightforward with ticks and tick-borne diseases. Should a different rodent that’s a poor Borrelia burgdorferi reservoir predominate in a fragmented patch of green space, fewer infected ticks—and thus a lower Lyme risk—might result, says Eisen. “It really depends on what your host community looks like,” she says.
Warmer, wetter winter conditions would make it easier for ticks to survive, says Micah Hahn, an environmental epidemiologist at the University of Alaska Anchorage and first author of the study. And even if their environment dries out, Hahn says that ticks can persist by finding a moist patch in an otherwise parched landscape. “If it gets dry, they climb down and burrow until it gets wet,” she says. “They can find tiny, tiny microhabitats even in less suitable areas.
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