Mild winter, dry spring mean mosquitoes bite us earlier, coyotes spreading Lyme disease

Scientific research is shining new light on the biology behind insect-borne disease threats to our health this summer.

Bug experts around the country predicted swarms of mosquitoes and ticks after a milder than normal winter, and they've largely been proven right.

Conventional wisdom is that fewer hard-frozen days during the winter means that more pests survive into the following spring, and that's true in many parts of the country.

In Tulsa, this year's mosquito population is actually closer to average, according to officials with the Tulsa City-County Health Department.

But other conditions also can make a difference, like a dry spring that can deny skeeters places to hatch, for instance.

One recent study showed that the severity of winter determines when at least one species of mosquito turns to feeding on humans and other mammals rather than birds.

A team led by entomologist Nathan Burkett-Cadena of the University of South Florida spent eight years trapping a breed known as Culex erraticus at various spots around Alabama.

The species is one of several breeds that spread the virus that causes Eastern equine encephalitis. The researchers analyzed the blood meals inside each of the insects they caught to identify which bird or mammal species they'd bitten.

In a report published in the March issue of Biology Letters, they said that after a mild winter, the mosquitoes were more likely to start feeding on mammals in May or June, but when winter was harsh, the insects didn't start mammalian feeding until August.

The timing's important because birds also host many viruses that mosquitoes can pass along to humans, including West Nile virus, once they do turn to feeding from mammals.

Although the researchers aren't sure why the timing changes, Burkett-Cadena speculated it might key on the timing of bird nesting. C erraticus favors blood meals from herons, and those birds tend to mate and hatch their broods earlier in the spring after a mild winter -- making them more mobile and harder to bite when female mosquitoes carrying eggs go after a blood meal.

Another study, published online in mid-June, finds an even more elaborate food-chain link to the continued spread of Lyme disease across the Eastern and Midwestern U.S. in the past three decades.

While the past mild winter has caused health officials to put people going outdoors in those regions this spring on high alert against ticks and tick bites, the report in the Proceedings of the National Academy of Sciences suggests that the long-term reason behind the spread of Lyme lies in a shift of dominant predators, specifically, coyotes crowding out native foxes.

Biologists once thought a surging deer population was behind the spread of the bacterial infection. But a team from the University of California, Santa Cruz, documented that Lyme has continued to increase even in areas where deer numbers have stabilized or even declined due to stepped up hunting and Bambi birth control efforts.

Instead, the key bacterial reservoir for Lyme that attracts most ticks lies in small mammals like white-footed mice, shrews and chipmunks, all known to carry the disease.

And one of the biggest factors in how many of these small critters there are in the woods is how many foxes are around to eat them.

Foxes are in decline across much of the east and Midwest. And besides a loss of habitat in some areas, the main reason may be the arrival of coyotes as the new top predator.

Not only do coyotes kill foxes, but foxes also are less likely to build dens in areas frequented by coyotes, said Taal Levi, lead author of the study and a recent doctoral graduate in environmental studies from UCSC. And fox and coyote hunting menus and styles are different. Foxes consume small game almost exclusively, and will often catch and store more than they can eat or immediately carry back to kits. Coyotes are more an eat-and-run predator, and, being larger, are likely to tackle a greater variety of animals, but over a much larger area.

Foxes have a much smaller range than coyotes, so they hunt their territories more intensively, the researchers point out, meaning foxes are much more effective at keeping populations of small, tick-bearing mammals in check.

After studying extensive animal population datasets from five states and mathematical models of the spread of Lyme disease, the team found the major influence seemed to be the growing abundance of coyotes over foxes.

Although deer carry the adult ticks that host Lyme, the cycle of infection seems to start with the smaller mammals and the nymph ticks they carry that often bite humans to infect them, but are so small they're never noticed.

 

(Contact Scripps medical and science reporter Lee Bowman at BowmanL@shns.com .)

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