As coyotes take over their ranges in North America, red fox populations are plummeting, and researchers have found one surprising result: The drop is fueling the spread of Lyme disease.

Lyme disease cases have increased enormously in recent years: From 1997 to 2007, the number of cases increased by 380 percent in Minnesota, 280 percent in Wisconsin and 1,300 percent in Virginia. Researchers used to think the increases were due to increasing deer populations, since deer are an important host to the disease-causing bacteria. However, the new data show these increases were independent of deer population levels.

"Increases in Lyme disease in the Northeastern and Midwestern United States over the past three decades are frequently uncorrelated with deer abundance and instead coincide with a range-wide decline of a key small-mammal predator, the red fox, likely due to expansion of coyote populations," the researchers write today (June 18) in the journal Proceedings of the National Academy of Sciences.

"We found that where there once was an abundance of red foxes, there is now an abundance of coyotes," said study researcher Taal Levi, who completed the study as a part of his graduate work at the University of California, Santa Cruz.

Tick troubles

Lyme disease is caused by a bacteria spread by ticks. The ticks pick up the bacteria from infected mice or deer, and if they bite a human, we can get the disease too. If caught early, it can be treated with antibiotics to kill the bacteria.

Lyme disease progresses in three stages: The first causes itching, chills, fever and headache for the first few weeks; second, an infected person starts having muscle pain and joint aches; third, these muscle and joint issues can continue for years after the initial infection, causing abnormal movement, weakness and speech problems.

Levi and his colleagues studied data from five states, and used this information to make mathematical models of the disease's spread. In their models, the researchers saw that the loss of the red foxes would result in an increase in Lyme disease, even with deer populations remaining steady.

Coyote complications

The red fox feeds on small mammals such as mice, shrews and chipmunks, animals that, like deer, can play host to the Lyme-disease-carrying ticks. As the red fox population declines, as it has been for the past 30 years, researchers have seen increases in the populations of these small mammals.

And the researchers have reason to believe this change is in the works: The foxes have been outcompeted by growing populations of coyotes in the regions, which are becoming the top predators in areas where mountain lions and wolves have gone extinct. The coyotes are more dangerous to the foxes, lowering their numbers, the researchers said.

"A new top predator has entered the Northeast and has strong impact on the ecosystem," Levi said in a statement. Coyotes can and will kill foxes and more significantly, he said, "foxes often don't build dens when coyotes are around."

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