New study unravels mystery of why deadly tick disease appears to be surging, yet fatalities have not

http://www.eurekalert.org/pub_releases/2015-10/bc-nsu102315.php

Public Release: 26-Oct-2015
New study unravels mystery of why deadly tick disease appears to be surging, yet fatalities have not
Burness Communications

A mild disease spread by the aggressive Lone star tick that is now colonizing large swaths of the United States is being mistaken for Rocky Mountain spotted fever, according to a new study from scientists at the U.S. Centers for Disease Control and Prevention. The findings may indicate a key reason reports of infections with the potentially fatal pathogen appear to be surging but deaths are not, according to researchers. The study was presented today at the Annual Meeting of the American Society of Tropical Medicine and Hygiene (ASTMH) and published in the American Journal of Tropical Medicine and Hygiene.

The study probes a major mystery of the tick world: Why did reports of Rocky Mountain spotted fever jump from 1.7 to 14.3 cases per million between 2000 and 2012, even as the death rate dropped from as high as 10 percent of infections to less than 1 percent? Populations of the ticks known to carry Rocky Mountain spotted fever did not increase. Researchers say part of the answer lies with the expansion of Lone star tick populations, which do not carry Rocky Mountain spotted fever but can carry a mild form of spotted fever that is apparently being confused with the more serious disease.

"We found that the expanding range of the Lone star tick tracked very closely to the reported rise in Rocky Mountain spotted fever infections," said F. Scott Dahlgren, MSPH, the lead author of the study. "You also see other signs of a milder disease at work. In many areas reporting higher infection rates, you didn't see a rise in hospitalizations or deaths that would normally accompany a true outbreak of Rocky Mountain spotted fever."

The study revealed America's shifting landscape of tick populations and the variety of infections they can carry that are considered members of the "spotted fever group" family of tick-borne diseases. The researchers also found that in addition to the rise of the Lone star tick, another "aggressive biter of people" called the Gulf Coast tick is also expanding its range and carrying with it yet another form of spotted fever that, while not fatal, can cause serious illness.

Dahlgren said that many diagnoses of Rocky Mountain spotted fever today are mostly caused by Rickettsia ambylommii, a closely related bacteria that is commonly carried by Lone star ticks that causes a milder form of the illness. Over the last few decades, human encounters with Lone star ticks have spiked in the U.S. as the species took advantage of warmer winters and rising deer populations to expand from its southern base to as far north as Maine and as far west as Nebraska.

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