Toxic ticks spread illness as planet warms
YALE (US)—A new study suggests fighting infectious disease could prove more challenging on a warming planet. Scientists have made a link between climate and the severity of Lyme disease in certain regions of the United States. Rising temperatures may lead to stronger, more persistent strains of the tick-borne illness, according to the findings.
Deer ticks, which carry and transport Lyme disease, live for two years and have three stages of life—larval, nymphal, and adult—with one blood meal during each stage in order to survive. If the source of the first meal (a mouse, bird, or other small animal) is infected with the bacterium that causes Lyme disease, the tick also becomes infected and passes it on to its next meal source—be it wildlife or human.
But, as the team from Yale University demonstrates, it’s the seasonal cycle of feeding for each stage of the tick’s life that determines the severity of infection in a given region. The researchers found that this cycle is heavily influenced by climate.
In the moderate climate of the Northeastern United States, larval deer ticks feed in the late summer, long after the spring feeding of infected nymphs. This long gap between feeding times directly correlates to more cases of Lyme disease reported in the Northeast, say the scientists.
“When there is a longer gap, the most persistent infections are more likely to survive,” explains Durland Fish, professor of epidemiology at Yale School of Public Health and corresponding author of the study. “These persistent bacterial strains cause more severe disease in humans, leading more people to seek medical attention and resulting in more case reports.”
But in the Midwest, where there are greater extremes of temperature, there is a shorter window of opportunity for tick feeding, and therefore a shorter gap between nymphal and larval feedings. Because of this, report the scientists, Midwestern wildlife and ticks are infected with less persistent strains, which correlates with fewer cases of Lyme disease reported in the Midwest.
The clear implication of this research, say the researchers, is that as the planet warms the Upper Midwest could find itself in the same situation as the Northeast: longer gaps between nymphal and larval feeding, and therefore more stubborn strains of Lyme disease.
“Other diseases, such as malaria, have been projected to expand in response to climate change,” explains project leader Maria Diuk-Wasser, assistant professor of epidemiology at Yale School of Public Health and senior author. “But this is the first study to show how the severity of disease can also be related to climate.”
The research was funded by the U.S. Centers for Disease Control and Prevention, the U.S. Department of Agriculture Agricultural Research Service, and the G. Harold and Leila Y. Mathers Foundation.
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