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During its two-year life span, a deer tick goes through three stages: larval, nymphal and adult. To survive, a tick must obtain a blood meal during each stage.
If the source of the first meal (mouse, bird or other small animal) is infected with the bacterium that causes Lyme disease, the tick also becomes infected. The tick can then pass the infection to its next meal, which could be wildlife or a human, during its nymph stage.
The seasonal cycle of feeding for each stage of a tick's life determines the severity of infection in a given region, according to the study in the April issue of Applied and Environmental Microbiology.
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.
When there is a longer gap, the most persistent infections are more likely to survive, the study's co-author, Durland Fish, a professor of epidemiology at the Yale School of Public Health, said in a Yale news release. 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, greater extremes of temperature mean a shorter time in which ticks can feed and, therefore, a shorter gap between nymphal and larval feedings.
Midwestern wildlife and ticks tend to be infected with less persistent strains, which correlates with fewer cases of Lyme disease in the Midwest.
As the planet warms, the researchers said, the Upper Midwest could more closely resemble the Northeast: longer gaps between nymphal and larval feeding and stronger, more persistent strains of Lyme disease.
Other diseases, such as malaria, have been projected to expand the geographic region in which they occur in response to climate change, said Maria Diuk-Wasser, assistant professor of epidemiology at Yale and senior author of the study.
But she said this was the first study to show how the severity of disease can also be related to climate.
One of the first symptoms of Lyme disease is often a rash at the site of the bite. Though treatable with antibiotics, the disease can cause fatigue, chills, fever, headache, muscle and joint aches and swollen lymph nodes.
-- Jennifer Thomas
SOURCE: Yale University, news release, April 21, 2009
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