The blacklegged tick is responsible for 95 percent of the Lyme disease cases in the United States. Photo: Graham J. Hickling

Ticking off reasons for Lyme disease spread

Lyme disease is the No. 1 vector-borne disease in the northern hemisphere, with more than 30,000 cases reported each year, according to Centers for Disease Control and Prevention (CDC) data. The blacklegged tick (also called the deer tick) — the only tick species that spreads the Lyme disease bacterium to humans in the eastern United States — is actively spreading to new areas, and confirmed cases of the disease will continue to increase as time goes by.

Even though the tick is established in various regions of eastern North America, the majority of confirmed disease cases and evidence of the pathogen occur predominantly in the northern part of the tick’s territory. Understanding the factors that limit the disease to the northern regions may help predict how the disease will spread so that public health measures can be directed toward mitigating disease risk.

Jean TsaoThe public’s ability to recognize the importance of other tick-borne diseases, especially in the Southeast, has been hindered by an excessive focus on Lyme disease and contradictory pieces of information about it. Public health officials, ecologists, medical entomologists and citizens all stand to benefit from reduced disease risk in the future because of the work of Michigan State University AgBioResearch scientist Jean Tsao and collaborators from several universities and research programs across the country.

Early findings from Tsao’s research suggest that ecological factors important to the Lyme disease story can’t be defined by findings from one geographical area, so quantifying the abundance and ecology of the tick and the disease-causing pathogen is taking place at multiple test sites located across the eastern United States and the Southeast. Sampling will continue for another year; the following year will be devoted to analyzing data and running lab assays on infections.

“Findings from this research can be used to help reduce the confusion and answer questions about whether Lyme disease exists in the Southeast and, if it does, at what level,” said Tsao, associate professor in the MSU Department of Fisheries and Wildlife. “Data will provide details about where, when, how much and how many infected ticks exist in the Southeast. If we can show that the risk is miniscule, then discussions about tick-borne diseases can shift from Lyme disease to other ones caused by highly abundant ticks in the Southeast. The CDC and public health officials can use the data, too, to increase risk awareness of tick-borne diseases other than Lyme disease.”

There is currently no vaccine for Lyme disease, and using practices such as spraying pesticides or reducing the population of the deer herd to reduce the potential for contact with an infected tick aren’t socially acceptable.

“If we find out that certain hosts are more important than others, perhaps new interventions can be developed, but part of it is the public’s willingness to implement certain practices,” Tsao said. “Having data available will help authorities make decisions about reducing disease risk and managing its spread.”

Tsao’s research has already demonstrated that tick density is much lower in the South, and ticks there express different host-seeking behaviors that reduce their contact with humans. Though adult ticks in the South, like their northern cousins, appear to feed mainly on deer, the nymph (teenage) stage feeds mostly on lizards, with only some feeding on common northern hosts such as mice, chipmunks and shrews.

“Is it because of the climate, or is it that they just like lizards better?” Tsao asked. “What we’ve found is that the ticks behave differently and there’s a lot of genetic diversity. We may need to revisit the hypothesis that there is more than one tick species, or perhaps the [same] species just behaves differently in the Southeast.”

The project has yielded sufficient data verifying lower risk of Lyme disease in the Southeast, but surveying will continue to generate a fuller sense of the degree of variability among all the various factors at each field site.

“My students, collaborators and I are extremely fortunate and excited to be able to work on such a multifaceted project to understand how Lyme disease risk varies geographically,” Tsao said. “Furthermore, findings at the regional and national levels also will help us better understand and, therefore, better mitigate the risk of Lyme disease for Michigan citizens as the tick spreads into more areas in the state.”