Tick bites transmit Lyme disease. But even knowing where these ticks live doesn’t necessarily mean you can predict disease in humans. It is only one part of a broader picture that includes human behavior and the habits of the parasite’s carriers.
Researchers at the University of California, Santa Barbara have discovered that the environment of the small mammals that ticks feed on could explain human Lyme disease rates, at least in California. As a result, scientists and health officials may be able to predict future disease risks by studying the response of these animals and their tick parasites to climate change and land use. The results appear in the journal Environmental Research Letters.
“This study is unique because it attempts to determine the links that climate, mammals, ticks and humans have, which requires different types of data, research techniques, and academic backgrounds,” said co-author Sam Sambado, a PhD student in the Department of Ecology. and evolution and marine biology.
Lyme disease is primarily caused by bacteria Borrelia burgdorferi by tick bites. However, the western black-legged tick is not born with bacteria. You can contract the pathogen only by feeding on an infected host, which acts as a reservoir for the microbe.
The ecology of tick hosts must influence the distribution of disease in humans, but contact is not always direct. “It’s difficult to relate the environment to epidemiology — or where people get sick — because humans change their behavior based on risk,” said first author Andy MacDonald, assistant professor in the Brain School of Environmental Science and Management. Where people go, how they interact with the landscape, and whether they take precautions against tick bites all influence where people catch Lyme disease.
The researchers had two questions in mind. First, what environmental factors influence the infection rate in tick populations? Second, can this knowledge be used to predict human infection?
For this project, the team used geo-referenced locations where infected ticks were collected across California. They then used machine learning to correlate tick infestation rates with different environmental characteristics and included habitat suitability for various small mammals that could serve as a reservoir for ticks. B. burgdorferi.
MacDonald explained that small mammals are more likely to catch the infection and then pass it on to an uninfected tick. Large mammals do not build up significant levels of pathogens, and reptiles’ immune systems may kill bacteria. Meanwhile, the role of birds is not well understood, but scientists believe they are not significant contributors to the amount of Lyme disease in tick populations in North America.
When the dust settled, the algorithm showed a clear link between infected ticks and one of their frequent hosts. The most important predictor, by far, of B. burgdorferi In ticks, McDonald said, is the environment of the dark-footed woods. “This species acquires infection from ticks easily, and infects new ticks easily as well.” Equally as Woodrat.
Remarkably, there is no real evidence that the bacteria harm young mammals. “It appears that animals that have evolved with these parasites, and the pathogens that transmit them, get over these infections quite well,” McDonald said. put another way, B. burgdorferi Infections are just a part of life like Woodrat.
The authors also investigated the effect of biodiversity. Hypothetically, areas of low diversity contain species that readily acquire and transmit bacteria. They’re probably the ones who live fast and die young, and don’t put much energy into their immune systems. Surprisingly, the variety appeared to have no effect on the incidence of infected ticks.
This study considered a large, heterogeneous region — the entire state of California — so individual species important for disease transmission stood out in the results. “The woodlice in particular—and the gray squirrel to some extent, too—interferes with where the ticks live,” McDonald said. “It’s the overlap in the distribution of ticks and those small mammals that I think is a major reason we see them come out of the models so much.”
In other words, there is nothing accidental about mammals predicting infection in ticks; It all comes down to chance. Namely, the range of gray squirrels and dark-legged woodpeckers overlaps with the habitats in which ticks tend to live, and it’s possible that the Lyme disease transmission cycle evolved through this close association.
Turning this environmental information into epidemiological insights can be challenging. “We wanted to understand whether this environment is really predictive of human disease,” McDonald said. “This is often not the case because humans respond behaviorally to disease risk.” For example, people may avoid high-risk areas, separating human infection from the underlying disease environment.
However, the distribution of infected ticks was a strong predictor of where people in California were infected. This contrasts with studies in the eastern United States, where ecology is not closely related to epidemiology. McDonald believes this may be because Lyme disease is relatively rare in the Golden State, so there is much less awareness of the disease, risk factors, and symptoms among the public and physicians alike. The findings point to a direct way to combat Lyme disease in California: education. The CDC has resources for preventing, recognizing, and treating Lyme disease.
The strong relationship between Lyme disease ecology and epidemiology also means that researchers in the western United States can focus on the environment to understand how the human disease is likely to change in the future. “Stressors such as climate change and land use are altering entire ecosystems, but they affect individual species at different rates,” Sambadou said. “Understanding what creates these differences will be an important challenge for all researchers, especially those working in multispecies systems such as Lyme disease.”
The authors conducted a planned follow-up study to determine how human behavior affects Lyme disease on the West Coast. They also plan to apply the technique to other diseases. They are studying the ecology of West Nile virus in California’s Central Valley as well as malaria and leishmaniasis in the Amazon in collaboration with a team from Lima, Peru.
“I think this kind of approach — trying to relate environment to epidemiology — can be really beneficial for many disease systems,” MacDonald said.