Home Career Human Lyme disease infections linked to environmental factors across California — ScienceDaily

Human Lyme disease infections linked to environmental factors across California — ScienceDaily


Tick ​​bites transmit Lyme disease. But even knowing where these ticks live, it is not necessarily possible to predict the disease in people. This is only part of a larger picture that includes human behavior and the habits of parasite carriers.

Researchers at the University of California, Santa Barbara have found that the ecology of the small mammals that ticks feed on may explain the rate of human Lyme disease, at least in California. As a result, scientists and public health officials will be able to predict future disease risk by studying the response of these animals and their tick parasites to changing climate and land use. The results are published in the journal Environmental Studies Letters.

“This study is unique because it attempts to quantify the relationships between climate and mammals and ticks with humans, which requires different types of data, research methods and academic training,” said co-author Sam Sambada, a doctoral student in the Department of Ecology, Evolution and Marine biology.

Lyme disease is mainly caused by bacteria Borrelia burgdorferi by tick bites. However, the western tick is not born with this bacteria. It can become infected with the pathogen only by feeding on an infected host, which serves as a reservoir for the microbe.

The ecology of tick hosts must influence the spread of disease among humans, but the connection is not always straightforward. “It’s hard to link ecology to epidemiology — or where people get sick — because people change their behavior based on risk,” said first author Andy MacDonald, an assistant professor in the Bren School of Environmental Sciences and Management. Where people walk, how they interact with the landscape, and whether they take precautions against tick bites all affect where people contract Lyme disease.

The researchers had two questions. First, what environmental factors affect the infection rate of tick populations? Second, can this knowledge be used to predict human infections?

For this project, the team used georeferenced locations where infected ticks were collected throughout California. They then used machine learning to correlate tick infestation rates with various environmental characteristics and incorporated habitat suitability for various small mammals that can serve as reservoirs for B. burgdorferi.

Small mammals are most likely to be infected by an uninfected tick and then pass on the infection, McDonald explained. Large mammals do not accumulate significant levels of pathogens, and the reptilian immune system can even kill bacteria. Meanwhile, the role of birds is less clear, but scientists believe they do not make a significant contribution to Lyme disease in tick populations in North America.

When the dust settled, the algorithm revealed an apparent link between infected ticks and one of their frequent hosts. “Definitely the most important predictor B. burgdorferi in ticks is the ecology of dark-footed woodrats, MacDonald said. – This species is easily infected by ticks and also easily transmits the infection to new ticks. Western gray squirrels also served as a reservoir for the bacterium, but not to the same extent as the wood rat.

Notably, there is no real evidence that the bacteria harm small mammals. “Animals that have evolved with these parasites, and the pathogens they transmit, seem to tolerate these infections very well,” Macdonald said. In other words, B. burgdorferi Infections are just part of a woodrat’s life.

The authors also explored the impact of biodiversity. Hypothetically, areas with low diversity have species that readily acquire and transmit the bacteria. They are probably the ones who live fast and die young without putting much energy into their immune system. Surprisingly, diversity did not seem to affect tick infestation rates.

This study looked at a large, heterogeneous area – the entire state of California – so individual species important to disease transmission stood out in the results. “The woodrat in particular — and the gray squirrel to some extent — matches where the ticks live,” MacDonald said. “It’s the overlap in the distribution of ticks and these particular small mammals that I think is the main reason we see them jump out of the models so often.”

In other words, there is nothing random about which mammals predict tick infestations; it all comes down to coincidence. Namely, the range of gray squirrels and black-footed woodrats overlaps with tick habitats, and the transmission cycle of Lyme disease likely evolved due to this close association.

However, turning this ecological information into epidemiological information can be difficult. “We wanted to understand whether this ecology actually predicts human disease,” McDonald said. “And often they don’t, because people are responding behaviorally to the risk of the disease.” For example, people can avoid high-risk areas by separating human infection from the ecology of the underlying disease.

However, the distribution of infected ticks was a strong predictor of where people became infected in California. This contrasts with studies in the eastern United States, where ecology is not closely related to epidemiology. MacDonald suspects this may be because Lyme disease is relatively rare in the Golden State, so the public and doctors are far less aware of the disease, risk factors and symptoms. The results show a simple way to fight Lyme disease in California: education. The CDC has resources on how to prevent, recognize, and treat Lyme disease.

The strong correlation between Lyme disease ecology and epidemiology also means that researchers in the western US can focus on ecology to understand how human disease may change in the future. “Stressors such as climate and land-use change alter entire ecosystems, but affect individual species at different rates,” Sambada 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 planned a follow-up study to determine how human behavior affects Lyme disease on the West Coast. This technique is planned to be applied to other diseases as well. They are investigating the ecology of West Nile virus in California’s Central Valley, and malaria and leishmaniasis in the Amazon, in collaboration with a team from Lima, Peru.

“I think this approach — trying to connect ecology with epidemiology — could be very useful for many disease systems,” McDonald said.

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