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Global Detective Cherie Briggs’ Work Helps Reveal Mysteries of Disease Ecology

Cherie Briggs. Image credit: Lourdes Velazquez.

Lyme disease has sickened both wildlife and humans across the United States. It was first described in the mid 1970s, affecting deer and leaving confounding symptoms in about 30,000 people a year, who had been bitten by infected blacklegged ticks.  

But why is the disease dramatically more prevalent in northeastern states than it is in the west?  

University of California, Santa Barbara Biology Professor Cherie Briggs, a disease ecologist, has answers for this geographic mystery: It’s what’s on the young tick’s menu.

“If you look at the western blacklegged tick that we have out here in most of the areas that have Lyme, about 70% of the juvenile ticks feed on lizards rather than feeding on mammals,” said Briggs. “And lizards have this ability to actually cleanse the tick of the Lyme pathogen. So, any tick that feeds on a lizard comes off uninfected. That is a great way to keep the Lyme prevalence low,” she said.  

Young ticks on the East Coast that feed on mice or birds don’t get that natural disinfection. Differences in the tick’s life cycle also play a part in how extensive the disease is.

“The timing in the East Coast really makes it easy for one generation of ticks to pick up the pathogen from the host that has been infected by the previous generation. Whereas in California, the timing makes that transmission really hard. It is all about ecology,” said Briggs.

Briggs, recently chosen as a 2019 AAAS Fellow “for distinguished contributions to the field of disease ecology,” collaborates with experts around the world, using theoretical models, and laboratory and field experiments to understand how disease affects the dynamics of animal populations.

Lyme disease is one of many zoonotic diseases, meaning the disease spreads from animals to humans, which also include deadly infectious diseases like SARS, MERS, and Coronavirus, which Briggs also researches. What causes the disease spread? People, livestock and plants are all coming in contact with things they never encountered in previous generations, according to Briggs.

“Human movement has kind of made these types of emergences inevitable,” she said. “A very large fraction of diseases that affect humans also have some type of non-human aspect to them.”

The Briggs lab also studies the devastating chytrid fungus, known around the world as “the frog apocalypse.” Since the early 2000s, about 500 amphibian species have been impacted. Some are now extinct.   

The most widespread form of the fungus, Batrachochytrium dendrobatidis (Bd), destroys the animal’s skin and usually leads to death. Bd is the only member of this ancient group of fungi that is known to attack vertebrates.

“Some of my colleagues [studying] frogs in the Sierras would show up one year and there would be thousands of frogs, and the next year there would be none of them left,” she said. 

The Briggs lab is working to determine why some frog populations can survive with this pathogen, while other populations are wiped out.

But, it’s difficult to treat animals in wild populations, according to Briggs. Some frogs have been exposed to the fungus in the lab, then cleared of the infection and released back into the wild. That process is yielding clues. Some scientists are working on the mechanisms of that immune response. There’s also been a lot of interest in exposing the animals to beneficial bacteria.

“On the skin of the frog, there is a whole community of bacteria, microbiomes. Fungi and bacteria kill each other off, so the idea is to give a particular species of bacteria to the frog to fight off the fungus,” said Briggs.

The global climate crisis is further complicating efforts to bring infectious diseases such as chytrid under control. Warmer temperatures in winter can increase a frog’s metabolism, leading to a weaker body condition.

Briggs said that while the state of California is trying its hardest to keep its climate policies in place, “I think it is going to be inevitable that the reality of climate change is going to force everybody to respond to it.”

The magnitude and urgency of chytrid devastation has prompted researchers to broaden their scope to find answers. Briggs works with chemists, physicists and computer scientists. And there is collaboration with The National Park Service, U.S. Fish and Wildlife, U.S. Forest Service and the San Francisco Zoo.

Briggs said more funding is also now available to study the ecological aspects of diseases, especially zoonotic diseases. She said a National Science Foundation program called Ecology and Evolution of Infectious Diseases has brought together groups of people who had not worked together before. That has rapidly increased studies that are benefitting both humans and wildlife.

She looks forward to more protection of endangered species and action on climate change from the scientific community.

“When I started out as a scientist, we wouldn’t do that type of thing. It seems like we really have to now,” said Briggs.

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