URI entomologist wins $2.6 million NIH grant to study evolution of Lyme disease bacteria in deer ticks
While the bacteria – Borrelia burgdorferi –
is the pathogen that causes Lyme disease in humans, its presence is quite
different in blacklegged ticks that pick up the bacteria from feeding on
white-footed mice. For the ticks, the bacteria doesn’t cause disease. It might
even be beneficial.
For the next four years, Couret’s team will research the
ecological factors driving the evolution of Borrelia burgdorferi in
blacklegged ticks thanks to a $2.6 million grant from the National Institutes
of Health. The grant is part of the prestigious Ecology and Evolution of
Infectious Disease (EEID) program, run by the NIH, National Science Foundation,
and U.S. Department of Agriculture.
“I am really interested in the factors that are driving the tick populations,” said Couret, an assistant professor of biological studies and the principal investigator on the grant. “Their populations vary year to year. Our preliminary data suggests that the survival of the ticks during some of their early life stages is improved based on whether they host these bacteria.”
For the four-year study, Couret is collaborating with Associate
Professor Sukanya Narasimhan of Yale Medical School, Associate Professor Jean
Tsao of Michigan State University, and Associate Professor Cynthia Lord of the
University of Florida – along with postdoctoral, graduate and undergraduate
trainees at each institution.
“One of my favorite aspects of this work is the research team. We
are all women and three of us are women of color,” said Couret, who is part
Indigenous, Afro-Cuban, and American. “I think that is – unfortunately –
somewhat rare in science.”
Blacklegged ticks, also called deer ticks, carry seven known
pathogens and are responsible for about 95% of the tick-borne diseases in the
U.S., including about 30,000 cases of Lyme disease reported each year. Deer
ticks can acquire the bacteria that causes Lyme disease during any of its life
stages – larvae, nymph or adult – during a blood meal from white-footed mice,
the primary carriers of the Lyme disease bacterium. (While the abundance of
deer ticks is casually associated with deer, these hosts do not transmit Borrelia burgdorferi to ticks, and deer are not
considered an important host for the maintenance of the bacteria in wildlife
populations.)
But the bacteria doesn’t lead to Lyme disease in either the mice
or the ticks. In pilot studies, Couret has seen changes in the ticks that
acquire the bacteria – including behavior, metabolism, respiration, and
survival. So there appears to be an advantage for those ticks, she said.
“That’s a shift in mindset,” said Couret, who joined URI in 2015
after earning her Ph.D. in the ecology of infectious diseases at Emory
University. “We mainly think of Borrelia burgdorferi as
a pathogen because it causes Lyme disease in humans. We are studying the
transmission cycle of the bacteria in nature between ticks and white-footed
mice. It’s possible that it’s not acting as a pathogen, but rather as a
beneficial symbiont of the tick, a partner. The bigger picture question is, if
we view Borrelia burgdorferi with this lens, can we better
understand its transmission dynamics?”
In understanding the transmission cycle of Lyme disease, the
researchers will explore the relationships of many influences on the bacteria
in the tick, including environmental factors, such as temperature and humidity;
ecological facets, such as the tick’s microbiome; and the bacteria’s
interactions with other organisms in the tick.
“We’re studying the effects of the bacteria on ticks at different
levels, from gene expression to behavior,’’ she said. “We’ll combine that
information to look at the evolutionary fitness of ticks, and model the impacts
of bacteria on annual tick populations. We also are considering the microbiome.
We want a really comprehensive view of the ensemble of ecological interactions
that influence ticks, Borrelia burgdorferi,
and their partnership.”
For the study, Narasimhan, a molecular biologist, will look at
gene expression to learn what is changing in the ticks that acquire the
bacteria, along with what is changing in the bacteria. Lord, a vector-borne
disease modeler, will incorporate the experiment results in a model that can
predict tick populations and rates of transmission of Borrelia burgdorferi.
Tsao, a tick ecologist, will study deer ticks in the Midwest,
another hot spot of Lyme disease. Paralleling Couret’s work in Rhode Island,
Tsao will study tick behavior and development in a semi-natural environment.
Tsao and Couret will also look at traits that may be affected by the presence
of Borrelia burgdorferi, effects of environmental conditions,
survival rates, and gene expression.
When it’s completed, the study will greatly expand our
understanding of the factors driving the maintenance of Lyme disease in
wildlife. Findings could eventually lead to ways to control the deer tick
population or inform disease prevention measures, Couret said.
Also, by characterizing the role of the microbiome as it relates
to tick-Borrelia interactions, the research could lead to novel methods of
biological controls, such as finding competing bacteria within the tick that,
when present, negatively impact Borrelia burgdorferi transmission.
A unique aspect of the grant is the heavy focus on providing
comprehensive mentorship for trainees, centering the experiences of those who
have been marginalized in science and supporting the team through professional
development across all four institutions involved. Called the Microbiome
Integrated Tick Ecology Network – or MITEY Network, as in mites – the mentoring
will send trainees to each partner University to sharpen science skills,
promote sustainable and productive writing practices and science communication,
support a growth mindset, and reduce imposter syndrome.
“We want to make sure it’s an inclusive research culture and
environment for our trainees,” Couret said.
