From UMass Amherst: In a residential backyard in Maine, Project ITCH researchers from the University of Massachusetts Amherst stumbled upon a surprise finding: rabbit ticks harboring a new type of bacteria related to a group of pathogens that can cause sometimes life-threatening spotted fever rickettsioses (SFR) infections in humans. The most common and deadly SFR is Rocky Mountain spotted fever, which has a death rate of 20-30% if not treated promptly with the antibiotic doxycycline.
The discovery of Rickettsia sp. ME2023 is a key piece of a puzzle that scientists at the UMass Amherst-based New England Center of Excellence in Vector-borne Diseases (NEWVEC) are pondering. Funded by a $10 million, five-year award from the Centers for Disease Control and Prevention (CDC), NEWVEC brings together academic communities, public health practitioners, residents and visitors across the Northeast to tackle vector-borne threats.
Identifying the new Rickettsia genotype was an unexpected bonus of NEWVEC’s Project ITCH (Is Tick Control Helping), which surveys residential properties across New England in an effort to develop best practices for tick control. As a service to the homeowners, the ticks collected are tested for pathogens, such as the bacteria Borrelia burgdorferi, which causes Lyme disease.
“We now know there is something different, something novel, than what was previously known,” says microbiologist Stephen Rich, executive director of NEWVEC and senior author of the research recently published in the journal Ticks and Tick-borne Diseases. “While these rabbit ticks don’t feed on people very often, there’s a possibility that they could spill over into systems where people could get exposed. We’re interested in figuring out the nature of this environmental risk.”
It was unusual for the researchers to encounter the rabbit ticks in the first place, though the species, Haemaphysalis leporispalustris, is found throughout North and South America. “We see mostly deer ticks and dog ticks biting people,” Rich says, “but rabbit ticks were collected in this one backyard in Maine.”
Lead author Guang Xu, research professor of microbiology on the NEWVEC team, tested the rabbit ticks for Rocky Mountain spotted fever-like bacteria and discovered they were infected. Xu conducted DNA sequencing tests that identified the distinct Rickettsia strain. It was unlike any other known strain but similar to one identified from rabbit ticks a few years ago in California, Candidatus Rickettsia lanei, which has caused severe cases of Rocky Mountain spotted fever-like illness. “Sequencing the DNA is the most definitive way to tell all these little bacteria apart,” Rich says.
NEWVEC collaborators at the University of Maine sent Xu more rabbit ticks to test. He found that 6.1% of 296 rabbit ticks collected from 38 towns across nine counties in Maine tested positive for the new Rickettsia genotype. “This wasn’t a needle in a haystack,” Rich says. “It looks like lots of the rabbit ticks there have this pathogen.”
Typically, across the country SFR infections are spread by several species of dog ticks, with the incidence climbing consistently in the past two decades, from 495 cases in 2002 to a peak of 6,248 in 2017, according to the CDC. (A new case definition took effect in 2020 and has resulted in an expected drop in incidence – to 1,292 in 2022.) Arkansas, North Carolina, Alabama, Missouri and Tennessee accounted for more than half the cases between 2018-2022.
SFR cases are rare in New England, but they do occur – even though the usual vector, dog ticks, have not been found to carry Rickettsia in this region. “So, it was a mystery,” Xu says. “Why are there some cases of Rocky Mountain spotted fever in New England? This finding may solve part of the puzzle. Maybe the rabbit ticks are the vector.”
Rich explains, “Although rabbit ticks don’t feed on people, there are ticks that feed on people that also feed on rabbits. So they could move this bacteria zoonotically from rabbits into people. For example, a deer tick could feed on a rabbit and pick up this infection from the rabbit tick and potentially infect a person with that.”
Rich is planning to collaborate with rabbit hunters in Massachusetts to collect more rabbit ticks from the region and advance understanding of its impact on public health.
“Leporispalustris is a very widespread tick. And the potential for this to be a widespread problem, not necessarily a common problem, exists,” Rich says. “So our research will be to dig in and figure it out.”
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