While mice have often been blamed as the primary host for Lyme disease — spread through tick bites — a new study found that shrews may be the main carriers on Martha’s Vineyard.
The findings were made through a cooperative effort between Tufts University and the Martha’s Vineyard Tick-Borne Illness Reduction Initiative.
“The bottom line is that shrews appear to be as important as mice in transmitting Lyme, not that mice are unimportant,” Heidi Goethert, the Tufts University research professor who led the study, said.
The study, published in the American Journal of Tropical Medicine and Hygiene on Nov. 12, analyzed 218 deer tick nymphs from Christiantown Woods in West Tisbury, collected between 2019 and 2023. These arachnids were provided by Patrick Roden-Reynolds, who leads the Martha’s Vineyard tick program. The researchers used a bloodmeal assay to distinguish which animals the tick drew blood from.
Of the 218 nymphal ticks, the researchers found 20 percent were infected by Borrelia burgdorferi, the bacterial agent of Lyme disease. Among the various mammals the tick nymphs fed on, shrews were found to have fed the largest proportion of larvae (around 41 percent) and contributed to the most infected nymphs (39 percent) on the Island.
Goethert said this was a surprising result, because it conflicted with prior knowledge of how Lyme was transmitted, especially because the research showed that shrews were the main hosts for five consecutive years.
“Our understanding of Lyme disease transmission in nature has revolved around the thinking that mice are the most important host,” Goethert said.
That brings into question the hypothesis that ticks only fed on shrews when mice were scarce.
“We knew that other animals, such as shrews and chipmunks, contribute to the transmission cycle, but they were always thought to be very minor contributors compared with mice, except for the short intervals during years when the mouse populations are low,” Goethert said, although admitting the caveat with the study was that the researchers did not know the mice population on Martha’s Vineyard.
Deer tick nymphs from another community, Nantucket, were also found to have had a “significant positive association” of feeding on shrews and getting infected by the Lyme disease agent.
“I’m hoping that researchers will approach new field studies with a more open mind,” Goethert said. “We have focused all our efforts solely on mice for so long that we have missed the contribution of other hosts. So, yes, I’d like them to look at shrews. It would be important to know how much they contribute to the transmission cycle on mainland sites, which have more diverse host communities.”
Still, that doesn’t mean researchers can discount white-footed mice, the species that had long been thought of as the main host of Lyme disease, since they are still a major carrier of Lyme disease.
“[Mice] on Martha’s Vineyard fed only 7 percent of the larvae, but were responsible for having fed 23 percent of the infected nymphs,” the study reads.
Although uncertain why, the study states, “Hosts that appear to feed fewer ticks overall disproportionately contributed to infected nymphs” on both Islands.
“We have always assumed that the importance of a host depends directly on the number of ticks it feeds,” Goethert said. “This work shows that we cannot discount hosts that feed seemingly fewer ticks.”
For perspectives on the study from the Vineyard, Roden-Reynolds said the omnivorous mice are typically “far more abundant” than shrews, which are carnivores and insectivores. Although he wasn’t sure how many shrews are on the Island, Roden-Reynolds said, the small hunters’ diet can limit their population compared with mice.
“A lot of the research in the past … have been focused on white-tailed deer and mice, which are the main players, but new research [is] going to look at the new players,” Roden-Reynolds said.
Thomas French, a consulting biologist who works with the Massachusetts Division of Fisheries and Wildlife, was surprised by the study’s outcome. However, he pointed out that all of the mammals on Martha’s Vineyard are fed on by ticks to some degree, even bats.
While the study did not specify what type of shrew the deer tick nymphs fed from, French said it was likely the northern short-tailed shrew. He said a northern short-tailed shrew is nearly as large as a white-footed mouse, and is more common than the other shrew species that lives on the Vineyard, the “much smaller … and probably less common, certainly less often seen” masked shrew.
“Shrews are a very understudied group,” French said.
Matthew Pelikan, a Vineyard naturalist and columnist of The MV Times’ “Wild Side,” concurred the short-tailed shrews are likely fairly common on the Vineyard, but they’re “secretive” animals that aren’t seen often.
“Most of the ones I’ve seen have been dead,” Pelikan said.
Lyme isn’t the only disease Tufts researchers had found in infected ticks feeding on shrews. In a 2021 study published in Communications Biology, 65 percent of 20 black-legged tick nymphs — collected from several parts of southern New England, including Martha’s Vineyard — infected with Powassan virus had been found to have fed on shrews, not mice. This led to the team’s conclusion that shrews were a likely host for the virus, which is a rare disease, but with cases on the rise.
Although shrews seem to carry some tick-borne illnesses, Goethert said she doesn’t consider them a public health concern.
“People have almost no exposure to shrews,” Goethert said. “Most have never seen one, and don’t even know what they look like.”
And the study’s results don’t really affect Vineyarders’ everyday lives, Goethert said, as there aren’t any “specific interventions that target shrews.”
“Personal prevention measures remain the best way to prevent exposure to all the tick-borne illnesses: perethrin-treated clothing, long pants, pants tucked into socks, and a daily tick check,” she said.
Goethert also said the Vineyard is lucky to have on-Island experts like Roden-Reynolds, who can assess residents’ yards for ticks and advise how to decrease tick exposure around their homes.
