For decades, scientists have relied on evidence that showed mice as the main source of Lyme disease in the Northeast. That may still be true in many places. But on Martha’s Vineyard, the top culprit is another tiny mammal: the shrew. That finding, based on six years of gathering ticks from the island, comes from researchers at Tufts Cummings School of Veterinary Medicine.
Study co-author Sam Telford said Lyme disease researchers have suspected that mice couldn’t be the only source of Lyme disease because control efforts that targeted mice produced some disappointing results. So Telford’s co-author Heidi Goethert, a research assistant professor at Tufts Cummings School of Veterinary Medicine, decided to try something new.
The mice/Lyme theory is based on studies of animals caught in traps. But it turns out that shrews, moles, squirrels, chipmunks and other possible carriers of the bacteria that causes Lyme disease are harder to trap than mice, or don’t respond to the same bait. So Goethert collected infected ticks instead and figured out which animals they’d been feeding on.
On Martha’s Vineyard, “shrews were in fact the most important host and not mice,” Goethert.
Goethert and colleagues conducted the same experiment on Nantucket. There, more infected ticks had fed on mice than shrews, but it was close: 35% to 31% respectively. On both islands, squirrels were also linked to infected ticks.
In addition, Geothert found that shrews are important in the transmission of two other tick-borne illnesses: babesiosis and Powassan virus.
“It seems like shrews in general are going to be much more important than we previously appreciated,” she said.
It’s not clear if shrews, squirrels and other mammals are important contributors in other parts of the Northeast with high rates of Lyme disease. The approach Goethert used in this study is relatively new. Telford said communities will need to determine which rodents and insectivores, including shrews, to target, in addition to the white-footed mouse.
For mice, there are products on the market and state-led programs aimed at reducing the spread of Lyme disease. Both Martha’s Vineyard and Nantucket allow deer hunting to reduce the numbers of these large animals where ticks live and breed. Telford said no one knows how to intervene with shrews.
“Stay tuned — we’re trying to get refunded by the National Institutes for Health to explore that,” said Telford.
Massachusetts’ state epidemiologist, Dr. Catherine Brown, said in an email that the study adds to our understanding of Lyme disease. But Brown said even when animal control efforts reduce tick populations in small areas, they don’t “consistently and successfully reduce cases of Lyme disease in humans.”
So, Brown said, the best way to avoid getting Lyme disease is to avoid getting bitten by a tick.
The study authors, including Patrick Roden-Reynolds, agree. Roden-Reynolds is the public health biologist for Martha’s Vineyard and Nantucket who collected tick samples for this research.
“A combination of doing tick checks, wearing clothing that covers exposed skin and then treating your clothing with permethrin or other repellants like DEET is effective,” said Roden-Reynolds who works with the Martha’s Vineyard Tick-borne Illness Reduction Initiative. In three years of being out in nature on the island, Roden-Reynolds said he hasn’t had a single bite.
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