Deer and mice are Lyme disease culprits
There are two animals on Block Island that are primarily responsible for spreading tick-borne disease: the white-tailed deer, which produce 1,000 larvae per tick lifecycle, and white-footed mice that carry infectious pathogens. The deer play a role in increasing the tick population while mice spread infectious diseases to ticks on the island.
That information was presented at the July 11 Ocean View Foundation event featuring Dr. Maria Diuk-Wasser, and a student, Samantha Kay. During a five-year research period, Diuk-Wasser, an Associate Professor in the Ecology, Evolution and Environmental Biology Department at Columbia University, and her students, examined Block Island’s ecological relationship to ticks, their hosts and tick-borne diseases.
While presenting her research, “Eco-epidemiology of Lyme Disease and Babesiosis on Block Island,” Diuk-Wasser discussed the environmental drivers for the emergence of tick-borne pathogen related infectious diseases on the island. She noted that Ixodes scapularis, the scientific name for a tiny, blacklegged tick, has been surviving and thriving, even in freezing temperatures, in the more densely vegetated localities on the island.
“It’s definitely increasing,” remarked Diuk-Wasser. “There’s something in the ecology that is driving it. If there are more ticks, there is going to be more of everything.” She said that although the white-tailed deer are responsible for increasing the tick population, other hosts, like mice and birds, are key contributors in spreading tick-borne diseases on the island.
Diuk-Wasser explained a tick’s two-year blood-feeding lifecycle. She said ticks hatch from eggs, and after the tick larvae (granular sized ticks) feed they molt into nymphs (medium sized ticks) that come out to feed in June. After the ticks molt again, they become adults (larger sized ticks) in the fall.
Diuk-Wasser said that before an adult tick dies it feeds on a deer for what’s called its “last meal.” When a female adult tick feeds on a deer they lay thousands of eggs, which hatch into larvae that feed on the deer, and then drop off. And, so goes the tick's lifecycle.
“The deer is essential to produce the ticks,” said Diuk-Wasser. “Most of the infections will be from nymphs. Adult ticks are bigger, and you can see them. June is the worst month for nymphs. That’s when most of the nymphs are out and looking for hosts.”
According to Diuk-Wasser, although the deer on the island produce a large number of ticks, they don’t transmit diseases. “The deer does not get the infection from the tick,” she said. “The blood of the deer actually kills the Borrelia in the tick." Borrelia is a paraisitic bacteria that is pathogenic to humans. "The Borrelia circulates in the other animals. The other animals maintain the infection.” The ticks acquire Borrelia from small, infected mammals, and occasionally birds.
“Deer serve a complex role here” on the island, said Kay, who is a M.A. student at Columbia University. “The deer are increasing the tick population density, and they’re also decreasing the nymphal infection prevalence. So, they’re increasing the number of ticks, but they’re decreasing the prevalence of the infected ticks.”
Diuk-Wasser said, “White-footed mice are super hosts. There are plenty here” on the island. “They produce a lot of infected ticks. The more mice you have, and the less of the other hosts, you get more infection. We expect there are a lot of infected mice and ticks on the island.”
“White-footed mice are super spreaders,” said Kay. “Eight of ten tick larvae that feed on a mouse will come off infected. That’s really high.” She said in contrast, “The bird population transmits less Borrelia than the mice. Totally contrastingly, are the white-tailed deer, which are not susceptible to Lyme disease. If they can’t obtain it, they can’t spread the infection.”
With that said, Diuk-Wasser believes “that the number of ticks produced is the most important factor. The deer are producing a huge number of ticks. But if they’re helping to reduce the infection, we want to know.” She said the deer population on the island could be reduced, but then the infection occurrence might not change. “It’s not good news,” she said, “because it means that reducing the deer may not be enough, or as efficient as we hoped.”
After the presentation, Diuk-Wasser was peppered with questions from the assemblage, some from people who have been infected with tick-borne diseases. About a dozen people raised hands after island resident Lotte Wolfe asked how many people in the audience had been infected with tick-borne disease.
Seasonal resident Jan Wampler, who has had Lyme disease, asked Diuk-Wasser if eliminating the deer would help in eliminating tick-borne diseases. “Yes,” she said. “But that’s not my decision. You won’t be able to eliminate them.” It’s been noted by the Deer Task Force that eliminating the island’s deer is impracticable. Diuk-Wasser also said it is "impossible" to eliminate the mice, but they could be treated with a vaccine.
Diuk-Wasser said she is willing to present her findings to the Town Council. “This is the last year of my research,” she said, noting that she needs funds to continue her basic operation on the island. “I don’t know if I can continue with my studies next year. And, we’re at a point where we can plan something to try to address the problem. I think we know enough.”