For regions that experienced a boom in mouse populations last year, scientists say 2017 could see a surge in cases of Lyme disease.
Caused by bacteria transmitted through the bite of an infected tick, Lyme disease causes serious neurological symptoms if left untreated. While Lyme disease occurs primarily in the Northeast and upper Midwest, it is slowly progressing across the country with cases along the West Coast as well.
According to the Centers for Disease Control and Prevention, Lyme disease is the most common vector-borne illness in the US, with 28,453 cases confirmed in 2015— actual infections are probably much higher. In this case, the vector is ticks.
With no capability to stop the spread of Lyme disease across the country, scientists focus on understanding factors that contribute to its spread and virulence. According to Dr. Richard Ostfeld, disease ecologist with the Cary Institute of Ecosystem Studies, the single most influential factor in Lyme disease is not ticks, or even deer—it is white-footed mice.
In the summer of 2016, Ostfeld noted an explosion in the mouse population in the Hudson River Valley. Because of that, Ostfeld expects incidence of Lyme disease to be higher than would otherwise be expected this summer. "We're anticipating 2017 to be a particularly risky year for Lyme," Ostfeld told NPR.
White-footed mice serve as a reservoir for the bacteria that causes Lyme disease, and the story about Lyme increasingly centers around their habitat and lifecycle. In connecting the dots for transmission of Lyme disease, these small mice are responsible for infecting the ticks that infect humans. In addition to the Lyme bacteria, Borrelia burgdorferi, white-footed mice carry the parasite that causes babesiosis, and the bacterial infection anaplasmosis.
There is an unnaturally close relationship between white-footed mice and ticks. Unlike most mammals that groom themselves to remove ticks, such as raccoons, white-footed mice are comfortable being infested with upwards of 100 ticks on their face and ears. Ticks do not impact the health, survival, or reproductive success of white-footed mice. Eventually getting their fill, the ticks become infected with Lyme bacteria and other pathogens, then drop off the mice.
Ticks move through four life stages and require a blood meal at each phase, often taking two or three years to move through adulthood. Once infected, they can pass pathogens on to each host on which they feed.
In a study published in the journal Ecological Applications, Ostfeld was part of a research team that distinguished factors influencing incidence of Lyme disease. Those factors include ticks, mice, weather, land use, and even acorns.
The larger story starts with land clearance by European settlers. Today, with occupation, farming, and regrowth, the area remains beautiful, if fragmented. Yet the loss of homogeneous forest reduced the number of predators of white-footed mice and created ideal environmental conditions for their spread.
In that same landscape, deer have thrived and ticks have ample hosts on which to survive. In years with an abundance of acorns in the fall, tick larvae emerge to find a large number of squirrels and mice feeding on and storing acorns. The ticks have an ample food supply and so do mice and other hosts.
[T]hat means that two years following a good acorn crop we see high abundance of infected ticks, which represents a risk of human exposure to tick borne disease.
Depending on your region, ticks could be active even in the winter. Take steps to protect yourself by using permethrin repellent products on your clothes, tuck your pants into boots, and apply DEET repellent on exposed skin. Walk to the center of trails, avoid debris piles, brush, and grasses by the side of the path where ticks wait outstretched to climb aboard. Once indoor, shower immediately and do a tick-check, especially under arms, in the groin area, and in hair. If you suffer fever, rash, swollen lymph nodes, or aches in the days and weeks following exposure, check with your doctor.
With climate warming, the territory of white-footed mice and ticks expands north and westward. Given the mild weather this winter, for residents of the Northeast and upper Midwest? The mice, ticks, acorns, weather—and the experts—predict 2017 is going to be a bad year for Lyme disease.