The following appeared in the 2020 Jan/Feb issue of ADIRONDAC MAGAZINE
Photographs courtesy of Cary Institute/Pamela Freeman
The black-legged ticks that infect us with Lyme and other tick-borne diseases disappear in winter, right? Not entirely.
Although we are less likely to get a tick bite in winter, compared to spring or summer, tick encounters are still a risk on winter days when temperatures are above freezing. There are plenty of hearty ticks lurking in a state of (almost) suspended animation, waiting for a mild day so they can resume questing for a blood meal.
Adult ticks are the main risk in winter. Larval ticks that hatched last summer, but failed to find an animal host to feed on, become dormant and remain on the forest floor in a quiescent state. The same is true of nymphal ticks, which are normally most active in spring and early summer. Larvae and nymphs are unlikely to activate until day length and temperature increase dramatically.
In contrast, adult ticks most actively seek hosts in late fall. Females that have already fed are overwintering in soil pores or under leaf litter, slowly converting host blood into eggs that they will lay next spring or summer. Some adults who did not get a meal will die from starvation or other causes. But unfed survivors will activate any winter or spring day with above-freezing temperatures. In the Northeast, that means ticks can bite during any warmish spell in January, February, and beyond.
One argument for really cold weather
Risk of winter tick bites goes down when temperatures are below freezing, the ground is frozen, and there is snow cover. If temperatures rise above 35 degrees, and the ground thaws, hungry adult ticks can awaken from dormancy and go looking for a meal. The very same mild winter days that draw people and pets outdoors also put us at risk. Be on the safe side, and perform tick checks after outdoor activities any time the temperatures are above freezing.
Many have inquired if cold winters kill ticks. After monitoring tick populations and weather conditions for the past twenty-six years at Cary Institute, we find little evidence that winter cold snaps reduce the number of ticks that come out the following spring. Overall, the relation between weather and tick populations is still not well understood, but the picture is getting clearer with ongoing research.
While winter temperatures can affect tick activity, conditions in fall and spring play a larger role in determining tick abundance. When temperatures remain summer-like well into fall, ticks have more time to find a blood meal and survive to the next life stage. Late winter onset improves tick survival, leading to large numbers of hungry ticks come spring.
Anyone who has removed a tick knows they are tough. One of their weaknesses is a need for moisture. When spring conditions are warm and dry, ticks can die from dehydration, reducing tick abundance and risk of bites later in the year.
Increasing threat to public health
Expanding populations of black-legged ticks are an increasing threat to public health. Climate warming is partly responsible for the expansion of black-legged ticks northward and westward in New York State and elsewhere. It also contributes to tick movements upward in elevation, for instance to higher locations in the Adirondacks and Catskills that were formerly too harsh for ticks to survive.
When ticks move into new regions, they take residents and health-care providers by surprise. In newly invaded areas, people generally don’t engage in protective activities such as performing tick checks, wearing protective clothing, using repellents, and avoiding high-risk areas like brush piles, fallen logs, old stone walls, and patches of barberry shrubs. This puts them at high risk of being bitten by a tick and falling ill.
People often don’t suspect a tick-borne disease when they feel flu-like symptoms, and may not seek appropriate medical treatment. Even when they do, their doctors may not be aware of the symptoms of tick-borne infections. Early diagnosis and treatment vastly increase the probability of curing the disease with a simple course of antibiotics. Lyme disease that’s left untreated for weeks or months can be much more difficult to treat.
Basic research into the ecology of ticks—including which pathogens they spread, what animals they feed on, and how environmental conditions regulate tick numbers—is vital to our understanding of how these disease systems work and how to avoid illness. Applied research into controlling ticks and protecting human health is an important complement to this basic science.
What’s being done
Our group at CARY INSTITUTE OF ECOSYSTEM STUDIES in Millbrook, New York, together with Dr. Felicia Keesing at Bard College in Annandale-on-Hudson and partners at the Centers for Disease Control and Prevention, are designing and testing environmentally safe methods to control black-legged ticks in residential areas that have been hard-hit by tick-borne disease. As part of the TICK PROJECT, participating households record ticks found embedded in or crawling on people or pets. While sightings are the heaviest in spring and summer, we receive notifications year-round.
Our twenty-five years of data from Dutchess County, New York, show that as the climate warms, ticks come out earlier in the year, advancing the dates of greatest risk. Climate change will continue to expand the areas and seasons amenable to ticks. Researchers are working hard to predict impending risks, warn the public, and battle tick populations to protect human health.
Dr. Richard Ostfeld is a scientist at Cary Institute of Ecosystem Studies and an expert on the ecology of Lyme and other tick-borne diseases. He co-leads the Tick Project, a five-year study exploring ecological interventions with the potential to reduce Lyme disease incidence in residential neighborhoods.