Tick exposure shifts earlier in warm years, but not longer

Clinical takeaway: Warm springs may bring patients with tick exposure in before clinicians expect it. Neither a hard winter nor low local deer numbers are good indicators of a lower number of ticks in a given season. 

Most clinicians keep a rough seasonal estimate for tick-borne illness in mind, about late spring through midsummer. They also carry a set of local cues for whether a given year will produce a lot of tick exposure. 

More than 450,000 Lyme disease cases are diagnosed annually in the United States, and nymphal blacklegged ticks account for the great majority of them. Diagnosis rests heavily on exposure history, so local risk assessment matters. The dominant heuristic has been deer: more deer, more ticks, more Lyme. 

Severe winters have carried the opposite association, that cold kills ticks off. Both cues were tested against more than three decades of continuous monitoring at six forest plots in Dutchess County, New York, where researchers tracked ticks, small mammals, deer, acorn production, and weather from 1991 through 2025. 

Prior work at these sites had shown that nymphs begin seeking hosts earlier as the local climate warms. But the questing period also ended earlier, so the total length of the season did not change. Cumulative degree-days above freezing, a measure of overall annual warmth, was associated with fewer nymphs rather than more. At a given mouse density, the warmest years supported roughly 45% fewer nymphal ticks than the coldest. 

Deer abundance varied more than sixfold across the study, yet showed no statistical relationship with nymphal tick density across the full dataset. Harshly cold winter days and harshly warm spring days did not enter the best-supported models at all. Ticks appear to escape temperature extremes behaviorally by moving deeper into soil and leaf litter. What did predict nymphs was mice: a strong mouse year was associated with about 40% more nymphs the following summer, and mouse numbers rose after heavy acorn production by red oaks the year before. 

Annual nymphal abundance varied more than 200-fold over the study period. The proportion of infected nymphs did not move nearly as much. Roughly 15% to 30% carried the cause of Lyme disease, Borrelia burgdorferi, across the study period, and roughly 30% to 50% carried at least one of the zoonotic pathogens the team monitored. 

Small mammals were live-trapped at each plot, tick density came from drag-sampling and from ticks attached to trapped animals, and deer abundance from bowhunter sighting rates. The findings are correlational. These plots sit in relatively intact forest. In nearby fragmented forest where mice dominate the host pool, mouse abundance does track infection prevalence, so the results may not transfer to suburban edge habitat. 

An early warm spring is a good reason to raise tick-borne illness in April. But fewer ticks in a very warm year does not necessarily translate to lower infection risk: the share carrying a pathogen stayed within a relatively narrow band across three decades of wide swings in tick numbers, so a patient with a single attached nymph faces roughly the same odds regardless of the local count. Tick abundance and infection prevalence are set by different processes, which means a light tick year changes how often people get bitten, not what a bite is likely to carry. 

"You would expect that a warming climate, which makes life better for the mice, would eventually make life better for the ticks, but we have not seen that so far," said Richard Ostfeld, a disease ecologist at the Cary Institute of Ecosystem Studies and co-director of the research program. 

Source: Ostfeld RS, et al. (2026 Aug 17) Proc Natl Acad Sci U S A. The ecology of Lyme disease: Long-term data, surprises, and a synthesis