Ticks, of the type found in Sweden, can normally be active down to a temperature of about five degrees. But now a study from the Swedish University of Agricultural Sciences (SLU), SVA and Linköping University shows that ticks can be active at lower temperatures than that.

"We have found active ticks when it has been zero degrees or even minus degrees", says Petter Kjellander, professor of wildlife ecology at SLU, and one of the researchers behind the study.

"We do not claim that the ticks have come to life in such low temperatures, but that they have been on host animals when it was so cold outside.

Snow depth is irrelevant

During three winters – and at two locations in Sweden – researchers have mapped ticks on deer. The animals have been captured, had any ticks removed, and then released into the open. After a number of weeks, the same deer have been re-checked for ticks.

"We have seen that up to one in two deer have new ticks, which has enabled us to draw this conclusion. We have made measurements during several winters, which have had both rain and snow, and where the snow depth does not actually seem to have had any bearing on how many ticks have been found," says Petter Kjellander.

"Healthcare needs to be aware"

The researchers behind the study suspect that the ticks have come to life when, for example, deer have gone to bed and warmed up areas where ticks overwinter. This in turn means that the ticks apparently do not wake up at lower temperatures than previously known. However, the results show that the winter season itself does not prevent ticks from finding new hosts to drill into.

According to Petter Kjellander, healthcare needs to take into account that people can suffer from tick-borne diseases even in winter.

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