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As climate change profoundly alters ecosystems in North America, a small parasite is wreaking havoc: the winter tick. This tick, now more prevalent due to milder environmental conditions, is severely affecting the winter survival of young moose in eastern Canada.

Climate change is allowing species such as the winter tick—an external parasite that feeds on the blood of large deer—to spread to new regions. Once rarer in eastern Canada, the tick is now well established there and is causing high mortality, especially among young moose.

The winter tick completes its entire life cycle by infesting a single host. In addition to moose, it can be found on other deer species, such as white-tailed deer and caribou, on which it has little effect.

Tracking young moose

Tick larvae are found in the layer of dead leaves on the ground during the summer. In the autumn, they search for a host to attach to as it passes by. Once they are on the host, they feed on its blood to develop into adults. Adults then mate on the host.

Towards the end of winter, female ticks consume the greatest amount of blood for reproduction. Once engorged with blood, the females detach themselves from the host and lay their eggs in the plant litter on the ground.

Our research team is working to understand the links between winter ticks, moose and environmental conditions in order to better predict how these relationships will evolve based on anticipated climate conditions. Our approach involved capturing and tracking young moose in five populations in areas ranging from southern New Brunswick to north of the St. Lawrence River.

We focused our efforts on moose aged 8 to 13 months whose low fat reserves, fast metabolism and high tick burden make them more vulnerable.

Ticks, moose: Recent co-existence

During each of the three years of our study (2020, 2022 and 2023), we captured about 20 young moose in each of the five populations studied. In addition to fitting each moose with a GPS collar, we reduced the tick load (or number) of half of the individuals using acaricides (a type of pesticide specifically designed to kill mites and ticks).

This allowed us to compare the behavior and winter survival of animals who live in the same environment but have varying degrees of infestation. The end of winter is a critical period for moose survival due to the depletion of their energy reserves. This is also the time when ticks consume the greatest amount of blood and when their effects on the condition and survival of moose are the most evident.

Our experience with a total of 280 calves allowed us to determine that the winter tick was responsible for the majority of the 67 winter deaths recorded and that most of these deaths would not have occurred in the absence of ticks.

Moose treated with an acaricide at capture, and therefore with low infestation levels, had a mortality risk approximately 94% lower (nine deaths out of 135 moose) than untreated moose with natural tick loads (58 deaths out of 145).

In addition, the tick load at capture increased the variation in blood indicators in untreated moose. The risk of mortality was also higher in regions where wolves, the moose’s main predator, were present, while the size of moose at capture reduced the risk of mortality. Finally, the survival rate of young males was generally lower than that of females of the same age.

Moose are more affected by winter ticks than other large deer species. This is because ticks and moose have only recently begun to co-exist. The northward expansion of the parasite, facilitated by climate change, has created spring conditions that are more conducive to its reproduction.

The fact that this cohabitation is so recent has meant that moose have not had enough time to develop mechanisms to get rid of the parasite. Other species, such as white-tailed deer, have cohabited with ticks for longer and have developed behaviors to get rid of them before they’re weakened.

Better planning for better management

Nevertheless, the long-term effect of climate change on ticks remains uncertain. On one hand, earlier snowmelt and later snowfall favor the survival of ticks and increase their chances of finding a host.

These two conditions therefore increase the likelihood that ticks will find a host and reproduce. But hot, dry summers like the summer of 2025 are harmful to ticks, as low humidity reduces the survival rate of their eggs.

There are limited management measures available to reduce the abundance of winter ticks. Since ticks must infest moose in order to reproduce, the abundance of the two species is closely linked. Therefore, reducing moose population density is one avenue to consider in problematic areas to limit the spread of ticks. Alternatively, forest management aimed at modifying soil temperature and humidity could also influence tick survival and abundance.

Our ongoing work aims to understand how forest structure influences the presence of ticks and to identify management practices that could limit their survival. We are also developing a model to predict tick infestations based on various environmental factors. These models will make it possible to better predict the impact of infestations on moose and adjust the way we manage forests and moose populations.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Citation: Milder winters, more parasites: Are moose at risk from winter ticks? (2025, December 16) retrieved 16 December 2025 from https://phys.org/news/2025-12-milder-winters-parasites-moose-winter.html

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