Ticks are nature’s hitchhikers. They latch on to migratory birds, deer, mice, and other creatures, feed to their heart’s content, and then drop off to moult.
One study suggests that between 50 million and 175 million ticks may be dropped by migratory birds each year across Canada. Wherever they land becomes their new temporary home. Depending on the tick’s age and species, it may lay hundreds or thousands of eggs.
Whether it’s able to lay eggs — and whether they’re able to grow to maturity — is determined in part by what the climate is like where it touches down. In colder weather, ticks tend to grow more slowly, says Kateryn Rochon, an assistant professor of entomology at the University of Manitoba.
“The life cycle is lengthy when you get north because of temperatures, but as that changes and the conditions get better, the ticks can grow more quickly,” says Rochon. “It's a tough world out there. It's very likely not to make it to reproduction. But if the conditions are getting better and better, there's better likelihood of survival, better likelihood of finding a mate, and better chance of making it to reproduction.”
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It’s not unusual for ticks to land in areas in the north. What is new is that populations of them are now able to survive and, in many cases, thrive in places they couldn’t before. Species such as the black-legged tick and the American dog tick — both increasingly common in Ontario — carry diseases that can be passed on to humans. The black-legged tick (also called the deer tick), for example, is a carrier of Lyme disease, which, if not detected quickly, can cause fever, headaches, and nerve and tissue damage.
“The thing is, what makes it that all of a sudden ticks have been able to establish?” says Rochon.
Warm weather, moisture, and the availability of food create ideal conditions for the arachnid. Increasingly high temperatures have been linked to increased survival rates for the black-legged and other species of tick.
Although not all ticks flourish at the same temperature, warmer is generally better because it promotes faster growth, says Nicholas Ogden, senior research scientist with the Public Health Agency of Canada.
A tick larva attaches to a host and feeds. After absorbing sufficient energy, it drops off into leaf litter and ages into a nymph. The nymph then finds a host, where it will feed before falling off and developing into an adult.
“Development from egg to larva, engorged larva to nymph, engorged nymph to adult, adult to egg-laying adult are all processes the length of which depends on the ambient temperature,” says Ogden.
The warmer it is, the faster the ticks can move through these stages. A more compressed life cycle means that more of them will survive and establish populations.
And with those populations comes Lyme disease. In 2009, there were just 144 documented cases in Ontario; in 2017, there were 2,025.
Given the fact that temperatures are expected to rise still further, researchers predict that there could be a moderate risk of Lyme disease in all parts of Ontario by 2050.
“What happens first of all is that the ticks spread, and people get tick bites, which isn't a great problem until the bugs that cause Lyme disease start to become established and transmission cycles start to become established,” says Ogden. “What we have seen is the emergence of not just the tick with climate change, but Lyme disease. We're seeing exponentially increasing number of cases.”