As the climate warms, our Biosphere is changing.
Our new normal may mean a good habitat for species that used to be confined to more southerly environments. This is the case with the striped bass, and recreational fishers seem to be welcoming the increased abundance of this sportfish.
Another invader which is increasing in abundance is not so popular. Ticks are becoming more prevalent in this province, and according to the most recent risk map (March 2019), part of our Biosphere has been designated by the province as “higher risk” for Lyme disease, transmitted by the black-legged tick. The story of tick abundance, the prevalence of Lyme disease and their relationship to climate change is complicated and includes many players.
The black legged tick and all of its close relatives have three life stages and all three require one blood meal before moulting to the next stage. It is during these blood meals that the spiral-shaped bacterium which causes Lyme disease is passed from one animal to another. It is important to note that not all black-legged ticks carry the Lyme bacterium. Each tick life stage tends to prefer a different type of animal for its meal.
Let’s start the story with the tick egg which is laid on the forest floor in May or June. That is probably why May has been declared National Lyme Disease Awareness Month. Optimal conditions for the tiny eggs include adequate moisture and warmth. These favourable environmental conditions are progressively increasing in our forests due to climate change.
In mid to late July, the eggs hatch and the larva seeks a blood meal from a small mammal or bird. Although white-footed mice provide the most common first meal for black-legged ticks, they have been known to feast on shrews, chipmunks, rats, squirrels, deer mice, raccoons, robins, and song sparrows. Any port in a storm!
At this stage the larval ticks can move farther afield as they hitchhike on birds or mammals on the move. The newly-hatched larva does not harbour the Lyme-producing bacteria but may pick it up from this first animal host. It attaches to that first host and feeds for three to five days at which point it drops off and moults into a nymph. Nymphs seek sustenance from small mammals and birds. During this meal they can pick up the Lyme-producing bacteria even if they managed to avoid it during their first feed.
Larger scale transport is likely at this stage if the host flies or runs some distance while the nymph is attached. If the nymph fed in the autumn, after it falls off the host, it will moult into an adult which can then overwinter in forest floor litter where the temperature is above 4oC. In our forests these conditions are possible near the forest floor even under the snow.
So, every season is tick season and, despite the ‘estimated risk” maps, ticks do occur throughout our province. Lyme disease can be transmitted to humans at the tiny nymph or larger adult stage if that human is unlucky enough to be targeted as a feeding station. If the unfortunate nymph doesn’t manage to find a suitable meal ticket in the autumn, it can overwinter and then carry on with its food quest when spring arrives.
After a good feed either in the fall or next spring, the nymphs moult into adults. Adults prefer to feed on white-tailed deer which is how they acquired the label deer ticks. Abundant deer are required to maintain an endemic deer tick population. However, the tiny insects have also been documented to feed on other large mammals such as humans, dogs, cats, raccoons, bears, and horses. Any port in a storm, again. The entire life cycle takes between two and four years, depending on the time it takes each life stage to find a host.
Not only is leaf litter important for overwintering ticks, it also provides optimal conditions for other seasons because it prevents ticks from drying out. Thus, they are usually more abundant on forest floors than they are on forest edges or grasslands. Transmission of Lyme disease to humans or other animals such as your beloved family dog is through a bite by an infected nymph or adult tick.
Transmission is not immediate but requires the tick to feed for at least 24 hours. So, you can still enjoy hikes without worrying. Periodic visual inspection of yourself and your dog will reveal the presence of ticks before they have attached for that period. Enjoy the natural surroundings of our Biosphere.
Dr. Annamarie Hatcher is a consulting ecologist and a board member of the Bras d’Or Lake Biosphere Reserve Association. For more information about the Bras d’Or Lake Biosphere Reserve Association, please visit http://blbra.ca/ or our Facebook page (https://www.facebook.com/blbra/).