Ticks are small, bloodsucking arachnids that live as external parasites on terrestrial mammals, birds, amphibians and reptiles. Ticks belong to the order Ixodida, a group of parasitic mites in the superorder Parasitiformes. This superorder also includes the varroa mite, a destructive parasite of honeybees. About 900 tick species are known worldwide, of which 48 have been found in Canada. Although they are most diverse in tropical regions, ticks continue to be significant pests to humans and other mammals as far north as the Canadian tundra.
Ticks are quite large compared to other mites; adults are typically 1–5 mm long when unfed and up to 20 mm when fully engorged with blood. Most ticks are brown or reddish-brown, and some are marked with white patterns on their backs. Larvae are usually less than 1 mm long and have six legs, while nymphs and adults are progressively larger and have eight legs. Two families of ticks are found in Canada; the hard ticks (Ixodidae; 41 species) have a hard plate on their back called a scutum, while the soft ticks (Argasidae; 7 species) do not. The scutum's shape, size and colouration are often used to tell species of hard ticks apart. Ticks have highly modified mouthparts, including a specialized piercing structure called a hypostome. The hypostome is covered in rows of backwards-directed teeth, resembling a round file or rasp, which anchor the tick in place while it feeds. Nymphs and adults of hard ticks have conspicuous, forward-projecting mouthparts, while the mouthparts of soft ticks project downwards and are thus hidden from above.
Distribution and Habitat
Ticks are found worldwide and feed on various mammals, birds, amphibians and reptiles. In Canada, they are primarily found below the treeline in temperate and boreal regions but may survive anywhere they can reliably find a suitable host.
Reproduction and Development
Ticks' life cycles involve four stages: egg, larva, nymph and adult. Eggs are laid on the ground or in sheltered locations, and larvae must seek out their hosts after hatching. In most cases, ticks find their hosts by climbing vegetation and waiting for an animal to brush past so they can grab on with their outstretched forelimbs. This behaviour is called questing, and it requires physical contact between the tick and the host, as ticks do not jump or drop onto their hosts. Larvae, nymphs and adults of both sexes feed on blood. Most species take a single blood meal before dropping to the ground, moulting and then seeking a new host. Therefore, most ticks complete their life cycle on three separate animals. However, a few species complete their entire life cycle on a single animal, while others feed on one host until the nymph stage and then seek a second host as adults. Adult ticks mate after their final moult, then females feed and produce eggs. The life cycles of most soft ticks differ in that they feed intermittently and attack many hosts in a lifetime, usually crawling onto their hosts at night as they sleep.
Ticks feed on the blood of vertebrates, especially mammals and birds, and can significantly impact wildlife populations through blood loss and disease transmission. For example, winter ticks (Dermacentor albipictus) pose a major threat to moose in the Northeastern United States, where they may be present in the thousands on a single moose. Their excessive numbers frequently cause death from blood loss and starvation. Since the winter tick completes its life cycle on a single host animal, it can remain active throughout the winter thanks to the warmth of its host.
Nearly all other ticks spend the winter in an inactive state, waiting for warmer temperatures to move around and find new hosts. For this reason, cold winters naturally limit tick populations to manageable levels. However, due to the impacts of climate warming, ticks are increasingly active throughout the year, and their ranges have steadily expanded northward, making them a threat to more and more Canadians.
In Canada, ticks can be a health concern for humans, livestock and game animals alike. Ticks can harbour and spread various disease-causing pathogens to their hosts through their saliva. These include bacterial diseases such as Lyme disease, Rocky Mountain spotted fever and tularemia, as well as diseases caused by protozoa (e.g., babesiosis) and virus (e.g., Colorado tick fever, Powassan encephalitis). Lyme disease, in particular, has received increasing attention due to the expanding populations of the ticks that spread it: the black-legged tick (Ixodes scapularis) in the East and the western black-legged tick (Ixodes pacificus) in the West. Lyme disease is caused by the bacterium Borrelia burgdorferi, which is spread to humans and other animals by infected ticks while they feed. Over 15 per cent of black-legged ticks in established populations usually carry the bacterium. In most cases, infected ticks must remain attached to their host for at least 24 hours to transmit the bacterium. In Canada, there have been over 17,000 reported cases of Lyme disease from 2009 to 2022, with more than half of those reported from 2019–22.
Besides transmitting disease, some tick species can cause a condition known as tick paralysis, which is caused by toxic components of their saliva. Tick paralysis normally only occurs after a tick has been attached to its host for several days. In Canada, most cases occur in restricted parts of South-central British Columbia and Manitoba, primarily affecting livestock.