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Ticks join new world order in infectious disease

Though only two varieties exist, they are responsible for a wide range of diseases

Black-legged tick.

By scientific estimate, ticks have been around for 100 million years. They represent the most famous blood-sucking arachnids, (eight-legged organisms).

The world is now in unchartered territory when it comes to infectious diseases. Over the past century, the number of new infectious diseases cropping up each year has nearly quadrupled. The number of outbreaks per year has more than tripled. In the past 25 years 15 new human diseases have appeared in North America and the disease agents that have existed forever move globally with increased virulence. Climate change is helping the northern extension of vectors and the diseases they carry. Ticks are among the agents responsible for the startling aberrations in disease prevalence we see today.

Ticks come in two varieties: soft ticks (Argasidae) and hard ticks (Ixodidae). Hard ticks get their name from the large and tough shield that they carry on their backs. Soft ticks lack this shield and have a leathery “skin” instead. Another difference between soft and hard ticks is the location of their mouth parts. Hard ticks have their mouthparts on the front of their body, giving the impression of a head, while soft ticks carry their mouthparts on their underside.

Most ticks go through four life stages: egg, six-legged larva, eight-legged nymph, and adult. After hatching from the eggs, ticks must eat blood at every stage to survive. The lifecycle of blacklegged ticks (causes Lyme disease) generally lasts two years during which they go through the four life stages. Generally, adult female hard ticks breed while on the host animal and then drop to the ground to lay eggs. A female lays several thousand eggs at a time, which will eventually hatch into the larval stage, known as seed ticks. Some species of tick larvae can live up to 540 days without feeding. Once a larva finds a host, it feeds for approximately five days then drops off to molt into a nymph.

Ticks do not fly or jump. Instead, they wait for a host, resting on the tips of grasses and shrubs in a position known as “questing.” While questing, ticks hold onto leaves and grass by their lower legs. They hold their upper pair of legs outstretched, waiting to climb onto a passing host.

Depending on the tick species and its stage of life, preparing to feed can take from 10 minutes to several days. Many species also secrete a cement-like substance that keeps them firmly attached during the meal. The feeding tube can have barbs, which help keep the tick in place. Ticks also can secrete small amounts of saliva with anaesthetic properties so that the animal or person can’t feel that the tick has attached itself and often feed unnoticed. Pathogens are ingested then transmitted during the next feeding. Once infected, a tick can transmit infection throughout its life and can transmit infectious organisms to succeeding generations in the life cycle.

The longer the tick is attached, the greater the risk of acquiring disease from it.

Ticks become efficient carriers of disease organisms because they readily adapt to climate change; often involve multiple species during a lifecycle, including humans; feed for long periods of time; and infectious agents pass from stage to stage in the lifecycle.

The list of diseases ticks transmit include:

  • Lyme disease: Lyme disease is the most prevalent tick-borne disease of humans and animals in Canada and the United States. Cases are common in Northeast, the upper Midwest, and California. Lyme disease, caused by a bacterial spirochete Borrelia burgdorferi, occurs across Canada in horses, cattle, dogs, and cats.
  • Anaplasmosis: Cattle and humans (different diseases) — Anaplasmosis in cattle is a growing concern across the Midwestern U.S. Anaplasmosis in cattle is an infectious disease of the red blood cells caused by the rickettsial bacteria Anaplasma marginale. Most commonly transmitted by ticks, A. marginale primarily causes disease in cattle. Other domestic and wild ruminants such as bison, deer, elk, sheep and goats can be infected, but clinical disease is uncommon. Needles and surgical instruments can also transmit the disease.
  • Babesiosis: Bovine babesiosis (BB) is a tick-borne disease of cattle. Tick fever drastically changed the movement of cattle from the southern U.S. to Canada during the early days of ranching and remains a threat in cattle moving into the U.S. from Mexico.
  • Ehrlichiosis: Dogs, humans
  • Rocky Mountain Spotted Fever (RMSF): Rocky Mountain spotted fever is a potentially fatal rickettsial disease of dogs and humans caused by R. rickettsii, and R. parkeri. Dogs serve as an important sentinel function for humans.
  • Hepatozoonosis: Dogs
  • Hemoplasmosis: Dogs, cats
  • Tick-borne encephalitis: Humans
  • Tick paralysis: Dogs, horses, humans — Tick paralysis in animals is caused by a salivary neurotoxin produced by certain species of ticks. Paralysis is usually caused by feeding adult females, but can be induced by large numbers of larval or nymphal ticks. Tick paralysis occurs worldwide.
  • Red meat allergy: Meat from any kind of mammal — beef, lamb, pork, goat, and even whale and seal — can cause an allergic reaction. Recently, scientists identified meat allergies initiated by a bite from the Lone Star tick. Both beef and pork are involved. The Lone Star tick is found predominantly in the Southeast from Texas, to Iowa, into New England, but has now appeared in Canada. A meat allergy to the alpha gal secreted by ticks can develop at any time in life. Poultry may also be involved. Symptoms may vary from simple rashes and respiratory signs to severe anaphylaxis.

There’s no answer to the question why the Creator made ticks. Most people agree that they’re just there. The tick’s place in the great web of life seems to be to transmit disease and in doing so, control populations.

About the author


Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).



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