Many tick species can spread tick paralysis.
Ticks vary in neurotoxic capabilities, so no one knows how many are needed to induce paralysis. In some cases, it has been as few as two ticks.
Sometimes, the tick itself is the causative agent of disease. In addition to transmitting many pathogens, some tick species secrete noxious substances capable of causing disabling or lethal toxic conditions, termed toxicoses, in their vertebrate hosts.
The best known of these toxicoses is tick paralysis, which is an ascending motor paralysis generally assumed to be induced by substances introduced into the host with the saliva when the ticks attach and feed. Tick paralysis is relatively common in livestock and pets, causing injury or death to thousands of animals each year. Cases also occur in humans and may lead to death if the attached tick is not removed.
In North America, tick paralysis in humans and domestic animals has been reported most frequently from British Columbia and in areas of Montana, Idaho and Washington near the Canadian border.
Many tick species are important as the causative agents of tick paralysis in different parts of the world. In North America, they are Dermacentor andersoni and Dermacentor variabilis, as well as Amblyomma americanum. (The genus Dermacentor is a large one, with many members that have been implicated not just in the spread of tick paralysis, but also ehrlichiosis and other diseases.)
The predominant symptom in tick paralysis is disruption of motor coordination. After a brief (5 - 7 day) incubation period, paralytic symptoms appear while the tick is still attached. Paralysis affects the lower extremities first and gradually ascends through the body. Paralysis of chest muscles leads to death from respiratory failure. In most cases, removal of the attached tick (or ticks) terminates the condition and allows for a complete recovery.
For unknown reasons, only certain individuals in the population of paralysis-causing ticks are capable of being neurotoxic, even in species that are notorious for causing the disease. Perhaps for this reason, data on the number of ticks needed to induce paralysis are ambiguous. Studies of D. andersoni show that as few as two female ticks were sufficient to induce paralysis in young puppies; larger numbers were required for paralysis in larger animals. Tick paralysis is actually only one expression of a continuum of toxicoses that may result from tick parasitism. Tick bites also may cause allergic hypersensitivity and toxemias. Toxic reactions to tick bites have been known for many years. In contrast to tick paralysis, toxic reactions develop within a few hours.
Many other tick-borne diseases of medical significance are also known to exist. These include tularemia, Q fever, and others. Specifics of these diseases, which are less common in North America, are intentionally omitted in this cours