Dermacentor variabilis, also known as the American dog tick or wood tick, is a species of tick that is known to carry bacteria responsible for several diseases in humans, including Rocky Mountain spotted fever and tularemia (Francisella tularensis). It is one of the best-known hard ticks. Diseases are spread when it sucks blood from the host. It may take several days for the host to experience symptoms.

Though D. variabilis may be exposed to Borrelia burgdorferi, the causative agent of Lyme disease, these ticks are not competent vectors for the transmission of this disease. The primary vectors for B. burgdorferi are the deer ticks Ixodes scapularis in eastern parts of the United States, Ixodes pacificus in California and Oregon, and Ixodes ricinus in Europe. D. variabilis may also carry Anaplasma phagocytophilum, the causative agent of human granulocytic anaplasmosis, and Ehrlichia chaffeensis, the causative agent of human monocytic ehrlichiosis.

Dermacentor ticks may also induce tick paralysis by elaboration of a neurotoxin that induces rapidly progressive flaccid quadriparesis similar to Guillain–Barré syndrome. The neurotoxin prevents presynaptic release of acetylcholine from neuromuscular junctions.