American Dog Tick

Dermacentor variabilis — the American dog tick — is the most familiar 'wood tick' in North America and one of the most clinically important tick species on the continent. The species is widespread across all of eastern and central North America from southern Canada south through the eastern US to Mexico, and west to the Rocky Mountains. Adults are 3-5 mm long (engorged females reach 1.5 cm), reddish-brown, with the species' diagnostic features: ADULT FEMALES have a small ornate WHITE-OR-GRAY DORSAL SCUTUM (the hard plate behind the head) decorated with intricate pattern; ADULT MALES have white-and-gray ornate patterns covering most of the dorsal surface. The species is the primary North American vector of ROCKY MOUNTAIN SPOTTED FEVER (RMSF) — caused by the bacterium Rickettsia rickettsii, transmitted through tick saliva during prolonged feeding. RMSF is one of the most LETHAL TICK-BORNE DISEASES in NA: case fatality rates are 5-10% even with prompt antibiotic treatment (doxycycline) and 20-30% without treatment. The disease causes high fever, severe headache, characteristic rash (small red spots that begin on wrists and ankles and spread to trunk and palms/soles — the rash is diagnostic but appears 3-5 days after fever onset, often delaying treatment), and progressive systemic symptoms including organ damage. Despite the common name, RMSF actually occurs predominantly in the SOUTHEASTERN US and central states (the Rocky Mountain name is from where the disease was first described in the late 1800s, but the disease center has shifted east over the past century). The species also transmits TULAREMIA (caused by bacterium Francisella tularensis) and is occasionally implicated in TICK PARALYSIS — a rare but serious condition caused by neurotoxins in tick saliva that cause PROGRESSIVE ASCENDING PARALYSIS in the host. Tick paralysis begins with weakness in the legs, ascends to the trunk and arms, and can cause respiratory failure if the tick is not removed. The paralysis resolves rapidly (within hours to days) once the engorging tick is removed — making prompt tick removal essential. Despite the 'dog' in the common name, the American dog tick readily attaches to humans, dogs, livestock, deer, raccoons, and many other vertebrate hosts. The species is one of the most-encountered ticks in NA outdoor recreation and is the focus of major CDC tick-borne disease prevention programs.