Dermacentor variabilis, often known as the American Dog Tick, is a 3-host tick that occurs mainly in the eastern United States (although it occurs outside this region as well), where adults are commonly encountered on dogs. These ticks generally feed on smaller mammals as larvae and nymphs and on larger mammals as adults. Although often found on dogs, these ticks will also feed on larger animals, such as cattle, horses, and even humans. Adults are vectors for Rocky Mountain spotted fever, tularemia, and canine tick paralysis. (Chan and Kaufman 2008 and references therein)
The 8-legged adult male and female D. variabilis ticks are typically brown to reddish-brown in color with gray/silver markings on their scutum (dorsal shield). Females have a very short scutum, present just behind the mouthparts, while the male scutum covers the majority of its dorsal surface. Once engorged with blood, a typical 5 mm female can enlarge to 15 mm in length and 10 mm in width. Larvae are yellow before blood-feeding and gray to black when engorged. Nymphs are a pale, yellowish brown before feeding; they become slate gray when engorged. Both larvae and nymphs have red markings near their eyes and lack any white on the scutum. Dermacentor variabilis ticks require a blood meal before progression from 6-legged larva to 8-legged nymph, and from nymph to adult, and adult females require a blood meal for egg production (typically laying between 4,000 and 6,500 eggs on the ground). Between each of these stages, the developing tick drops off its host and must then locate a new host for its next meal. Because larval and nymphal D. variabilis rarely bite humans, the adult tick is the primary stage of concern for humans. (Chan and Kaufman 2008 and references therein)
After hatching, larvae remain on the ground or climb onto vegetation, where they wait for small mammals, such as mice, to serve as hosts for their first blood meal. This host location behavior is known as "questing". Under favorable conditions, larvae can survive up to 11 months without feeding. After contacting and attaching to a host, larvae require from two to 14 days to complete blood feeding. After feeding, larvae detach from their host and fall to the ground, where they digest their blood meal and molt into the nymphal stage. This process can take as little as a week, although it often takes longer. Nymphs can survive six months without a blood meal. After successfully questing for their second host, which is typically a slightly larger mammal (such as a raccoon or opossum), the nymphs feed for three to 10 days. After engorging, they fall off the host, digest their blood meal and molt into an adult. This process can take anywhere from three weeks to several months. Adults can survive two years without feeding. Questing adult ticks climb onto a grass blade or other low vegetation, cling to it with their third pair of legs, and wave their other legs as a potential host approaches. As the hosts brush the vegetation, the ticks grab onto the passing animal. The complete life cycle requires at least 54 days to complete, but can take up to two years depending on host availability, host location, and ambient temperature. Mating occurs on the host and the female engorges within six to 13 days, after which she drops from the host to lay her eggs and then dies, thus completing the cycle. (Chan and Kaufman 2008 and references therein)
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