Environmental Dermatology

What’s Eating You? Clinical Manifestations of Dermacentor Tick Bites

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Dermacentor ticks are hard ticks found throughout most of North America and are easily identified by their large size, ornate scutum, and prominent dorsal pits. They are important disease vectors and are implicated in transmission of Rocky Mountain spotted fever (RMSF), Colorado tick fever, tularemia, and erlichiosis. They also are an important cause of fatal tick paralysis.


 

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Background and Distribution

The Dermacentor ticks belong to the family Ixodidae (hard ticks). The 2 best-known ticks of the genus are Dermacentor andersoni (Rocky Mountain wood tick)(Figure, A) and Dermacentor variabilis (American dog tick)(Figure, B). The Dermacentor ticks are large ticks with small anterior mouthparts that attach to a rectangular basis capituli (Figure, A). Both ticks exhibit widely spaced eyes and posterior festoons as well as bifid coxa 1 (the attachment site for the first pair of legs) and enlarged coxa 4. As adults, these ticks display an ornate hard dorsal plate, or scutum, with numerous pits. Female ticks have a much smaller scutum, allowing for abdominal engorgement during feeding.1 Although D andersoni tends to have a brown to yellow hue, the specimens of D variabilis display a somewhat silver color pattern.

Dermacentor andersoni is characterized by wide-spaced eyes, posterior festoons, and an ornate scutum, with small anterior mouthparts that attach to a rectangular basis capituli (A). Dermacentor variabilis is characterized by wide-spaced eyes, posterior festoons, dorsal pits, and an ornate scutum with a somewhat silver color pattern (B).

Dermacentor ticks can be found throughout most of North America, with the northern distribution limits of both species previously occurring in the province of Saskatchewan, Canada. Although the range of D andersoni has remained relatively stable within this distribution, the distribution of D variabilis recently has expanded westward and northward of these limits.2 The ranges of the 2 species overlap in certain areas, though D andersoni primarily is found in the Rocky Mountain and northwestern states as well as southwestern Canada, whereas D variabilis can be found throughout most parts of the United States, except in the Rocky Mountain states.3 Within these regions the ticks can be found in heavily wooded areas, but they most commonly inhabit fields with tall grass, crops, bushes, and shrubbery, often clustering where these types of vegetation form clearly defined edges.4 The diseases transmitted by the Dermacentor ticks include Rocky Mountain spotted fever (RMSF), Colorado tick fever, tularemia, tick paralysis, and even human monocytic erlichiosis, though Amblyomma americanum is the major vector for human monocytic erlichiosis.

Rocky Mountain Spotted Fever

Both species of ticks are known to serve as vectors for RMSF, but D variabilis is the major vector in the United States, especially in the eastern and southeastern parts of the United States. Overall, the majority of cases occur in North Carolina, South Carolina, Tennessee, and Oklahoma,5 with North Carolina having the highest incidence. In endemic areas, RMSF should be suspected in any patient with fever and headache, and empiric treatment with antibiotics should be started while awaiting the results of diagnostic tests. Serologic testing with indirect fluorescent antibodies is widely available and is considered the best method for detection; although the sensitivity is poor during the first 10 to 12 days of infection, it increases to 94% during days 14 to 21.6 Therapeutic decisions should be influenced by clinical suspicion and epidemiologic data. Treatment should be started promptly and should never be delayed until confirmatory tests are available. Doxycycline is considered the gold standard therapy in both adults and children, with a typical treatment duration of 10 days. The only other recommended agent for pregnant women in the first or second trimesters or patients with severe hypersensitivity reactions to tetracyclines is chloramphenicol.7

Colorado Tick Fever

Colorado tick fever, also known as mountain fever, is an arboviral infection transmitted by D andersoni. Its distribution coincides with the tick’s natural geographic range in the western United States and Rocky Mountains. Colorado tick fever causes an acute febrile illness consisting of chills, headaches, myalgia, retro-orbital pain, and malaise, which tend to occur within 3 to 5 days of the tick bite. Some cases may be accompanied by a nonspecific rash that may be morbilliform or petechial in appearance. Notably, approximately half of all patients will experience transient resolution of symptoms for 24 to 48 hours followed by a recurrence of fever, a phenomenon that has been referred to as saddleback fever. Routine laboratory findings may include leukopenia, thrombocytopenia, and a peripheral smear with atypical lymphocytes. Reverse transcription polymerase chain reaction is both sensitive and specific for detecting viral loads in the blood during the first week of infection, though testing does not alter management, which is largely supportive.8

Tularemia

Tularemia is a relatively rare disease but has been documented in every US state except Hawaii.9 The disease is caused by Francisella tularensis, a small, aerobic, gram-negative coccobacillus transmitted via inhalation, bitingflies, or tick bites; the most common ticks to transmit the disease include D andersoni, D variabilis, and A americanum.10 Clinical manifestations depend on the form of exposure, with tick bites most often resulting in an ulcerated skin lesion at the site of the vector bite accompanied by regional lymphadenopathy and systemic symptoms such as fever, chills, myalgia, and headache.11 Mucosal manifestations such as pharyngitis, conjunctivitis, and other ocular lesions also are commonly seen. Diagnosis most frequently is made using serology because F tularensis is both challenging and dangerous to culture; in fact, because of the high risk of contagion, F tularensis should only be cultured in biosafety level 3 laboratories. Polymerase chain reaction assays can be used on tissue samples with decent sensitivity (78%) and specificity (96%); however, these assays cannot distinguish between Francisella subspecies and are not readily available to most clinicians.12 First-line therapy for the treatment of tularemia is streptomycin given as twice-daily intramuscular injections over the course of 7 to 10 days. Alternative agents include gentamicin, ciprofloxacin, imipenem, doxycycline, and chloramphenicol.10 Because tularemia is relatively rare, a high index of suspicion is necessary to reduce the morbidity and mortality associated with the disease.

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