A 65-year-old woman presented to the dermatology clinic in July with a pruritic rash of 2 days’ duration that started on the back and spread diffusely. The patient gardened regularly. Physical examination showed inflammatory papules and pustules on the back (Figure 1), as well as the groin, breasts, and ears. There was a punctate black dot in the center of some papules, and dermoscopy revealed ticks (Figure 2). Removal and microscopic examination confirmed larval-stage lone star ticks (Figure 3). The patient was prescribed topical steroids for pruritus as well as oral doxycycline for prophylaxis against tick-borne illnesses.
A 54-year-old man presented to the same clinic in July with pruritic lesions on the back, legs, ankles, and scrotum of 3 days’ duration that first appeared 24 hours after performing yardwork. Physical examination revealed diffusely distributed papules, pustules, and vesicles on the back (Figure 4). Some papules featured a punctate black dot in the center (similar to patient 1), and dermoscopy again revealed ticks. Removal and microscopic examination confirmed larval-stage ticks. The patient was treated with topical steroids and oral antihistamines for pruritus as well as prophylactic oral doxycycline.
Ticks are well-known human parasites, representing the second most common vector of human infectious disease.1 Ticks have 3 motile stages: larva (or “seed”), nymph, and adult. They can bite humans during all stages. Larval ticks, distinguished by having 6 legs rather than 8 legs in nymphs and adults, can attack in droves and cause an infestation that presents as diffuse, pruritic, erythematous papules and pustules.2-4 The first report of larval tick infestation in humans may have been in 1728 by William Byrd who described finding ticks on the skin that were too small to see without a microscope.5
The ticks in both of our cases were lone star ticks (Amblyomma americanum). The larval stage of A americanum is a proven cause of cutaneous reaction.6,7 A PubMed search of articles indexed for MEDLINE as well as a Google Scholar search using the terms tick, seed tick, or tick bite in combination with rash, eruption, infestation, papule, pustule, or pruritic revealed 6 reported cases of larval tick infestation in the literature (including our case); 5 were caused by A americanum and 1 by Ixodes dammini (now known as Ixodes scapularis); all occurred in July or August.3,7-10 This time frame is consistent with the general tick life cycle across species: Adults feed from April to June, then lay eggs that hatch into larval ticks within 4 to 6 weeks. After hatching, larval ticks climb grass and weeds awaiting a passing host.4
Larval tick infestation remains a frequently misdiagnosed etiology of diffuse pruritic papules and pustules, especially in urban settings where physicians are less likely to be familiar with this type of manifestation.3,9-11 Larval ticks are submillimeter in size and difficult to appreciate with the naked eye, contributing to misdiagnosis. A punctate black dot may sometimes be seen in papules; however, dermoscopy is critical for accurate diagnosis, as hemorrhagic crust is a frequent misdiagnosis.
In addition to symptomatic therapy, both of our patients received doxycycline as antibiotic prophylaxis for tick-borne illnesses given that a high number of ticks had been attached for more than 2 days.12,13 Antibiotic prophylaxis for tick-borne illness is controversial. The exception is Lyme disease transmitted by nymphal or adult I scapularis when specific conditions are met: the bite must have occurred in an endemic area, doxycycline cannot be contraindicated, estimated duration of attachment is at least 36 hours, and prophylaxis must be started within 72 hours of tick removal.13 There are no official recommendations for the A americanum species or for larval-stage ticks of any species. Larval-stage ticks acting as vectors for disease transmission is not well documented in recent literature, and there currently is limited evidence supporting prophylactic antibiotics for larval tick bites. The presence of spotted fever rickettsioses has been reported (with the exception of Rickettsia rickettsii and Ehrlichia chaffeensis) in larval A americanum ticks, suggesting a theoretical possibility that they could act as disease vectors.3,8,11,14-17 At a minimum, both prompt tick removal and close patient follow-up is warranted.
Human infestation with larval ticks is a common occurrence but can present a diagnostic challenge to an unfamiliar physician. We encourage consideration of larval tick infestation as the etiology of multiple or diffuse pruritic papules with a history of outdoor exposure.