Environmental Dermatology
What’s Eating You? Sand Flies
As thousands of Americans descended upon Brazil for the Olympic games in the summer of 2016, the mosquito-borne Zika virus became a source of...
Local Reactions
A tick bite may induce local hypersensitivity, leading to a red papule or plaque at the bite site, followed by swelling, warmth, and erythema. A cellular immune reaction induces induration and pruritus. Hard ticks are less likely than soft ticks to cause a serious local reaction.11,12
A variety of clinical and histologic features are observed following an arthropod bite. Histologically, acute tick bites show a neutrophilic infiltrate with fibrin deposition. Chronic reactions demonstrate a wedge-shaped, mixed infiltrate with prominent endothelial swelling. Eosinophilic cellulitis, or Wells syndrome, reveals tissue eosinophilia and flame figures.13 Tick mouthparts may be identified in the tissue. B-cell hyperplasia is seen in Borrelia lymphocytoma and is more common in Europe, presenting as erythematous to plum–colored nodules on the ear and areola.14
Lyme Disease
Disease manifestations vary by location. Lyme disease is associated with Borrelia burgdorferi and the recently identified Borrelia mayonii in the United States15; in Europe and Asia, acrodermatitis chronica atrophicans is associated with Borrelia afzelii and neuroborreliosis, with Borrelia garinii. Lyme disease is the most common tick-borne illness in the United States.16 The I ricinus species complex is the most common vector harboring Borrelia species.17 At least 36 hours of tick adherence is required for disease transmission.18 The incubation period is 3 to 20 days (median, 12 days).19
Clinical Findings
Erythema migrans is the most characteristic sign, seen in 80% of cases of Lyme disease. The typical rash is a centrifugally spreading, erythematous, annular patch with central clearing at the site of the tick bite.20 Atypical rashes include vesicular, indurated, ulcerated, and follicular variants.21 Histopathology commonly shows a superficial and deep perivascular lymphocytic infiltrate with plasma cells, histiocytes, and eosinophils.22 Typically, the rash resolves in 3 to 5 weeks.18
Early disseminated Lyme disease can present with any of the following findings: multiple erythema migrans; neurologic involvement, including cranial nerve palsy and meningitis; and Lyme carditis, which may result in atrioventricular block.23,24 Late findings include arthritis, encephalopathy, and polyneuropathy. A late cutaneous manifestation, acrodermatitis chronica atrophicans, is rare in the United States but occurs in as many as 10% of Lyme disease cases in Europe. An initial inflammatory response manifests as blue-red erythema and edema of the extensor surfaces of the extremities, commonly on the dorsal hands, feet, elbows, and knees. Firm fibrotic nodules may develop later over the olecranon and patella.23,24
The term chronic Lyme disease has been used to describe the persistence of symptoms after treatment; however, large clinical trials have not detected a difference in symptom frequency between patients with a history of Lyme disease and matched controls.25,26 Many patients with chronic Lyme disease may instead have posttreatment Lyme disease syndrome, described as nonspecific symptoms including fatigue, arthralgia, and decreased mental acuity following treatment of confirmed Lyme disease. Symptoms generally improve within 1 year.27
Laboratory Testing
The gold standard for laboratory diagnosis of Lyme disease is 2-tiered serologic testing. First, an enzyme immunoassay or immunofluorescence assay is used to screen for antibodies. A Western blot follows if the result of the screen is positive or equivocal. Western blot testing for IgM and IgG is used when illness duration is less than 4 weeks; after 4 weeks, a Western blot for IgG alone is sufficient.27,28 The 2-tiered test has 99% specificity. Sensitivity increases with duration of disease (29%–40% with erythema migrans; 42%–87% in early disseminated disease; 97%–100% in late disease).29,30 A false-positive result can occur in the presence of infectious mononucleosis, an autoimmune disorder, and syphilis. If serologic testing is negative and suspicion remains high, testing should be repeated in 2 to 4 weeks.31 When a patient in a Lyme-endemic area presents with typical erythema migrans, serologic testing is unnecessary prior to treatment.32
Management
Treatment of Lyme disease centers on antibiotic therapy (Table). First-line treatment of early disseminated disease is doxycycline for 14 days (range, 10–21 days).27 In pregnant women, children younger than 8 years, and tetracycline-allergic patients, amoxicillin or cefuroxime axetil for 14 days (range, 14–21 days) may be used.33 For erythema migrans without complications, doxycycline for 10 days is effective. Complications that require hospitalization are treated with intravenous ceftriaxone.27 Re-treatment in patients with posttreatment Lyme disease syndrome is not recommended.34 Prophylaxis with a single dose of doxycycline 200 mg may be indicated when all of the following conditions are met: (1) the patient is in an area where more than 20% of Ixodes ticks are infected with B burgdorferi, (2) the attached tick is I scapularis, (3) the tick has been attached for more than 36 hours, and (4) treatment is begun within 72 hours of tick removal.27
