Isolated Thigh Lesion: Bug Bite-or Herpes?

My patient is a woman in her midtwentieswho recently presented with anerythematous, slightly pruritic lesionon her thigh. Over several days, a lineof erythema progressed from the lesionto the groin; a tender "knot" later erupted in the groin. Shesuspected that she had been bitten by an insect.Examination revealed a solitary 3-cm erythematouspatch on the mid-posterior thigh that included a collection ofpinpoint pustules; my impression was that the lesion was herpetiform.A faint red linear streak led from this lesion to thegroin, where there was a single, tender 1.5-cm lymph node inthe inguinal region. She had no fever or other constitutionalsymptoms.Initially, I believed these findings most likely representedan insect bite that had become secondarily infected and thatthis infection had progressed, with lymphangitis and regionalhymphadenitis. However, I thought this could also be anatypical presentation of herpes zoster (even though therewere no other lesions), so I ordered a viral culture. The culturewas negative for varicella-zoster virus but positive forherpes simplex virus (HSV) type 2.How often does HSV-2 infection present as an isolatedlesion that involves a dermatome distant from the oral and genital regions? Can genital herpes spread to an adjacentdermatome? What is the likelihood that a positive result ona herpes viral culture will be a false-positive?---- MDInfection with either HSV-1 or HSV-2 can manifest at almostany mucocutaneous region of the body. Extragenitallesions are a recognized complication in 10% to 22%of primary HSV-2 infections and also occur in 3% to 5%of recurrences.¹ During the course of primary HSV infection,tender lymph nodes and extragenital lesionsdevelop after the onset of genital lesions, often during thesecond week.²Although it is not common, patients can present withonly extragenital lesions. These are typically located adjacentto the genitalia on the buttocks, groin, and thigh("boxer shorts" distribution), or at distant sites such as thefingers. Extragenital lesions occur more frequently inwomen than in men.In a prospective study, half of the patients who hadnongenital lesions during their first episode of genital HSVinfection subsequently had recurrences at a nongenitalsite.³ Nearly 7% of patients whose primary HSV-2 infectioninvolved only genital lesions later experienced nongenitalrecurrences--chiefly on the buttocks and legs.HSV establishes latency in the regional ganglion;when reactivated, it migrates via the peripheral sensorynerves to mucosal sites. Sensory fibers that originatefrom the sacral and lower lumbar spine innervate the genitaliaas well as a significant part of the lower body, includingthe buttocks, posterior thighs, and perianal region.⁴Because of this shared neural network, prodromal symptomsand lesions both on the genitalia and in the surroundingboxer shorts distribution can be consideredgenital herpes.This patient may have newly acquired genital herpeswith ganglionic spread to the posterior thigh, but morelikely she has a recurrence of a previously unrecognizedgenital herpes infection. It is difficult to distinguish betweenprimary and recurrent genital herpes on clinicalgrounds; however, this can be done by obtaining acuteand convalescent titers of HSV antibodies. Approximately30% of patients who present for evaluation of a first episodeof genital herpes and who deny a history of genitalulcers have serologic evidence of past HSV-2 infection.Positive herpes cultures can be confirmed in the laboratorywith type-specific monoclonal antibodies. Whenthis is done, false-positives are uncommon. Use a laboratorythat participates in proficiency testing.Order type-specific herpes antibody tests for all patientswho present with nongenital herpes. Counsel themabout the likelihood of their having genital herpes andeducate them about symptoms and transmission.---- Rachna Gupta, MD
Virology Research Clinic
University of Washington
Seattle

References:

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