Increased detection of rickettsialpox in a New York City hospital following the anthrax outbreak of 2001: use of immunohistochemistry for the rapid confirmation of cases in an era of bioterrorism.

BACKGROUND

Rickettsialpox is a self-limited febrile illness with skin lesions that may be mistaken for signs of potentially more serious diseases, such as cutaneous anthrax or chickenpox. The cluster of cutaneous anthrax cases from bioterrorism in October 2001 likely heightened awareness of and concern for cutaneous eschars.

OBJECTIVES

To apply an immunohistochemical technique on paraffin-embedded skin biopsy specimens for diagnosing rickettsialpox, and to compare the reported incidence of rickettsialpox before, during, and after the cluster of cutaneous anthrax cases.

DESIGN

Case series.

SETTING

Dermatology department in a large tertiary care hospital in New York City.

PATIENTS: Eighteen consecutive patients with the clinical diagnosis of rickettsialpox from February 23, 2001, through October 31, 2002.

MAIN OUTCOME MEASURES: Results of immunohistochemical testing of skin biopsy specimens and of serological testing.

RESULTS

Immunohistochemical testing revealed spotted fever group rickettsiae in all 16 eschars and in 5 of the 9 papulovesicles tested. A 4-fold or greater increase in IgG antibody titers reactive with Rickettsia akari was observed in all 9 patients for whom acute and convalescent phase samples were available; 6 patients had single titers indicative of rickettsialpox infection (> or =1:64). Of the 18 patients, 9 (50%) presented in the 5 months following the bioterrorism attacks.

CONCLUSIONS

Rickettsialpox remains endemic in New York City, and the bioterrorism attacks of October 2001 may have led to increased awareness and detection of this disease. Because rickettsialpox may be confused with more serious diseases, such as cutaneous anthrax or chickenpox, clinicians should be familiar with its clinical presentation and diagnostic features. Immunohistochemical staining of skin biopsy specimens, particularly from eschars, is a sensitive technique for confirming the clinical diagnosis.