Development of dermal hypersensitivity to coccidioidal antigens associated with repeated skin testing

Four antigens prepared from Coccidioides immitis were found to evoke no dermal reactions in 1,245 volunteers on first testing, but 2 months later, upon retesting, 18.6% of the subjects reacted to 1 or more of the antigens. In no subject did other findings indicate an intercurrent coccidioidal infection. Conversion of a skin test to reactive significantly (p less than 0.018) increased with subject age. No association was found with either years of residence within areas endemic for C. immitis or occupational exposure, associations that would be expected if conversion was due to latent hypersensitivity from a prior coccidioidal infection. Possible explanations for our findings include either boosting of immunity to a noncoccidioidal stimulus or primary sensitization from the skin testing itself. Until this phenomena is understood more fully, physicians should be reluctant to accept interval conversion of coccidioidal skin tests as a means of diagnosing recent coccidioidal infection.