Spherulin and coccidioidin: cross-reactions in dermal sensitivity to histoplasmin and paracoccidioidin.

Until recently coccidioidin has been the only antigenic preparation available for detecting delayed dermal sensitivity induced by an experience with Coccidioides immitis. It is prepared from autolysates of the mycelial phase (saprophytic) of the fungus. A more sensitive reagent, spherulin, was developed in 1969 from the spherule phase (parasitic) of the organism. Use of spherulin showed that coccidioidin failed to detect approximately 30% of individuals specifically sensitive to C. immitis. However the potential of spherulin to detect cross-sensitivity induced by Histoplasma capsulatum was unknown. This information was considered to be germane because of the capacity of coccidioidin to detect a histoplasmal experience. Accordingly, both reagents as well as paracoccidioidin were compared simultaneously in 365 Columbian soldiers from areas endemic for histoplasmosis but not for coccidioidomycosis. At standard strength both preparations detected nonspecific responses in 0.8% to 3% of the histoplasmin negative and positive subgroups, respectively. At 10-times standard strength both preparations cross-detected histoplasmin sensitivity comparably; 5.1% to 7.1% of histoplasmin-positive subjects reacted with the coccidioidal antigens. No pattern of cross-reactivity was observed between paracoccidioidin sensitivity and sensitivity to the coccidioidal antigens.