A Study of the Whole Yeast Cell Agglutination Test in Rabbits Experimentally Infected with Histoplasma Capsulatum

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Abstract
Summary: It was found that whole yeast cells of Histoplasma capsulatum grown on brain heart infusion agar at 37°C could be used as the antigen in a tube agglutination test with sera from rabbits experimentally infected with H. capsulatum. Very low or negative agglutinin titers were obtained with sera from infected rabbits using the commonly used incubation temperatures of 25, 37 or 52°C. Overnight refrigeration of tubes (4°C) resulted in slightly increased titers. Greater agglutinin titers were demonstrable in tubes that had been centrifuged, and it was found that an incubation period was not necessary using this technique. The method of choice was, therefore, to centrifuge the tubes immediately after mixing antigen with serum. Yeast cells from 3 strains of the fungus were shown to be useful agglutinogens, exhibiting approximately equal combining power. An additional strain was found to agglutinate spontaneously in normal human and rabbit sera. The methods used for inactivation (heat, formalin, and phenol) were found to have an effect upon the agglutinating ability of cells from the different strains, but no one method seemed to be the best for all strains tested. In sera from rabbits experimentally infected with H. capsulatum, agglutinin titers were found to rise rapidly during the first week of the disease, reach a maximum by the end of the second week, and become negative at the end of a 9 week period. Complement fixation tests using whole cell antigens were also positive after 1 week, but reached a peak at 6 weeks and remained positive in high titers at the end of 13 weeks. The histoplasma agglutination antigen when tested in sera of rabbits experimentally infected with Blastomyces dermatitidis, Sporotrichum schenckii, Candida albicans and Coccidioides immitis, gave positive results only with B. dermatitidis. No differences in specificity of the antigens were noted after treatment with formalin and phenol or heat inactivation. A macroscopic slide agglutination test is described which proved of value in “screening” rabbit sera for the presence of antibodies against yeast cells of H. capsulatum.