Varying virulence in rabbits infected with different filamentous types of Histoplasma capsulatum.

Histoplasma capsulatum filamentous primary isolates and their subcultures are separable into two distinct colonial types (A and B) having different microscopic characteristics. Yeast forms of the A and B types and the parent (P) strains from which they are derived are microscopically indistinguishable. Critically standardized inocula of living P, A, and B yeasts from one strain of H. capsulatum (G-184) were injected intravenously into 12 rabbits. Each type produced progressively debilitating disease, but in varying degrees. Of the 12 animals, 6 died within 2 to 14 weeks. A persisting copious nasal exudate, beginning at or before 1 week, was cultured weekly at 26 C on Mycosel (BBL) agar. Pure cultures of A and B filamentous type colonies were recovered from exudates of animals receiving A and B yeasts, respectively, whereas both filamentous types were isolated from rabbits injected with P yeasts, with B predominating. Only A and B yeasts thus maintained their filamentous integrity during animal passage. It was noted that dissemination of H. capsulatum through the nares of infected rabbits represents a possible hazard to laboratory personnel heretofore unrecognized. It is also a possible means of cross-infecting or sensitizing or cross-infecting and sensitizing animals housed in the same room, if A and B yeasts prove not to be antigenically identical.