Resistance of Hamsters in Infections with Histoplasma Capsulatum

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Abstract
Summary: Of 108 mature hamsters inoculated subcutaneously with 5 × 104 yeast cells of Histoplasma capsulatum all developed a disseminated infection. Dissemination appears to have occurred rapidly in all animals: spleens from each of the five animals sacrificed on the 2nd day following injection were positive by culture. Furthermore, during the 6-week period of study either spleen or liver, or both, yielded positive cultures in all instances. Daily composite culture and tissue section indices rose gradually during the first 2 weeks of infection at which time they reached a plateau which was roughly maintained throughout the remainder of the 6-week test. Individual daily indices showed extensive variation throughout the plateau period. Certain patterns of resistance and susceptibility were discerned among the responses of the individual animals to infection. These patterns ranged from apparent early resistance with minimal spread of the infection, to late resistance following extensive dissemination; and from extreme susceptibility resulting in death within the first 2 weeks of infection, to a progressive deterioration during the course of infection. Evidence suggesting some degree of effective resistance was found in 51% of the animals infected 2 weeks or longer. Large yeasts were found in 45% of the infected livers. In all such livers well circumscribed lesions were present. Zones of inhibition of organism growth surrounding the tissue on culture medium were observed increasingly during the last 2 weeks of the investigation, as well as an increasing number of negative cultures of tissue found positive in tissue section. Possible bases for this growth inhibition and its relation to cultural diagnosis of human infection are discussed.