Wood, animals and human beings as reservoirs for human Cryptococcus neoformans infection.

Abstract

Cryptococcus neoformanswas first cultivated by Sanfelice from fermenting peach juice in 1894. At the same time, Busse and Buschke reported, separately, the first case of human disease caused by this yeast. During the next years, the fungus was isolated only from lesions or secretions of men or animals. In 1951, Emmons reported the isolation of C. neoformans from soils of Virginia and, in 1955, demonstrated that virulent strains of C. neoformans were found commonly and abundantly in pigeon manure under roosting sites. Since cryptococcosis is not contagious, and virulent strains of C. neoformans are not isolated from skin, mucosa or feces of man with sufficient frequency to support the concept of an endogenous source of infection, Emmons concluded that the exposure of men and animals to pigeon excreta could explain adequately the epidemiology of cryptococcosis [1]. This saprophytic source of C. neoformans has been recognized in many areas of the world, but many questions about the ecology of C. neoformans and the epidemiology of cryptococcosis still remain unanswered. C. neoformans, the anamorph of Filobasidiella neoformans, is an encapsulated basidiomycetous yeast with two varieties - var. neoformans (serotypes A, D, and AD) and var. gattii (serotypes B and C) - with distinct life cycles, physiology, ecology and genetics [2,3]. Infection caused by C. neoformans var. gattii is restricted to some geographical areas, mainly tropical and subtropical regions, occurring predominantly in non-immunocompromised individuals [4,5], whereas reported cases of opportunistic cryptococcosis throughout the world are usually due to C. neoformans var. neoformans. In nature, C. neoformans var. gattii has been isolated from plant debris of Eucalyptus trees in some endemic areas of cryptococcosis due to the variety gattii [6], but it has not been demonstrated in avian droppings and soils contaminated by avian excreta, the major saprophytic source of C. neoformans var. neoformans. The ecology of C. neoformans was recently reviewed by Sorrell and Ellis [7]. Human infection by C. neoformansis thought to be acquired by inhalation of airborne propagules from an environmental source [3]. However, evidences for an epidemiological association between exposure to saprophytic sources of C. neoformans and human infection are circumstantial. C. neoformans yeast cells in soils and in avian nesting areas possess minimal capsule, may be smaller than 2 µm, and are easily aerosolized. On the other hand, basidiospores of F. neoformans, which have been demonstrated to be pathogenic in animal experiments, have characteristics that favor them to be more readily deposited in the lungs than the yeast cells: they are smaller (1.8 x 2.5 µm in diameter), easily aerosolized, and much more resistant to desiccation than yeast cells [8]. Human infection and exposure to natural sources of C. neoformans var. neoformans