Basic background information:
Cryptococcus neoformans is an encapsulated yeast, with variable size and shape (round to oval, 3.5 to 8.0 microns in diameter)(2). Narrow based budding is observed between the parent and daughter cells (blastoconidia or buds). Cryptococcus neoformans is the most common human pathogen of the group(4). Cryptococcus albidus(5) and C. laurentii(3) have been reported to cause disease. Four serotypes (A,B,C,D) of Cryptococcus neoformans have been identified with serotypes A and D most commonly associated with disease(4).
Cryptococcus neoformans classically is associated with desiccated pigeon feces, although the organism can be found in the feces of other birds including turkeys and starlings as well as in bat feces(4). While these carry the fungus in the gastrointestinal tract, they do not develop invasive infections. In desiccated feces, the organism is a small, easily aerosolizable, poorly encapsulated form(4). Defects in cellular immunity or underlying disease are found in up to 85% of patients with cryptococcosis. Predisposing factors to infection with C. neoformans include corticosteroid therapy, notably in transplantation immunosuppression as in this patient, leukemia and lymphoma (particularly Hodgkin's disease). Infection with C. neoformans is frequently seen as the AIDS defining disease in patients with HIV infection(7). Cryptococcosis may occur in patients with underlying diseases which have endogenous increases in corticosteroid production or require exogenous corticosteroid therapy, for example, sarcoidosis, Cushing's syndrome, and collagen vascular diseases.
Primary infection with Cryptococcus neoformans usually follows inhalation of the fungus with infection of the lungs. A transient colonization of the tracheobronchial tree may result, or more extensive pulmonary involvement may develop. This can take the form of a self-limited pulmonary disease or a progressive pulmonary disease. Either can be associated with dissemination to other sites. The final result of pulmonary cryptococcal infection may be a cryptococcoma or a residual pulmonary nodule(1). The primary infection may also take the form of a painless skin lesion (pustule, ulcer, cellulitis, or subcutaneous skin mass), probably due to direct inoculation of the yeast. Meningoencephalitis is the most common manifestation of the disseminated infection and the most severe. The onset of symptoms may be insidious, as in this patient, but in untreated patients, the fatality rate is high(7). The prostate, kidneys, lymph nodes, bone and skin may be involved in disseminated cryptococcosis. Arthritis and osteomyelitis may occur either as a isolated site of infection or following dissemination.
The major virulence factors of C. neoformans include the capsule, polysaccharide and glycoprotein cell products, and production of melanin and mannitol. The thick capsule of C. neoformans is made up of polysaccharides including glucuronoxylomannan, galactoxylomannan, and a mannoprotein. The capsule decreases the efficiency of phagocytosis by neutrophils and macrophages. In addition to capsule production, the organism produces vast quantities of the capsular polysaccharides and glycoproteins which are released into the blood stream. This may act to inactivate complement in the immunodeficient host. Melanin and mannitol production contributes to the organisms virulence by deactivating products of the host's oxidative defenses(6).
Host resistance to C. neoformans is mediated by phagocytosis, cell mediated immune response, and antibody production with complement fixation. In the immunocompetent host, neutrophils and macrophages phagocytose the organism making these an important first line of defense against C. neoformans. Perforin release by NK cells and T cells also may help in direct destruction of the organisms. Macrophages acting as antigen presenting cells cause the stimulation of specific T lymphocytes; (Th1) lymphocytes which act in producing a cell-mediated immune response or helper (Th2) lymphocytes which in turn result in stimulation of B-lymphocytes and antibody production. Antibody production, which is variable, allows for opsonization and complement fixation(6).
Cryptococcus neoformans will produce mucoid, flat or raised, colonies with smooth borders on most media, including 5% sheep blood agar and Sabouraud dextrose agar at 37 degrees C. On Niger - birdseed agar the colonies appear brown in color. Cycloheximide, commonly added to fungal media to inhibit bacterial growth, inhibits growth of the organism. Narrow based budding organisms are usually easily identified and hyphae are rarely present. India ink stain (nigrosin) is useful to show the thick capsule which is seen as a halo around the organism. When subcultured the organism will convert to a poorly encapsulated form or lose the capsule entirely. The organism also produces large quantities of urease, a property that is used for rapid presumptive identification(1,4). C. neoformans assimilates dextrose, maltose, sucrose, galactose, inositol, xylose, trehalose, dulcitol, and variable raffinose and cellobiose. C. neoformans is the only known yeast to produce phenoloxidase, which degrades caffeic acid to melanin producing a brown-black color (Caffeic Acid Disk Test). Latex bead agglutination tests are a fast and sensitive way of identifying C. neoformans capsular antigen in clinical specimens. The latex beads are coated with polyclonal rabbit antibodies specific for the capsular polysaccharide. A positive reaction is seen as agglutination of the latex beads(7). Serologic titers can be performed but variable results are seen due to the organisms irregular stimulation of antibody production. Histologically, the yeast is seen as a grey yeast form surrounded by a clearing of surrounding tissue, a halo representing the capsular material. Mucicarmine staining is helpful is delineating the capsule.
The patient was successfully treated with amphotericin B and 5-flucytosine as well as several debridements of his distal left clavicle.
Contributed by Christine Dorvault, M.D., William A. Pasculle, Sc.D., John P. Anhalt, Ph.D., M.D. and Charles A. Richert, M.D.