Epidemiology

C neoformans is distributed worldwide. Most cases of cryptococcosis involve serotypes A and D. Serotypes B and C are restricted to tropical and subtropical areas and are isolated from certain species of eucalyptus trees and the air beneath them. C neoformans var neoformans, which is recovered from aged pigeon feces, bird nests, and guano, is invariably serotype A or D. Although serotypes A and D exist in high concentrations in the pigeon feces, the fungus does not infect the birds. In moist or desiccated pigeon excreta, C neoformans may remain viable for 2 years or longer. In saprobic environments, C neoformans grows unencapsulated; however, unencapsulated strains regain their virulence following reacquisition of their polysaccharide capsule. C neoformans var gattii usually causes disease in patients with intact cell-mediated immunity.

Naturally occurring cryptococcosis occurs in both animals and humans, but neither animal-to-human transmission nor person-to-person transmission via the pulmonary route has not been documented. Transmission via organ transplantation has been reported when infected donor organs were used. C neoformans var neoformans causes the vast majority of cryptococcal infections in immunosuppressed hosts, including patients with AIDS, whereas C neoformans var gattii causes 70-80% of cryptococcal infections among immunocompetent hosts.

Although C neoformans var neoformans is found worldwide, C neoformans var gattii is usually identified in subtropical areas such as Australia, South America, Southeast Asia, and Central and sub-Saharan Africa. In the United States, C neoformans var gattii is found in Southern California.

As noted above, C neoformans var gattii may be found in association with several different trees, such as river red gum trees (E camaldulensis) and forest red gum trees (E tereticornis). Infection is acquired by inhalation of air-borne propagules that infect the lungs and may extend via fungemia to involve the CNS.

In 1999, C neoformans var gattii emerged on Vancouver Island, British Columbia, Canada. Infections were reported among residents and visitors to the island, as well as among domesticated and wild animals. Disease has been most often identified in cats, dogs and ferrets. Marine mammals have also been identified to carry the infection. Vectors can disperse the spores from an endemic area to a previously unaffected area. This may have been the route of spread in the case of Vancouver Island. Since 2003, cryptococcal disease has become a provincially notifiable infection in British Columbia. Isolates have been identified in coastal Douglas fir and coastal western hemlock biogeoclimatic zones.

The incidence of infection related to age, race, or occupation does not significantly differ. Healthy persons with a history of exposure to pigeons or bird feces and laboratory workers exposed to an aerosol of the organism have a higher rate of positive delayed skin reaction to cryptococcal antigen or cryptococci. Occasionally, laboratory accidents result in transmission of C neoformans, but pulmonary and disseminated disease is rare in this setting. Accidental cutaneous inoculation with C neoformans causes localized cutaneous disease.