Introduction

Cryptococcosis is an important mycosis of humans and animals. It is the most common systemic mycosis of cats, which are approximately 10 times more susceptible than dogs. Cryptococcosis is most commonly caused by two encapsulated yeast species that are dimorphic, basidiomycetous fungi of the genus Cryptococcus: Cryptococcus neoformans and Cryptococcus gattii. Cryptococcus gattii is considered an emerging pathogen of immunocompetent humans in the north-western United States, in contrast to C. neoformans, which is an opportunistic pathogen in humans. Other Cryptococcus species have rarely been associated with disease in dogs and cats, including Cryptococcus albidus and Cryptococcus laurentii. Infection is acquired following inhalation from the environment and is not transmitted between animals. Occasionally, outbreaks of infection in humans and animals occur as a result of exposure to common environmental sources. Growing evidence suggests that the local epidemiology of cryptococcosis in dogs and cats may differ considerably between geographic locations worldwide.

Life Cycle

The life cycle of Cryptococcus spp. involves vegetative growth as a haploid budding yeast, combined with the ability to transition to a filamentous form (Filobasidiella species), also known as a teleomorph. The teleomorphs of C. neoformans and C. gattii are F. neoformans and F. bacillosporus, respectively. Two mating types of Cryptococcus spp. exist, alpha and a, the alpha type being vastly more prevalent in environmental and clinical samples. Under optimum conditions and in the presence of mating pheromones, the two mating types can fuse and adopt a dikaryotic filamentous state, which is known as the perfect state. This is followed by production of basidia, which are small club-shaped structures on which basidiospores form. Cells of the alpha mating type can also respond to desiccation, nitrogen limitation, and pheromone signals and undergo asexual reproduction via haploid fruiting, which also involves filamentation and spore formation. This may explain the relative high prevalence of the alpha mating type. When inhaled, basidiospores are thought to give rise to mammalian disease. The mating of Cryptococcus spp. has never been observed in nature, but can be demonstrated under special laboratory conditions. Unlike other dimorphic fungi, the yeast phase of Cryptococcus is found under routine laboratory conditions and in mammalian host tissues.

Epidemiology

Cryptococcus spp. were initially classified into 4 serotypes, A, B, C and D, based on antigenic differences in the cryptococcal polysaccharide capsule, and a hybrid serotype AD was also identified. Cryptococcus neoformans contains serotypes A and D, and C. gattii contains serotypes B and C. With the introduction of a variety of genotyping methods, considerable genetic diversity has been identified within these serotypes. The results of PCR fingerprinting using microsatellite M13 or minisatellite (GACA)₄ primers, or amplified fragment length polymorphism (AFLP) analysis, have shown that the Cryptococcus species complex can be divided into 8 molecular types. To some extent, the types correlate with Cryptococcus species and serotypes, and consist of VNI/AFLP1 and VNII/AFLP1A/1B (C. neoformans var. grubii, serotype A); VNIII/AFLP3 (AD hybrid); VNIV/AFLP2 (C. neoformans var. neoformans, serotype D), and VGI/AFLP4, VGII/AFLP6, VGIII/AFLP5 and VGIV/AFLP7 (C. gattii, serotypes B and C). Differences in epidemiology, pathogenicity, clinical features, and drug susceptibility have been associated with the species, variety, and molecular types. Cryptococcus gattii can be easily distinguished from C. neoformans in the microbiology laboratory using canavanine-glycine-bromthymol blue agar, although few veterinary diagnostic laboratories in the United States at the current time perform differentiation. Unfortunately, many publications and laboratory reports still refer to all Cryptococcus spp. isolates as C. neoformans. Molecular typing is generally performed within specialized diagnostic or research laboratories. Multilocus sequence typing, whereby multiple genes are sequenced, has been recommended as a consensus typing method for cryptococcal isolates worldwide.

Cryptococcus neoformans

Cryptococcus neoformans has a worldwide distribution. The major environmental niche for C. neoformans is believed to be weathered bird (especially pigeon) guano, in which it mates abundantly. It can also be found in decaying plant matter in hollows of certain trees. Cryptococcus neoformans passes through the gastrointestinal tract of pigeons, but systemic infection of pigeons is rare. Pigeons can also carry the fungus on their feathers, crops, or cloaca. Pigeon guano provides an environment rich in many nitrogen-containing compounds including creatinine that favor cryptococcal growth. Cryptococci may remain viable for years in pigeon lofts, where accumulations of guano are protected from drying or sunlight. Macaws, swans, and parakeets have also been associated with C. neoformans carriage. The vast majority of human cryptococcal infections in human immunodeficiency virus-infected patients result from infection with C. neoformans, especially C. neoformans var. grubii. Cryptococcus neoformans is the most common cryptococcal species isolated from humans, dogs, and cats in southeastern Australia, accounting for more than 70% of infections. The majority of dogs with cryptococcosis in North America are also infected with C. neoformans, whereas infection of cats with this species appears to be rare.

Cryptococcus gattii

Originally thought to be a tropical or subtropical pathogen, C. gattii has also emerged as an important pathogen in temperate regions following outbreaks of disease in apparently immunocompetent individuals in British Columbia, Canada, and the Pacific Northwest of the United States. Although to date it has been most commonly reported from Australia, New Zealand, Canada, and Central and South America, its presence has been reported in a growing list of other geographic regions including Austria, China, Congo, Germany, Greece, France, India, Italy, Japan, South Korea, the Netherlands, Spain, South Africa, the United Kingdom, and the United States. In Australia and North America, infections have been reported in dogs, cats, goats, horses, llamas, koalas, echidnas, marine mammal species, and exotic birds. Studies to date have indicated that worldwide, genotypes VGI and VGII are prevalent, with VGIII and VGIV being less common. The outbreaks of disease in Canada and the Pacific Northwest have been associated with certain highly virulent molecular types of Cryptococcus gattii, primarily VGIIa, and to a lesser extent, VGIIb. In northern California, almost all cats with cryptococcosis have been infected with C. gattii genotype VGIII. This molecular type has also been recognized recently in humans from southern California. In contrast, the prevalence of C. gattii infection is lower in cats from southeastern Australia, with most C. gattii isolates belonging to VGI. VGII predominates in southwestern Australia and the Northern Territory. Cryptococcus gattii VGI in Australia is associated with dead plant material in eucalyptus tree hollows. VGI is also found in countries where Australian eucalyptus trees have been exported. Nevertheless, the isolation of C. gattii from eucalyptus trees is rare outside Australia, despite extensive environmental sampling. Imported eucalyptus trees have not been associated with the environmental presence of C. gattii in Spain, central Africa, or Canada. Cryptococcus gattii has now been associated with over 50 tree species, the majority (77%) of which are angiosperms (hardwoods that are often deciduous). In British Columbia, Canada, C. gattii is consistently found on living tree bark and in air, and has also been found in freshwater and seawater. Decayed wood hollows appear to be a specific ecologic niche for the organism. It has been suggested that C. gattii is an environmental pathogen that maintains its virulence attributes in the absence of a recognized host animal species. Interestingly, approximately one-quarter of the cats with cryptococcosis seen at the author's teaching hospital are kept exclusively indoors. It is possible that organisms may be brought indoors on fomites such as shoes or soil used for indoor plants.

Implications for Companion Animal and Human Health

In humans, the lung is thought to be the primary site of infection with Cryptococcus spp. The nasal cavity has been suggested as the primary site of infection in cats and dogs, although some cats and dogs develop pulmonary or disseminated involvement without evidence of lesions within the nasal cavity. Differences in clinical presentation have been identified in humans infected with C. gattii versus those infected with C. neoformans, although host immune responses appear to play the predominant role in determining clinical presentation. Patients infected with C. neoformans tend to be immunocompromised, in particular with acquired immunodeficiency syndrome (AIDS). Infections tend to be widely disseminated to a variety of organs, with a high prevalence of fungemia and funguria. In contrast, patients infected with C. gattii are often immunocompetent. Fungemia and funguria are generally absent, and infections are frequently associated with mass lesions in the cerebrum and/or lungs (cryptococcomas). Patients infected with C. gattii are frequently treated for longer with antifungal drugs, antigen titers are higher, and morbidity, mortality, and severe neurologic sequelae are more frequent among patients infected with C. gattii. Mouse models have shown that C. gattii may inhibit the migration of neutrophils to sites of infection, as well as the production of protective cytokines. As a result, it appears that C. gattii strains thrive in immunocompetent hosts by evading or suppressing immune responses that normally limit the progression of disease caused by C. neoformans.

Evidence from cats and dogs from northern California infected with C. neoformans and C. gattii suggests that to some extent, similar patterns of infection may exist in companion animals. Overall, feline infections with C. gattii are associated with a better long-term prognosis, but infections are often difficult to cure with antifungal drugs, and high antigen titers tend to persist. The inflammatory response to infection is often absent or limited. Dogs tend to have widely disseminated disease, with a high prevalence of neurologic involvement and common involvement of sites such as the liver, myocardium, and kidneys, often with a marked inflammatory response to the organisms. Differences may also exist in clinical presentation between dogs infected with C. gattii and dogs infected with C. neoformans. Differentiation between C. gattii and C. neoformans by veterinary microbiology laboratories is encouraged for further understanding of the epidemiology, pathogenesis, and response to treatment of these organisms.