Cryptococcus gattii has long been known to cause disease in both humans and animals but was historically considered to be restricted to tropical and subtropical regions. Recently, infections have been recognized in other areas of the world, particularly in the Pacific Northwest of North America. C. gattii can infect humans and other mammals, as well as birds and reptiles, and has been positively identified in many species so far. While not considered zoonotic, veterinary and human cases frequently occur in the same geographic regions. The pathogen itself can be dispersed from one area to another via mechanical vectors such as water and soil and has been shown to be anthropogenically transferred. The emergence of the organism into temperate climate zones, as well as its apparent increased virulence in North America, is a concerning issue for human medical and veterinary professionals.
Cryptococcus gattii is a basidiomycetous yeast found in the environment, sometimes in association with various plants and/or animals.13,14,18 It is closely related to Cryptococcus neoformans, and these two species are responsible for the majority of cryptococcal infections.3,18,28 Though closely related to C. neoformans, C. gattii differs in a number of important aspects including natural habitat, ecology, manifestation of disease, and epidemiology.27 Additionally, C. gattii has been classified as a primary pathogen, affecting both immunocompetent and immunosuppressed hosts.5 In comparison, C. neoformans has historically been considered an opportunistic pathogen, affecting mainly immunocompromised hosts, though evidence for this may be stronger in humans than in animals.27,18 Previously, it was thought that C. gattii was found only in tropical and subtropical regions such as Australia, South America, and Southeast Asia, with most historical research coming from Australia where the organism appears to be associated with eucalyptus trees and koalas.2,13-15,28 The distribution of C. gattii has now expanded to include much of the world, with a recent emergence of two apparently highly virulent strains in the Pacific Northwest of North America.2,3 While much is still unknown about this organism, a great deal of research has been done in recent years. This paper will summarize the current information on C. gattii and its relevance to zoo veterinarians.
Pacific Northwest Outbreak
The current Cryptococcus gattii outbreak started in 1999 on Vancouver Island, British Columbia, Canada, and spread to mainland British Columbia by 2004.1,3,10 From there, it continued to move south into Washington and Oregon. This region has a temperate climate, which was previously thought to be outside of the ecological niche of C. gattii,9 thereby prompting investigation into the organism and the epidemiology behind it; however, the origin of this unusual outbreak remains unknown.2,5
Molecular studies have identified four molecular types of C. gattii: VGI, VGII, VGIII, and VGIV.2 Most C. gattii isolates identified in the outbreak region (human, veterinary, and environmental) are VGII.2,10 VGII comprises three molecular subtypes: VGIIa, VGIIb, and VGIIc, with VGIIa appearing to be the most virulent form causing the majority of C. gattii outbreak-associated infections.1,5,9 Interestingly, this isolate is different from the strains identified in endemic areas in other countries such as Australia, where VGI is the most common molecular type identified.5,27 The subtype VGIIc (which behaves similarly to the virulent VGIIa) has, thus far, only been found in the United States.2
Various types of virulence testing have been performed to investigate the apparently altered pathogenesis of the outbreak strains of C. gattii affecting North America. To address this question, several groups have examined melanin production, mitochondrial morphology, intracellular proliferation rate, and virulence in murine models.2,9,21 The details of this research are beyond the scope of this paper; however, these results have shown that the outbreak strains of C. gattii (specifically VGIIa and VGIIc) are, in fact, more virulent, although the exact factors involved have not yet been identified.2
It is unknown how these virulent strains moved from Vancouver Island to mainland British Columbia and then to the northwestern United States, but a number of theories exist. One hypothesis is that the organism has been present in the environment for a number of years and that recent environmental changes have allowed for an ecological shift to occur, resulting in the organism being either more common or more pathogenic.1,10,22 A few C. gattii VGIIa isolates have been identified from older samples (one from a human patient in Seattle in the early 1970s, one from an environmental sample in San Francisco, California, in 1990, and one from a patient with HIV infection in Southern California) which may support this theory.2,26,29 The California isolates are similar to but genetically distinct from the outbreak isolates, and it is not known if one strain originated from the other or if they are a result of genetic drift or some other mechanism.2 The Seattle sample, which is identical to the outbreak strains, suggests that VGII has been present in the environment for decades; however, this cannot be proven since the travel history of the patient is unknown and no environmental samples were taken at the time.2,11
Another theory is that since C. gattii has been cultured from asymptomatic animals, it is possible that migrating “carrier” wildlife may bring the organism from one place to another.12 C. gattii has been also been shown to colonize soil, trees, fresh and sea water, and it can be cultured from air samples.5,11 There is evidence suggesting that humans may play a role in dispersal of C. gattii from one of these environmental sources to another location. One study showed that C. gattii can survive on wheel wells and shoes (sometimes for extended periods of time of up to several months or even years) and be transported to new locations via these mechanical vectors.12 This study also found that the highest concentrations of C. gattii were isolated in areas with heavy human traffic, particularly popular outdoor parks, again implicating a human role in dispersal.4,12 Furthermore, it was found that any disruption of soil or trees contaminated with C. gattii, such as construction, deforestation, and even gardening, greatly increases the concentration of C. gattii isolated from the surrounding air.12 Other research has shown that the disturbance of soil or trees in C. gattii endemic areas is a major risk factor for animals to acquiring C. gattii infection.7 When trees contaminated with C. gattii are cut down and processed into byproducts such as wood chips, the organism can survive in these materials for long periods, potentially even after they’ve been shipped to other locations.12,22 The theory that the organism may be imported with plant species has not yet been proven but is still plausible given the rest of these findings.9,12,22
Human cases of C. gattii infection became notifiable in British Columbia in 2003.5 In the U.S., although surveillance began in Washington and Oregon in 2005, coordinated national surveillance at the CDC began only in 2008 as a collaboration between the CDC and state health authorities and the BCCDC.3 Since C. gattii became reportable in Washington and Oregon only in 2011, it is likely that many past cases of C. gattii were not identified.3,5 To date, 86 human cases and 65 veterinary cases have been reported in the United States.3 It has been identified as a cause of disease in many mammalian species, ranging from dogs and cats to porpoises and dolphins.6,24,28 However, the number of veterinary cases has almost certainly been underestimated for a number of factors; primarily that animals with severe disease are more likely to be euthanized without complete diagnostic testing.18 Additionally, it is impossible to know how many wild animals have been affected. One study investigated nasal colonization of wild mammals in British Columbia and found a 2% incidence in the animals sampled, similar to what had been found in domestic animals at the time; however, further research in wild animals is needed.6 Overall incidence of C. gattii is likely at least somewhat underreported, since historically clinical cryptococcal cases have not been routinely identified to the species level, with most cases reported simply as C. neoformans.3,5
The disease caused by C. gattii is similar to other cryptococcal diseases. It is primarily acquired via inhalation of desiccated yeast cells or spores, though the infectious propagule of C. gattii is still unknown.2,5 After being inhaled, the organism settles mainly in the upper and lower respiratory systems and, not uncommonly, invades the central nervous system.5,8,18 This results in various clinical signs, but typically pneumonia is the result of respiratory infection and meningitis the result of CNS infection. C. gattii may remain localized in the lungs or other organ systems or it may become disseminated.5,16,18,27 It also tends to form cryptococcomas (mass-like lesions) more readily than other cryptococcal species.5,27 Since monitoring and surveillance efforts in the U.S. have been increased, there has been an ∼35% case fatality rate among humans.3 The case fatality rate is reportedly higher in animals; however, this is difficult to substantiate since many veterinary cases in domestic animals and wildlife have likely gone unnoticed and companion animals may be euthanized without attempting treatment.6
Diagnosis of C. gattii is similar to any other cryptococcal or fungal disease in general. Diagnosis may be made via characteristic cytology (large capsule, narrow-based budding, many organisms present, etc.), culture, serum or CSF antigen titers, urine antigen test, or histopathology.18 While growth on CCB (L-canavanine, glycine, 2-bromothymol blue) agar can differentiate C. gattii from C. neoformans, routine testing performed in most labs will not speciate the two organisms.2,17 Molecular typing is necessary to identify molecular type and genotype.2
In both humans and animals, early detection of the disease and aggressive antifungal therapy provides the best chance for survival.18 Since early signs can be nonspecific, veterinarians and human medical doctors need to have a high index of suspicion for C. gattii, yet another reason why increased awareness of the disease is critical.2,18 Like most other fungal diseases, long-term treatment is required (months to years). In human treatment protocols, the treatment depends on whether or not there is CNS involvement.25 In veterinary species, treatment is often cost prohibitive, and animals may be alternatively euthanized; therefore, it is difficult to assess which treatments are effective.18 Most sources recommend amphotericin B as a first line agent, often in combination with other antifungals.18,25 When cryptococcomas have formed, surgical excision or debulking may be recommended as well.18
Discussion and Relevance to Zoo Veterinarians
Many medical and veterinary professionals are still unaware of the existence of C. gattii or the current outbreak, and much is still largely unknown about its incubation period, evolution to hyper-virulence, and drivers of dispersal. Furthermore, with evidence indicating that C. gattii may live for long periods of time in the environment and may be transported from one location to another via plant material, byproducts, or by humans acting as mechanical vectors, there is a significant need for increased awareness of this pathogen.6,13
While few studies have examined the impact of C. gattii on zoos specifically, there are a number of relevant issues that zoo professionals should be aware of. Since C. gattii has the ability to cause potentially fatal disease in a wide spectrum of species and in immunocompetent individuals, many animals housed in zoos are potentially susceptible. Due to what is currently known about how the organism can be dispersed, the potential exists for it to be imported with animals or plants from distant countries, or imported locally via wood chips used in many parks and zoos for landscaping and animal enclosures.13,23 Travel-associated cases of C. gattii have also been diagnosed, which is especially relevant to zoo veterinarians given the amount of movement of animals between zoos and importation of wild-caught animals.4,19,20 There have been clinical cases of C. gattii in zoo animals, several in wild and captive koalas, one of which was housed at a zoo in Japan in 2002.24 This animal had been imported from Australia and it is assumed that the animal acquired the infection prior to its relocation to Japan.24 Several more cases in zoo animals have been identified recently in North America and reports are currently in preparation (J. Sykes, personal communication). Given these cases and the high virulence of the Pacific Northwest strain, zoo veterinarians should not only be aware of the disease but consider preventative actions when appropriate. For example, testing animals for C. gattii colonization prior to relocation from endemic areas, and particularly the Pacific Northwest, may be warranted. Regardless, if an animal shows signs suspicious of cryptococcal disease, rapid response is necessary, and state veterinary officials should be notified. Every effort should be made to speciate any confirmed cryptococcal infection for better understanding of C. gattii and its true presence and impact.16
I would like to thank Drs. Julie R. Harris, Karen Bartlett, and Jane Sykes for their comments and contributions to this paper.
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