An immunohistochemical method that differentiates Cryptococcus neoformans varieties and serotypes in formalin-fixed paraffin-embedded tissues

Preclinical Models for Cryptococcosis of the CNS and Their Characterization Using In Vivo Imaging Techniques

Infections caused by Cryptococcus neoformans and Cryptococcus gattii remain a challenge to our healthcare systems as they are still difficult to treat. In order to improve treatment success, in particular for infections that have disseminated to the central nervous system, a better understanding of the disease is needed, addressing questions like how it evolves from a pulmonary to a brain disease and how novel treatment approaches can be developed and validated. This requires not only clinical research and research on the microorganisms in a laboratory environment but also preclinical models in order to study cryptococci in the host. We provide an overview of available preclinical models, with particular emphasis on models of cryptococcosis in rodents. In order to further improve the characterization of rodent models, in particular the dynamic aspects of disease manifestation, development, and ultimate treatment, preclinical in vivo imaging methods are increasingly used, mainly in research for oncological, neurological, and cardiac diseases. In vivo imaging applications for fungal infections are rather sparse. A second aspect of this review is how research on models of cryptococcosis can benefit from in vivo imaging methods that not only provide information on morphology and tissue structure but also on function, metabolism, and cellular properties in a non-invasive way.

Ferret Dermatology

Skin diseases commonly affect pet ferrets, with neoplastic, endocrine, and parasitic diseases being the most common. This review includes clinical presentation, diagnostic workup, and treatment of diseases with a dermatologic presentation in ferrets.

Important Mycoses of Wildlife: Emphasis on Etiology, Epidemiology, Diagnosis, and Pathology—A Review: PART 1

Simple Summary

The number of wild animals is steadily declining globally, so the early diagnosis and proper treatment of emerging diseases are vital. Fungal diseases are commonly encountered in practice and have a high zoonotic potential. This article describes aspergillosis, candidiasis, histoplasmosis, cryptococcosis, and penicilliosis, and is only the first part of a detailed review. The laboratory methods (fungal isolation, gross pathology, histopathology, histochemistry, cytology, immunohistochemistry, radiography, CT, PCR, or ELISA) used in the diagnosis and the clinical details that provide a complete view of the mycoses are presented.

Abstract

In the past few years, there has been a spurred tripling in the figures of fungal diseases leading to one of the most alarming rates of extinction ever reported in wild species. Some of these fungal diseases are capable of virulent infections and are now considered emerging diseases due to the extremely high number of cases diagnosed with fungal infections in the last few decades. Most of these mycotic diseases in wildlife are zoonotic, and with the emergence and re-emergence of viral and bacterial zoonotic diseases originating from wildlife, which are causing devastating effects on the human population, it is important to pay attention to these wildlife-borne mycotic diseases with zoonotic capabilities. Several diagnostic techniques such as fungal isolation, gross pathology, histopathology, histochemistry, cytology, immunohistochemistry, radiography, CT, and molecular methods such as PCR or ELISA have been invaluable in the diagnosis of wildlife mycoses. The most important data used in the diagnosis of these wildlife mycoses with a zoonotic potential have been re-emphasized. This will have implications for forestalling future epidemics of these potential zoonotic mycotic diseases originating from wildlife. In conclusion, this review will highlight the etiology, epidemiology, diagnosis, pathogenesis, pathogenicity, pathology, and hematological/serum biochemical findings of five important mycoses found in wild animals.

Evaluation of the clinical performance of 2 point-of-care cryptococcal antigen tests in dogs and cats

Background

Point‐of‐care (POC) Cryptococcus antigen assays may provide veterinarians with a more rapid, patient‐side diagnosis when compared with traditional laboratory‐based latex agglutination tests.

Objective

To determine the sensitivity and specificity of 2 POC lateral flow cryptococcal serum antigen tests, CrAg LFA (Immy, Norman, OK) and the CryptoPS (Biosynex, Strasbourg, France) for diagnosis of cryptococcosis in dogs and cats, using the cryptococcal antigen latex agglutination system (CALAS) as the reference standard.

Animals

102 serum samples from 51 dogs and 40 cats.

Methods

Specimens were classified as CALAS‐positive (n = 25) or CALAS‐negative (n = 77). The sensitivity and specificity of each POC assay was calculated by comparing the results to the serologic reference standard results.

Results

The CrAg LFA assay correctly classified 23/25 CALAS‐positive specimens and 69/74 CALAS‐negative specimens resulting in a sensitivity of 92.0% (confidence interval [CI], 75.0%‐98.6%) and specificity of 93.2% (CI, 85.1%‐97.1%). The CryptoPS assay correctly classified 8/10 tested CALAS‐positive specimens and 56/59 tested CALAS‐negative specimens resulting in a sensitivity of 80.0% (CI, 49.0%‐96.5%) and specificity of 94.9% (CI, 86.1%‐98.6%).

Conclusion and Clinical Importance

The POC assays appear to be a sensitive and specific alternative to the traditional CALAS assay with more rapid turnaround times, which may result in earlier diagnosis and treatment.

Pathologic, histochemical, and immunohistochemical findings in pulmonary and encephalitic cryptococcosis in a goat

We describe the pathologic, histochemical, and immunohistochemical findings associated with pulmonary and encephalitic cryptococcosis in a 3-y-old, mixed-breed, nanny goat from central-west Brazil. The goat had progressive neurologic signs over 30 d; cryptococcosis was diagnosed antemortem by cytologic evaluation of cerebrospinal fluid. Treatment was initiated, but the animal died spontaneously shortly thereafter. Grossly, there was a large space-occupying gelatinous mass (cryptococcoma) in the left lung and smaller masses in the cerebral temporal and frontal cortex, thalamus, hippocampus, basal nuclei, and mesencephalon with consequent internal hydrocephalus. Histologic evaluation revealed marked granulomatous cryptococcal pneumonia and meningoencephalitis. Intralesional narrow-necked budding cryptococcal yeasts were identified on special stains (Mayer mucicarmine and Grocott methenamine silver) in sections of lung and brain. Immunohistochemistry utilizing a panel of monoclonal antibodies that selectively label capsules of Cryptococcus spp. was consistent with C. neoformans var. grubii.

Atypical cutaneous cryptococcosis in four cats in the USA

BACKGROUND

Cryptococcosis is an uncommon fungal infection in humans and mammals. Occasionally, cryptococcosis manifests as cutaneous lesions, either as an extension of nasal disease or as stand alone lesions unassociated with the nose. Histologically, these lesions are typically characterized by abundant organisms with mild granulomatous dermatitis. Herein, four feline cases of atypical cutaneous cryptococcal infections are described.

METHODS

Skin punch biopsies from four client owned cats were submitted for histological evaluation between 2006 and 2015. Histological examination, including histochemical stains, was performed in all cases. Immunohistochemical stains and PCR were performed in three of four cases. Fungal culture was performed in two cases and transmission electron microscopy was performed in one case.

RESULTS

Grossly, the cutaneous lesions were papular to nodular with occasional ulceration and were located predominantly on the trunk. Histological examination revealed severe granulomatous to pyogranulomatous and eosinophilic dermatitis with rare, capsule-deficient yeasts. Immunohistochemistry, PCR and fungal culture confirmed Cryptococcus spp. to be the aetiological agent in these cases.

CONCLUSIONS AND CLINICAL IMPORTANCE

In cutaneous lesions, capsule-deficient strains of Cryptococcus spp. may induce a severe inflammatory response with rare intralesional organisms that may not be readily identified on routine haematoxylin and eosin stained slides. Special stains with careful examination and ancillary tests (PCR, immunohistochemistry, fungal culture or antigen testing) should be performed when pyogranulomatous and eosinophilic dermatitis is encountered without an identifiable cause.

Cryptococcosis in cats: ABCD guidelines on prevention and management

OVERVIEW

Cryptococcosis is worldwide the most common systemic fungal disease in cats; it is caused by the Cryptococcus neoformans- Cryptococcus gattii species complex, which includes eight genotypes and some subtypes (strains) with varying geographical distribution, pathogenicity and antimicrobial susceptibility. Cats acquire the infection from a contaminated environment. The prognosis is favourable in most cases, provided a diagnosis is obtained sufficiently early and prolonged treatment is maintained.

INFECTION

Basidiospores are the infectious propagules of Cryptococcus species as they penetrate the respiratory system and induce primary infection. Asymptomatic colonisation of the respiratory tract is more common than clinical disease. Avian guanos, particularly pigeon droppings, offer favourable conditions for the reproduction of C neoformans. Both Cryptococcus species are associated with decaying vegetation.

DISEASE SIGNS

Cryptococcosis caused by C neoformans or C gattii is indistinguishable clinically. The disease can present in nasal, central nervous system (which can derive from the nasal form or occur independently), cutaneous and systemic forms.

DIAGNOSIS

An easy and reliable test for cryptococcosis diagnosis is antigen detection in body fluids. Only isolation and polymerase chain reaction allow identification of the species genotype.

DISEASE MANAGEMENT

Amphotericin B, ketoconazole, fluconazole and itraconazole have all been used to treat cats. Surgical excision of any nodules in the skin, nasal or oral mucosa assists recovery. Continued treatment is recommended until the antigen test is negative.

PREVENTION

Efficient preventive measures have not been demonstrated. Vaccines are not available.

Selected emerging diseases in ferrets

Since their introduction as pets several decades ago, ferrets have become an increasingly popular household pet. Great strides have been made in improving their diet and understanding common diseases (eg, insulinoma, hyperadrenocorticism, lymphoma) that affect them. With the frequency with which these conditions are seen, it sometimes is easy to forget that ferrets can be affected by other diseases. Some of these diseases, such as cryptococcosis, are known, but may be increasing in incidence and range, whereas others, such as hypothyroidism and pure red cell aplasia, may be underrecognized or underreported. This review highlights new and emerging diseases not already well reviewed in the literature.

Bovine cryptococcal meningoencephalitis

Cryptococcal meningitis and encephalitis is reported in a 5-year-old bull. The clinical signs included multifocal neurological deficits manifested by hypermetria, ataxia, depression, circling, impaired vision, head pressing, low head carriage, wide-based stance, and falling to the side or backwards. At necropsy, 6 cavities with a gelatinous content were observed in the brain and brain stem, and the meninges were thickened and off white. Numerous yeasts, with a nonstaining capsule, and associated mild inflammatory reaction were observed histologically. Specimens for culture were not collected at necropsy, but the identity of the yeast in tissue sections as Cryptococcus neoformans var. grubii was supported by immunohistochemistry.

Feline Cryptococcosis

Disease summary Cryptococcosis, principally caused by Cryptococcus neoformans and Cryptococcus gattii, is the most common systemic mycosis of cats worldwide. Cats may be infected following inhalation of spores from the environment, with the nasal cavity suspected as being the initial site of colonization and subsequent infection. Other sites of infection in cats are the skin, lungs, lymph nodes, central nervous system (CNS), eyes and, occasionally, periarticular connective tissue. Cryptococcosis can be diagnosed using serology (antigen testing), cytologic examination of smears, histopathology or culture. Treatment of localized disease is generally successful using azole antifungal drugs; however, cats with CNS involvement or disseminated disease require additional treatment with amphotericin B, with or without flucytosine. The prognosis is variable, depending on host and pathogen factors. Some cats require long-term (>1 year) treatment or indefinite therapy.

Patient group Cats of any breed, gender and age may be affected. Retroviral status does not appear to be a risk factor for developing cryptococcosis and indoor cats are not protected from disease.

Global importance Feline cryptococcosis occurs worldwide, but is most frequently reported in Australia, western Canada and the western United States. Species and molecular type vary in different geographical regions and may affect clinical presentation and antifungal susceptibility patterns.

Clinical challenges Serologic tests that detect cryptococcal antigen in serum are sensitive and specific, but false negatives can occur in cats with localized disease. Long-term drug therapy can be expensive and has the potential for toxicity. The extent to which the pathogenicity and antifungal susceptibility is affected by molecular type is currently under study.

Evidence base This review draws on recent literature relating to epidemiology, CNS involvement and advanced diagnostic imaging to update clinicians regarding research findings relevant to clinical practice.

Pathogenesis of pulmonary Cryptococcus gattii infection: a rat model

A model of pulmonary cryptococcosis in immunocompetent rats was developed to better understand the virulence of Cryptococcus gattii. Six isolates were studied, representing four molecular genotypes (VGI-MATα, VGIIa-MATα, VGIIa-MAT a, VGIIb-MATα), obtained from Australia, Vancouver (Canada) and Colombia. These originated from human patients, a cat and the environment and were administered intratracheally (i.t.) or transthoracically into Fischer 344 or Wistar-Furth rats in doses varying from 10(4) to 10(7) colony-forming units (CFU) in 0.1 ml of saline. With the exception of animals given the VGIIa-MAT a isolate, rats consistently became ill or died of progressive cryptococcal pneumonia following i.t. doses exceeding 10(7) CFU. Affected lungs increased in weight up to tenfold and contained numerous circumscribed, gelatinous lesions. These became larger and more extensive, progressing from limited hilar and/or tracheal lesions, to virtually confluent gelatinous masses. Disease was localized to the lungs for at least 3-4 weeks, with dissemination to the brain occurring in some animals after day 29. The dose-response relationship was steep for two VGI isolates studied (human WM179, environmental WM276); doses up to 10(6) CFU i.t. did not produce lesions, while 10(7) or more yeast cells produced progressive pneumonia. Intratracheal inoculation of rats with C. gattii provides an excellent model of human pulmonary cryptococcosis in healthy hosts, mimicking natural infections. Disease produced by C. gattii in rats is distinct from that caused by C. neoformans in that infections are progressive and ultimately fatal.

Differentiation of Fusarium from Aspergillus species by colorimetric in situ hybridization in formalin-fixed, paraffin-embedded tissue sections using dual fluorogenic-labeled LNA probes

Fusarium and Aspergillus are 2 genera of fungal pathogens that can result in devastating disease particularly in immunosuppressed hosts. In tissue sections, these organisms can be extremely difficult to distinguish from one another. To differentiate between these 2 pathogens, a rapid (<3 hours) ribosomal RNA (rRNA) in situ hybridization (ISH) protocol using dual fluorogenic-labeled oligonucleotide probes composed of a mixture of DNA and locked nucleic acids (LNAs) was developed. This assay was able to differentiate between Aspergillus and Fusarium in formalin-fixed, paraffin-embedded tissue sections. ISH targeting rRNA can be used to identify the species of fungal pathogens in surgical pathology material and may be useful when pathogens are histologically observed but cultures are negative or have not been performed. ISH with dual-labeled LNA probes may be useful for detecting a variety of fungal pathogens in formalin-fixed, paraffin-embedded tissue specimens.

Spinal cryptococcoma in an immunocompetent cat

This report describes an unusual case of primary cryptococcoma in the proximal thoracic spinal cord of an 11-year-old immunocompetent cat from a farm on which there were large numbers of pigeons. This animal was referred for examination with progressive paralysis and shown to be free from feline immunodeficiency virus, feline leukaemia virus, feline coronavirus and Toxoplasma gondii. It died 2 months later. At necropsy, the only lesion detected was a malacic area, 4cm in length, in the spinal cord. Histopathological examination of the spinal cord revealed severe granulomatous inflammation associated with large numbers of encapsulated yeast cells. In addition to the granulomatous host response, necrosis, digestion chambers, Gitter cells, spheroids and lymphocytic perivascular cuffs were features of the malacic areas. Immunohistochemistry confirmed the presence of Cryptococcus neoformans var. grubii yeast cells.

Successful treatment of cryptococcal pneumonia in a pony mare

A 20-year-old Welsh Mountain Pony (212 kg) mare was initially presented for a chronic cough, fever, weight loss and low grade abdominal pain. She later developed dyspnoea, tachypnoea and exercise intolerance. The presence of multiple masses (up to 17 cm diameter) in the pulmonary parenchyma was established using lateral thoracic radiography and transthoracic ultrasonography. Encapsulated, budding yeasts were observed in smears made from transtracheal washings and needle aspirates of the pulmonary lesions. Cryptococcus gattii (synonym: Cryptococcus neoformans variety gattii; Cryptococcus bacillisporus) was cultured from the transtracheal washings and aspirates of the lung masses. The pony was successfully treated using daily intravenous infusions of amphotericin B (typically 0.5 mg/kg in 1 L 5% dextrose in water over 1 h, following premedication with 50 mg flunixin intravenously) over a 1 month period, until a cumulative dose of 3 g had been administered. Treatment was considered to be successful on the basis of progressive improvement in clinical signs, reduction in the size of pulmonary cryptococcomas, 48 kg weight gain and a reduction in the cryptococcal antigen titre from 4096 to 256, 1 year after cessation of treatment.

Long-term outcome of therapy for 59 cats and 11 dogs with cryptococcosis

OBJECTIVE

To determine the outcome of therapy in cats and dogs with naturally occurring cryptococcosis. Design Retrospective study of 59 cats and 11 dogs at the University Veterinary Centre Sydney from 1986 to 2004.

METHOD

Following detailed analysis of case notes potential associations between patient characteristics, cryptococcal species, retroviral status (cats), disease severity and type of therapy were examined in relation to duration and success of therapy. Treatment protocols based on amphotericin B, fluconazole and itraconazole were compared.

RESULTS

Seventy-six percent of feline patients were successfully treated. For cats, the presence of central nervous system disease was the only factor found to influence outcome. Cats with neurological involvement, disseminated disease or refractory disease treated with amphotericin B containing protocols did as well, on average, as cats with less severe disease treated with azole monotherapy. Amphotericin B was thus an effective agent for treating severe cases of cryptococcosis. The median cumulative dose of amphotericin B for cats cured at the first attempt was 16 mg/kg (range 7 to 23 mg/kg). The median duration of treatment required to effect a cure at first attempt was significantly shorter for fluconazole (4 months; range 1 to 8 months) than for itraconazole (9 months; range 3 to 24 months; P = 0. 0191; Mann Whitney U test). The success rate for treatment of canine cases was 55%. No factor appeared to influence disease outcome in dogs. Large cumulative doses of amphotericin B could be administered via the subcutaneous route in both species and generally with minimal nephrotoxicity. Recrudescence occurred in a significant proportion of animals, in some cases despite a reduction of serum latex cryptococcal antigen agglutination test to zero.

CONCLUSION

Although the prognosis of cryptococcosis should be described as guarded, a majority of the canine and especially feline patients can be expected to be cured, although treatment is protracted and expensive.

Retrospective study of feline and canine cryptococcosis in Australia from 1981 to 2001: 195 cases

A retrospective study of 155 cats and 40 dogs diagnosed with cryptococcosis between 1981 and 2001 was undertaken. Age, sex, breed, clinical findings, feline immunodeficiency virus and feline leukaemia virus status (in cats), species of Cryptococcus causing disease and region of domicile were recorded. Associations between variables were tested. Male and female cats were affected equally. Age ranged from 1 to 16 years, with a preponderance of cats aged between 2 and 3 years. Siamese, Himalayan and Ragdoll breeds were over-represented. Rural cats were more frequently infected with Cryptococcus gattii. Retroviral infection was not identified as a predisposing condition and was not correlated with either species of Cryptococcus or physical findings. Most cats had signs of nasal cavity infection, which was typically localised for a substantial period before invasion of adjacent structures or dissemination. Male and female dogs were affected equally. A marked preponderance of young, large breed dogs was noted. Border Collies, Boxers, Dalmatians, Dobermann Pinschers, Great Danes and German Shepherds were over-represented. Cryptococcus species involved was not affected by place of domicile. Although nasal cavity involvement was important, the canine cohort had a greater propensity to develop secondary central nervous system involvement and disseminated disease than feline cases. There were no clinical findings in either cats or dogs which could be reliably used to distinguish disease caused by Cryptococcus neoformans variety grubii from disease caused by Cryptococcus gattii. Both Cryptococcus species appear to be primary pathogens of cats and dogs, with the upper respiratory tract presumed to be the predominant primary site of inoculation in most but not all cases.

Clinicopathologic features of an unusual outbreak of cryptococcosis in dogs, cats, ferrets, and a bird: 38 cases (January to July 2003)

OBJECTIVE

To determine clinical and pathologic findings associated with an outbreak of cryptococcosis in an unusual geographic location (British Columbia, Canada).

DESIGN

Retrospective study.

ANIMALS

1 pink-fronted cockatoo, 2 ferrets, 20 cats, and 15 dogs.

PROCEDURE

A presumptive diagnosis of cryptococcosis was made on the basis of serologic, histopathologic, or cytologic findings, and a definitive diagnosis was made on the basis of culture or immunohistochemical staining.

RESULTS

No breed or sex predilections were detected in affected dogs or cats. Eleven cats had neurologic signs, 7 had skin lesions, and 5 had respiratory tract signs. None of 17 cats tested serologically for FeLV yielded positive results; 1 of 17 cats yielded positive results for FIV (western blot). Nine of 15 dogs had neurologic signs, 2 had periorbital swellings, and only 3 had respiratory tract signs initially. Microbiologic culture in 15 cases yielded 2 isolates of Cryptococcus neoformans var grubii (serotype A) and 13 isolates of C. neoformans var gattii (serotype B); all organisms were susceptible to amphotericin B and ketoconazole. Serologic testing had sensitivity of 92% and specificity of 98%.

CONCLUSIONS AND CLINICAL RELEVANCE

Serologic titers were beneficial in identifying infection in animals with nonspecific signs, but routine serum biochemical or hematologic parameters were of little value in diagnosis. Most animals had nonspecific CNS signs and represented a diagnostic challenge. Animals that travel to or live in this region and have nonspecific malaise or unusual neurologic signs should be evaluated for cryptococcosis.

A ten-year-old boy with a pulmonary nodule secondary to Cryptococcus neoformans: case report and review of the literature

Pulmonary cryptococcosis is an uncommonly recognized disease of childhood. Among immunocompetent and non-HIV-infected individuals, pulmonary cryptococcosis may be asymptomatic or present with chronic, nondescript symptomatology. In this report we describe a 10-year-old with malignant fibrous histiocytoma of bone and a pulmonary nodule secondary to Cryptococcus neoformans. We use this case as a background to review the pediatric literature regarding pulmonary cryptococcosis and to discuss the utility of immunohistochemistry for diagnosis of this clinical entity.

Cryptococcus neoformans var. gattii in the koala (Phascolarctos cinereus): a review of 43 cases of cryptococcosis

Details of 11 previously reported cases and 32 new cases of cryptococcosis in captive and wild koalas were analysed. Cryptococcus neoformans var. gattii accounted for all 29 cases in which varietal status was determined. No age or sex predisposition was observed. The respiratory tract was the primary focus of disease in 77% of cases. Although the lower respiratory tract was affected most commonly (60% of cases), 30% of cases had upper respiratory tract lesions and 14% had both. Dissemination was common, especially to the central nervous system (37% cases). Local extension to surrounding tissues was a feature of upper respiratory tract disease. Other tissues showing cryptococcal invasion included lymph nodes (19%), gastrointestinal tract (12%), kidneys (12%), spleen (9%) and skin (7%). Only three cases (7%) had no respiratory tract or central nervous system involvement, two cases of primary skin inoculation and one case of primary lymphadenopathy. Late presentation was a likely factor in the high proportion of cases with disseminated disease (40%). The proportion of koala cases with involvement of the central nervous system, lower respiratory tract and skin, parallels what has been reported for immunocompetent people. Cryptococcosis in the koala appears to be an excellent naturally occurring model for examination of the cryptococcal host-parasite relationship in all species.

Central nervous system cryptococcoma in a cat

This report describes the gross, histopathological, immunohistochemical and electron microscopic findings in a 4-year-old cat with systemic Cryptococcus neoformans infection. A 1-cm diameter pontine mass, pinpoint lesions in the cerebellum and in the right kidney were the main macroscopic findings. A presumptive diagnosis of cerebral neoplasia with metastasis was formulated. Light microscopy revealed a huge number of yeasts surrounded by a pyogranulomatous inflammatory reaction in the cerebral parenchyma and, to a lesser extent, in the kidney, while the meninges were not involved. The positive mucicarmine stain that coloured the capsule of the yeasts was indicative of cryptococcal infection. Immunohistochemistry confirmed the presence of C. neoformans var. grubii (C. neoformans serotype A). Electron microscopy revealed yeasts with a massive fibrillar capsule and lamellar cell wall free in the cerebral tissue and within macrophages.

Cryptococcus neoformans var. gattii in the koala (Phascolarctos cinereus): serological evidence for subclinical cryptococcosis

Cryptococcus neoformans var. gattii has been shown to have a strong association with eucalypts frequently used by koalas and, not surprisingly, it has been shown to colonize the nasal cavities of koalas. The progression from nasal colonization to tissue invasion is critical to understanding the pathogenesis of cryptococcosis in this species and provides a model for pathogenesis of cryptococcosis in other species. Cryptococcal antigenaemia was detected in twenty-eight healthy koalas from three different regions. This was interpreted as representing limited subclinical disease. One koala developed cryptococcal pneumonia 6 months after leaving the study, whereas another developed cryptococcal meningoencephalitis during the course of the study. Opportunistic necropsies on ten antigen-positive koalas resulted in discovery of small cryptococcal lesions in two (paranasal sinus and lung, respectively). Our data suggest that cryptococcal antigenaemia occurs commonly in koalas, especially in areas with a high environmental presence of C n. var. gattii. Subclinical disease appears most likely to manifest as a small focal lesion in the respiratory tract. Possible outcomes include elimination by an effective immune response, quiescence with possibility of later re-activation or direct progression to overt disease. Symptomatic and subclinical cases showed differences in levels of antigenaemia. The data presented have significant implications for koalas in captivity.