Abstract
The clinical review in this issue of JFMS by Trivedi et al on cryptococcosis in cats highlights the emerging global concerns about cryptococcosis. 1 Animals may be sentinel species when it comes to considering emerging or re-emerging diseases of people, and this seems particularly true of cryptococcosis. Professor Malik, in particular, has been instrumental in promoting the study of cryptococcosis in animals as having high value in discovering the ecological and epidemiological concepts, and the critical virulence factors, relevant to the causative organism. Comparative pathology (now more commonly referred to as ‘One Health’ or ‘One Medicine') is a valuable tool in the study of the pathogenesis of infectious diseases.
It is increasingly apparent that it is no longer appropriate simply to identify or even think of this fungus as Cryptococcus neoformans without an appreciation of the current taxonomical divisions of the C. neoformans—C. gattii species complex (Table 1). While our understanding of these divisions will no doubt evolve, it is essential for clinicians to appreciate the current taxonomy. The diseases caused by strains of each of these molecular types have significant differences that impact both the response to therapy and dissemination of the disease. Data on molecular types is also necessary for the identification of environmental risk factors and to elucidate the important virulence factors that affect the clinical presentation of the disease.
As knowledge evolves of the variation among pathogenic strains of cryptococci, so too does the challenge for the veterinary practitioner. This cytology image shows a VGI isolate with a large capsule from a koala with nasal cryptococcosis (pictured inset), which was diagnosed and treated by one of the editorialists (MBK) and colleagues for 12 months before it died of acute lymphoid leukaemia
The C. neoformans-C. gattii species complex: current accepted taxonomy
For example, currently VNI and VNII have different epidemiologies, as do molecular types VGI, VGII and, likely, VGIII. There are also subtypes of VGII, which behave differently in the environment and in disease. 2,3
The sections in the review on the life cycle of the fungus and the route of infection are useful and, while only brief summaries of the current knowledge, they provide the clinician with necessary information. The idea that cryptococcosis caused by C. gattii is a tropical/subtropical disease has long been an erroneous conclusion (seemingly ignoring the fact that a large proportion of disease caused by C. gattii occurs in temperate Australia); moreover, this review reinforces the fact that cryptococcosis caused by C. gattii has emerged over the past 10–15 years in new environmental niches. The classic example that conclusively broke the paradigm of C. gattii as being a tropical fungus was the emergence of C. gattii VGII as the primary cause of cryptococcosis in western Canada. This molecular type is now extending further south along the western coast of North America.
The emergence of cryptococcosis in western Canada established C. gattii VGII as a major pathogen (hitherto only important in northern and western Australia) but, most importantly, revealed a plasticity in environmental associations of VGII. 4,5 However, as a consequence of the lack of routine specific typing of these organisms, the precise epidemiology is still obscure.
Comparison of pathology of cryptococcosis in cats (based on four major case series)
∗Additional cases in cats from 2003-2006 in western Canada confirmed the predominance of CNS signs, as indicated in the original study (January to July 2003). 11 There were a total of 119 cases of feline cryptococcosis for which the original presenting clinical signs were recorded: 39.5% cats presented with CNS signs, 28% had pulmonary or other respiratory signs, 25% had skin or nasal granulomas, while 3.3% had mandibular lymph node enlargement, 2.5% presented with gastrointestinal signs and 1.7% had renal signs. Forty-six cases were typed; 42 of these were C. gattii VGIIa while four cases, all with skin granulomas, were C. neoformans var. grubii (SJL, unpublished observations)
The VGIIa isolate has been identified from archived tissue in the Pacific Northwest from 1971, which raises the possibility that this environmental niche is not new, but factors within the species or the environment have allowed for a more virulent strain to develop. 6 Similarly, the emerging prevalence of cryptococcosis caused by C. gattii VGIII emphasised in the review by Trivedi et al points to new environmental associations of this fungus in California. 5,7
The discussion on diagnosis, prognosis and treatment will make this review particularly useful to feline clinicians as it provides up-to-date practical information to demystify this disease and allow appropriate clinical intervention for those who may only encounter this disease infrequently. The review highlights the fact that the clinical manifestations of cryptococcosis in cats in Australia and California are somewhat different, presumably related to the differing prevalence of different members of the C. neoformans—C. gattii complex (in particular, the molecular types of C. gattii and the prevalence of VGII and VGIII) between these two disease populations. This is particularly important for the implementation of effective empirical therapeutic strategies in the different populations. It seems reasonably clear that VGII, at least, has a different antifungal susceptibility profile (likely intrinsic) compared with VGI (which predominates in eastern Australia). This was first identified in cases of animal cryptococcosis in the Canadian outbreak and has spawned recent research into the mechanisms of azole resistance in C. gattii. C. gattii VGII has a higher inherent resistance to fluconazole than VGI but still appears susceptible to ketoconazole. 8,9
The comparative pathology of cryptococcosis highlights both the species differences and similarities. The differences in disease presentation are common, which could reflect a difference in the microenvironment sampled by the different species or a fundamental difference in the pathogen or induced host response that promotes infection in some species. These differences, when elucidated, will likely identify a fundamental characteristic of early pathogenesis and will be useful as a therapeutic target. In the HIV pandemic, cryptococcosis was significantly associated with immunocompromised individuals but now is described equally often in immunocompetent individuals and further underlines the identification of C. gattii as a re-emerging pathogen. In people there is a predominance of pulmonary involvement, which may not be reflected in the clinical signs but is identified as cryptococcomas within the lung parenchyma and also within the brain. Dissemination to other tissues also occurs. It has been suggested that the different incidence in dogs as compared with cats reflects the anatomy of the nasal cavity. The incidence differential may have a more complex foundation related to immune responses since C. gattii is found in bark and tree habitats; if the infectious propagule is inhaled, then the rooting behaviour of dogs should result in increased incidence as compared with cats, which tend to a more fastidious nature.
The incidence in cats is, in fact, far higher than in other domestic species and fastidious grooming behaviour as well as the breathing pattern of cats could contribute to increased delivery of the fungus to the nasal cavity, or the difference may relate to species-specific host responses. Given that the organism utilises intracellular survival in its pathogenesis, and the primary host immune responses are predominantly cell-mediated, study of the cat's immune responses could provide insight into the pathogenesis of the disease in all species; in particular, comparative investigation of macrophage activity among host species will provide critical insights into the virulence of these fungal species. 10
As noted previously, CNS involvement is greater in the western Canadian and western US disease populations than that observed in Australia, which may reflect pathogen factors. However, it would be useful to tease out more definitively the differences in pathology among the different molecular types (Table 2).
The apparent regional differences of cryptococcosis in cats could be explained by the molecular type of C. gattii, but there is insufficient data presented in these studies to allow for the type of metadata approach required to build significant numbers of cases. Unfortunately, the differences are obscured by the number of unreported molecular types. While the existence of repeatable molecular type subdivisions has been known since at least 1997, the biological relevance of this has really only been emphasised in the past 8–9 years. Table 2 highlights the necessity to refine standard diagnostic laboratory identification of fungi from the C. neoformans—C. gattii species complex, biobank disease-cultured isolates and further examine the capacity to molecular type from less ideal DNA sources such as formalin-fixed paraffin-embedded tissues. There may be some capacity to do this from the ITS region as well as an MLST approach with shorter PCR products than the standardised MLST protocol. 13
The section on predisposing factors is particularly affected by previous studies’ failure to document the aetiological agent in sufficient detail (ie, molecular type). The exposure to disturbed soil as a predisposing factor in western Canada is related at least partially to the aetiological agent being solely VGII, rather than just regional variation.
Given the dependence on immune responses, retroviral status as a predisposing factor may also relate to the specific aetiology. The role of cell-mediated immunity indicates that Th-1 responses may be critical to fungal control, while, at least in humans, the Th1-Th2-Th17 balance is critical to the outcome. 14 This could relate to the retroviral infections in felines; Th1 responses decrease over time with HIV infection, which corresponds to the increased incidence of cryptococcal infections. Since many of our patients are not examined over long periods, due to loss of life either from disease or euthanasia, the role of the viruses over time is still unknown. These associations are not currently capable of being tested effectively from a metadata approach.
While infectious disease is the result of many interactions among host, pathogen and environment, 15 the evolving knowledge of the variation among pathogen strains is an area that feline clinicians need to understand. This review makes an excellent contribution to this process. As it points out, much remains to be learned. Continued development of molecular assays and speciation of the organism is a requirement for further advancement of our understanding of the pathogenesis of cryptococcosis.