GUIDELINE for Cryptococcosis in Cats

Published: 01/01/2013
Last updated: 06/11/2024
Last reviewed:

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The Cryptococcosis in cats guidelines were first published in the J Feline Med Surg 2013, 15: 611-618 by Maria Grazia Pennisi et al. The present guidelines were updated by Maria Grazia Pennisi and ABCD colleagues.

Key points

  • Cryptococcosis is a non-contagious, rare or sporadic disease, which occurs worldwide and is considered the most common systemic fungal disease in cats.
  • It is caused by the Cryptococcus neoformans-Cryptococcus gattiispecies complex, which includes eight genotypes and some subtypes (strains) with different geographical distribution, pathogenicity and antimicrobial susceptibility. However, they are clinically indistinguishable.
  • Cats acquire the infection from a contaminated environment.
  • Avian guano, particularly pigeon droppings, offer favourable conditions for the reproduction of  neoformans, but both C. neoformans and C. gattiispecies are associated with decaying vegetation such as Eucalyptus leaves.
  • Basidiospores are the infectious propagules of Cryptococcus and usually they penetrate the respiratory system and induce the primary infection there.
  • Asymptomatic superficial colonization of the respiratory tract is more common than clinical disease that develops when tissue invasion occurs.
  • Cryptococcosis can present in several different clinical forms, including nasal, central nervous system, cutaneous, and systemic forms.
  • An easy and reliable test for the diagnosis of cryptococcosis is antigen detection in blood serum, urine and body fluids. Biopsy samples can be collected from lesions and submitted for cytology, histopathology and PCR.
  • Only isolation and PCR provide identification of the species and the genotype involved.
  • Prognosis is favourable in most cases, provided a diagnosis is obtained sufficiently early in the course of disease and treatment compliance in patients and owners is good as a long treatment course (months) and follow-up (years) are required.
  • Treatment guidelines have not been established and the choice of the appropriate antifungal drug given depends on many factors, including owner compliance.
  • Amphotericin B, fluconazole and itraconazole are the most commonly used drugs. Dual treatments are used particularly in central nervous system and systemic forms.
  • Any possible surgical excision of nodules and masses from affected tissues and organs is recommended as valuable adjunct treatment in cats undergoing medical therapy.
  • Cryptococcosis is not a zoonotic disease and cats with the disease are not contagious to humans and other animals. They are considered sentinels of environment contamination.
  • The presence of avian guano, particularly pigeon droppings and some decaying vegetation substrates, such as Eucalyptus leaves, may be considered a risk factor, but efficient preventative measures have not been demonstrated.
  • Vaccines are not available.

Agent properties

Feline cryptococcosis is caused by basidiomycetous yeasts of the genus Cryptococcus belonging to the C. neoformans species complex and the C. gattii species complex. A previous classification distinguished five serotypes (A, B, C, D, AD) according to antigenic characteristics of the capsular polysaccharide (Takashima and Sugita, 2022). Updated nomenclature, based also on genotyping, differentiated two main species affecting cats: C. neoformans – including the varieties C. n. var. grubii (formerly serotype A) and C. n. var. neoformans (formerly serotype D) – and C. gattii (formerly serotypes B and C). Based on molecular typing and subtyping characterization, seven reclassified species of C. neoformans and C. gattii species complexes and three hybrids are now described (Takashima and Sugita, 2022). Isolates from C. neoformans species complex were reclassified as two different species: C. neoformans (formerly C. n. var. grubii) and C. deneoformans (formerly C. n. var. neoformans). C. gattii complex has been reclassified into five species with four belonging to serotype B (C. gattii, C. bacillosporus, C. decagattii, C. deuterogattii) and one to serotype C (C. tetragattii). The different species have different epidemiological characteristics, virulence and antimicrobial susceptibilities (Takashima and Sugita, 2022).

The cryptococcus fungus can differentiate into several morphological forms including yeast, chlamydospores, pseudohyphae and hyphae under certain conditions, but it is typically present in the yeast form in mammalian hosts, reproducing by mitosis in animal tissues (Alspaugh et al., 2000; Lin and Heitman, 2006). Other Cryptococcus species have been rarely reported in cats: Cryptococcus albidus (currently renamed Naganisha albida) may affect immunocompromised cats and Cryptococcus magnus has been isolated in cats with otitis (Kano et al., 2004, 2008). 

Epidemiology

Cryptococcosis affects humans, cats, dogs, ferrets, horses, goats, sheep, cattle, marine mammals, koalas and other marsupials, birds, reptiles, amphibia and fish (Danesi et al., 2021). It has a worldwide distribution and is observed more commonly in cats than in dogs (McGill et al., 2009). Cats are five to six times more likely to be affected by the disease than dogs, and three times more than horses (McGill et al., 2009).

In Europe, Cryptococcus spp. has been reported in Austria, Belgium, Bosnia and Herzegovina, Denmark, France, Germany, Greece, Italy, the Netherlands, Portugal, Spain, Sweden and the United Kingdom. However, updated data considering the prevalence of species currently described are lacking (Lester et al., 2011; Nunes Rodrigues et al., 2020; Glavinić et al., 2024). Unfortunately, Cryptococcus is not usually identified to the species and molecular level with routine diagnostic sampling, and data regarding the feline disease in Europe are from single case reports or small case series, since the disease usually occurs sporadically (Castella et al., 2008; Nunes Rodrigues et al., 2020; Vercelli et al., 2021; Glavinić et al., 2024).  Larger retrospective studies are available only from Canada, Australia and California (Craig et al., 2002; O’Brien et al., 2004; Duncan et al., 2005, 2006; McGill et al., 2009; Sykes et al., 2010).

The disease is rare or sporadic. However, a large-scale outbreak of cryptococcosis caused by C. gattii molecular type VGII (currently renamed C. deuterogattii), starting in 1999 and continuing for about 20 years, involving humans, terrestrial (dogs, cats, ferrets, llamas, horses, birds) and marine (porpoises Phocoenoides dalli) animals, was reported for the first time. Three unrelated outbreaks clones occurred on southern Vancouver Island, British Columbia, Canada and the American states of Washington and Oregon in the Pacific Northwest region, characterized by wet, mild winters and dry, warm summers. Twenty years after the onset of the outbreak, it has been hypothesized that a tsunami caused in 1964 by the great Alaskan earthquake (M9.2 on the Richter scale) was responsible for a large scale displacement of C. gattii from sea water into nearby coastal forests of the Pacific Northwest territory, requiring a long period of land adaptation of C. gattii VGIIa before it caused a One Health emergence (Engelthaler and Casadevall, 2019). C. gattii species complex has been detected in Western Europe, Southern Africa, the Pacific Northwest and Western California (Barrs et al., 2024a).  Cryptococcus neoformans. var. grubii (renamed Cryptococcus neoformans) has a worldwide distribution and is commonly isolated from affected individuals in various animal species. Cryptococcus deneoformans is observed mainly in Europe and causes cutaneous infections (Takashima and Sugita, 2022).

Cryptococcus neoformans species complex ecology is usually related to the presence of avian guano, particularly pigeon droppings, which offer favourable conditions for the mitotic amplification and reproduction of the fungus, but all Cryptococcus species have been associated with decaying vegetation such as Eucalyptus leaves (Fortes et al., 2001). Pigeons serve as Cryptococcus carriers that likely contribute to the worldwide distribution, as they carry Cryptococcus on their beaks, feathers, and legs (Pal, 1989). Importantly, animals, plants, soils and waterways are sources from which the potential pathogen may be acquired.

Prevalence

Environmental exposure and asymptomatic colonization of the respiratory tract are more common than clinical disease (Malik et al., 1997b; Connolly et al., 1999). Asymptomatic nasal carriage may occur in cats with many Cryptococcus species (Danesi et al., 2014). Asymptomatic carriage of C. gattii has been recognized in 4.3% of cats, 1.1% of dogs and in 2% of wild animals (squirrels) trapped in British Columbia during the outbreak that started in 1999 (Bartlett et al., 2003; Duncan et al., 2005).

Retrospective studies of feline cases tend to show a preponderance in males, although this finding was not confirmed in all studies (Malik et al., 1992; Flatland et al., 1996; Jacobs et al., 1997; Gerds-Grogan and Dayrell-Hart, 1997; Lester et al., 2004; McGill et al., 2009; Sykes et al., 2010). Pedigree breeds, such as Ragdoll, Birman, Siamese and Himalayan, were considered more often affected than domestic shorthair or longhair breeds, but this finding has not been confirmed in more recent studies (Malik et al., 1992; O’Brien et al., 2004; McGill et al., 2009; Sykes et al., 2010; Trivedi et al., 2011). In contrast with other animal species, where usually young adults contract the infection, cats of any age may be affected (Malik et al., 1992; McGill et al., 2009; Vercelli et al., 2021). No seasonal trend in the diagnosis of infection has been observed (McGill et al., 2009). Additionally, lifestyle does not seem to be a risk factor and the disease has been reported in indoor cats too.

Transmission

Small size infectious propagules, such as basidiospores (<2 μm) and desiccated yeast cells (<3 μm), are easily dispersed by air flow and can penetrate the respiratory system where primary infection takes place. Less frequently, penetrating injuries can directly inoculate infectious particles into the skin. The oral route is probably responsible for unique gastrointestinal lesions that sometimes arise with crypotococcosis (Reis et al., 2021).

Pathogenesis

Cryptococcus is an airborne pathogen, and the nasal cavity is usually the primary site of infection in cats and dogs. In most cases there is only subclinical colonisation without invasion of the epithelium (Duncan et al., 2005). When invasion of mucosal tissues occurs, progression to disease occurs locally and/or systemically. In both people and cats, the infection may follow ingestion of desiccated yeast cells or, more rarely, cutaneous inoculation of fungal forms by penetrating injuries. The incubation period varies from months to years, and the source of infection often remains unknown. The virulence (genotype) and burden of the infecting organisms influence the outcome of infection.

From the upper respiratory tract, the infection may spread locally to the central nervous system (CNS) through the ethmoid bone, and rarely also to the lower respiratory tract or systemically (Martins et al., 2011).

There are temperature-sensitive species which are unable to grow at temperatures > 37.0°C and may cause infections only at body sites where the temperature is lower (skin, nose, scrotum) (Bemis et al., 2000; Lin, 2009).

Immunity

Antibodies produced against capsular antigens are not protective. Persistent infections can occur because the capsule of Cryptococcus yeast forms inhibits phagocytosis and other virulence factors, such as melanin production, protecting the yeast cells from oxidative damage. Cryptococcus is therefore able to survive inside phagocytic cells – such as macrophages and neutrophils – and can be disseminated with these cells (Urban et al., 2006; Lester et al., 2011; Trivedi et al., 2011).

Some studies suggested that cryptococcosis has a higher prevalence or a less favourable outcome in FeLV- or FIV-infected cats (Gerds-Grogan and Dayrell-Hart, 1997; Jacobs et al., 1997), but this conclusion has not been shared by others (Malik et al., 1992; O’Brien et al., 2004, 2006; Norris et al., 2007; Sykes et al., 2010). Cryptococcosis has been reported in cats under chemotherapy or with concurrent opportunistic infections, so a role for poor immunocompetence in the pathogenesis of infection cannot be excluded (Trivedi et al., 2011; Graham et al., 2011), but differences in virulence exist. 

Clinical signs

Cryptococcosis caused by various species previously considered as C. neoformans and C. gattii species complexes is clinically indistinguishable. The disease can present in several different clinical forms, including the nasal form, CNS form (which can derive from the nasal form or occur independently), the cutaneous form and the systemic form. Geographical differences in the prevalence of some clinical presentations are postulated as a consequence of the distribution of genotypes with differing virulence.

The course of disease is generally chronic and progressive but acute onset presentations are reported with the CNS form and when the respiratory form causes pleural effusions.

Nasal form

The nasal form is the most common in cats, presenting as a chronic sino-nasal disease, either alone or together with contiguous spread to bones, subcutis, the skin, and regional (submandibular) lymph nodes (Malik et al., 1992; O’Brien et al., 2004; McGill et al., 2009). It induces naso-facial swelling followed by deep nonhealing ulceration draining gelatinous exudate, chronic serous, mucopurulent or haemorrhagic nasal discharge (monolateral or bilateral) stertor and inspiratory dyspnoea, sneezing and snuffling and submandibular lymphadenopathy (Figs. 1-3).

Fig.1 Nasal cryptococcosis - chronic monolateral nasal discharge and mild nasal deformity. Courtesy of Maria Grazia Pennisi

Fig.1 Nasal cryptococcosis – chronic monolateral nasal discharge and mild nasal deformity. Courtesy of Maria Grazia Pennisi

Fig. 2. Cryptococcal disease - severe naso-facial swelling and deformity. Courtesy of Maria Grazia Pennisi

Fig. 2. Cryptococcal disease – severe naso-facial swelling and deformity. Courtesy of Maria Grazia Pennisi

Fig. 3. Cryptococcal disease - ulcerated skin nodules on the face. Courtesy of Maria Grazia Pennisi

Fig. 3. Cryptococcal disease – ulcerated skin nodules on the face. Courtesy of Maria Grazia Pennisi

Anorexia and subsequent weight loss may also be a result of anosmia affecting cats with chronic nasal disease. Cryptococcosis is an important differential in cats with chronic nasal discharge, regardless of whether or not facial swelling and/or skin ulceration is present. In some cases, a protruding fleshy mass from one or both nostrils may occur. Nasopharyngeal granulomas (resembling polyps or cancer) presenting with stertor, inspiratory dyspnoea and open mouth-breathing have also been described (Malik et al., 1997a; McEwan and Sykes, 2022). Proliferative or ulcerated lesions in the oral cavity or pharynx may also develop (Nunes Rodrigues et al., 2020). Otitis media/interna with vestibular signs may occur (Beatty et al., 2000; Paulin et al., 2013).

Central nervous system form

Central nervous system involvement most likely arises following local dissemination through the cribriform plate; in such cases, sudden blindness due to optical neuritis appears together with seizures or behavioural changes (Vercelli et al., 2021). In other cases, it follows systemic dissemination and induces granulomatous encephalomyelitis with solitary or multiple lesions and monofocal or multifocal neurologic signs respectively (Belluco et al., 2008; Sykes et al., 2010; Huang et al., 2023; Jacobson et al., 2023a). Many cats show head or spine pain but other signs of meningeal involvement (hyperesthesia, nuchal rigidity) are not common (Sykes et al., 2010). 

Cutaneous form

Cutaneous forms are characterized by solitary or multiple dermal to subcutaneous skin lesions: the former are suggestive of direct inoculation, the latter of haematogenous spread from the primary site of infection. Nodules are more frequently reported, compared to papules and plaques (Myers et al., 2017; Nunes Rodrigues et al., 2020). The nodules are usually alopecic, non-pruritic, not painful, can be ulcerated and are commonly accompanied by regional lymphadenopathy.

Systemic form

Systemic forms may occur through haematogenous dissemination and manifest with signs of meningo-encephalomyelitis (see CNS form), uveitis, chorioretinitis, osteomyelitis and polyarthritis, systemic lymphadenitis or multi-organ involvement, including the kidneys (Figs. 4 and 5). Systemic lymphadenitis as a single sign is rare (Costa et al., 2022). Cryptococcosis, together with feline infectious peritonitis and feline leukaemia virus infection, was found to be one of the most important infectious diseases causing ocular lesions in a study describing ophthalmic and immunopathological findings in samples from cats at necropsy that had died due to a systemic infectious disease in Brazil (Wronski et al., 2023). Lower respiratory tract disease may follow the nasal form but the disease can also present as only pulmonary or mediastinal disease, with or without a pleural effusion, which are evident clinically and radiologically (Evans et al., 2018; Collins and Sunico, 2020). In this case dyspnoea can be severe with open mouth breathing and diagnostic procedures can be performed only after stabilisation and oxygen support. However, a chronic stage lipoid pneumonia, causing a large and poorly defined consolidated lung mass, has been incidentally diagnosed in a cat and cryptococcosis was confirmed after lung lobectomy (Newman and Schaible, 2019).

Nodules can be observed on the oral mucosa (Ferrari et al., 2023). Abdominal cavity involvement with peritonitis is considered extremely rare (Johnston et al., 2021; Teh et al., 2024). It has been described in a very severe case in a FIV and FeLV negative 13 year-old Ragdoll cat with chronic nasal disease followed by dissemination (Teh et al., 2024). The cat was examined about 14 months after the onset of upper respiratory tract disease, and it was emaciated, with severely obtunded mentation, pale, icteric and tacky mucous membranes, nonambulatory tetraparesis, increased breath sounds, systolic heart murmur (IV/VI), and signs of anterior uveitis (Teh et al., 2024).  It was euthanized and the autopsy revealed dissemination of cryptococcosis in both thoracic (lungs, trachea, heart) and abdominal (liver, spleen, kidneys, gallbladder, mesentery, adrenal glands, abdominal fluid) organs, CNS, eyes, thyroid, lymph nodes, and hindlimb muscles (Teh et al., 2024). A small amount of abdominal translucent dark yellow fluid was detected (8 mL) and analysis results were consistent with a high protein transudate (Teh et al., 2024).

Cranial venal caval syndrome with severe oedema of the head and neck was reported in a cat presenting a cryptococcal mediastinal mass compressing the vein, as well as the oesophagus and trachea (Letendre and Boysen, 2015).

Apathy and cachexia appear in cats with severe dissemination of disease during a prolonged chronic course. The systemic form, arising from haematogenous dissemination, may or may not follow the classical nasal disease form (Tisdall et al., 2007; Martins et al., 2011). 

Fig. 4. Cryptococcal disease - kerato-uveitis and cryptococcoma in the anterior chamber. Courtesy of Maria Grazia Pennisi

Fig. 4. Cryptococcal disease – kerato-uveitis and cryptococcoma in the anterior chamber. Courtesy of Maria Grazia Pennisi

Fig. 5. Thoracic radiography, ventro-dorsal view: diffuse, multiple, poorly defined nodules with blurred margins in the lung of a cat with systemic cryptococcosis. Courtesy of Maria Grazia Pennisi

Fig. 5. Thoracic radiography, ventro-dorsal view: diffuse, multiple, poorly defined nodules with blurred margins in the lung of a cat with systemic cryptococcosis. Courtesy of Maria Grazia Pennisi

Diagnosis

An in-depth review article on the diagnostic approach to feline invasive fungal infections, including cryptococcosis, has been published by Barrs et al. (2024a).

Laboratory changes

Abnormalities in blood tests are non-specific and, if present, show a chronic inflammatory process or they are related to the clinical signs. For instance, mild to moderate normocytic, normocromyc (non-regenerative) anaemia and monocytosis can be found in blood cell count (Barrs et al., 2024a). In case of seizures, elevated creatine kinase concentrations are found (Vercelli et al., 2021). In a case where abdominal fluid was collected at autopsy, the dark yellow translucent fluid was a high protein transudate (43 g/L), with 678 x 106/L nucleated cells and 4,000 x 106 /L erythrocytes (Teh et al., 2024).

Diagnostic imaging

Radiology and advanced diagnostic imaging techniques [computed tomography (CT) and magnetic resonance imaging (MRI)] are frequently used in the diagnostic process of respiratory and CNS signs (Evans et al., 2018; Cooley et al., 2022; Huang et al., 2023; Jacobson et al., 2023b; Wong et al., 2024). They provide useful information on extension and severity of lesions (particularly the head and thorax). Common abnormal findings are related to the presence of chronic rhinitis, frontal sinusitis and/or intranasal or intracranial focal solitary or multifocal masses or fluid-filled lesions (Sykes et al., 2010; Schlacks et al., 2019; Collins and Sunico, 2020). In dyspnoeic cats, lobular or mediastinal masses, pulmonary infiltrates (interstitial pattern with peribronchovascular distribution) can be observed (Evans et al., 2018; Collins and Sunico, 2020). If the cat’s clinical condition allows, ultrasonography can be performed carefully in dyspnoeic cats and ultrasonographically guided fine needle aspirate samples can be obtained (Evans et al., 2018; Schlacks et al., 2019; Newman and Shaible, 2019; Collins and Sunico, 2020).

Confirmation of diagnosis is not possible by imaging alone, but resolution of a mass lesion can be followed up by MRI in cats under medical therapy (Karnik et al., 2009; Hammond et al., 2011). MRI findings may also include meningeal enhancement, olfactory lobe, optic nerve and cribriform plate involvement (Sykes et al., 2010; Vercelli et al., 2021). 

Detection of the infectious agent

Direct detection

An easy and reliable test for cryptococcosis diagnosis is antigen detection in body fluids (serum, cerebrospinal fluid (CSF), abdominal fluid, or urine). However, when possible, samples are collected from lesions and submitted for cytology, histopathology, and polymerase chain reaction (PCR). Suitable samples include (i) pleural or peritoneal effusions, (ii) CSF, (iii) broncho-alveolar lavage fluid, (iv) fine needle aspirates from tissue nodules and masses or enlarged lymph nodes, (v) biopsies taken from any affected tissues.

Culture and PCR offer the opportunity to identify the infecting species.

Antigen detection

Antigen detection in blood serum is the first line test, if available, because it is fast, reliable and minimally invasive. Cryptococcal capsular antigen may be detected by the latex cryptococcal antigen agglutination test (LCAT) on serum, CSF or urine using commercial kits used in humans. An increased risk of cerebellar herniation after CSF collection is suspected and this invasive procedure should be considered only when CNS disease compatible with cryptococcosis has not been confirmed by using other suitable biological samples (Sykes et al., 2010). Jacobson et al. (2023a) found in a retrospective study that, after adjustment for treatment, CSF collection was associated with decreased survival time in cats with CNS cryptococcosis .

Various LCAT kits are commercially available and their sensitivity and specificity are improved by pre-treating samples with heat and a proteinase (pronase, often included in commercial diagnostic kits); these tests are considered good in cats (Teh et al., 2024). However, in some cases false negative results may occur and they are usually observed in case of high levels of antigen in the fluid causing the so called postzone phenomenon (Belluco et al., 2008; Teh et al., 2024). If this is the case, a positive result is obtained after dilution of the sample. Negative LCAT results may occur in the case of very limited lesions. A positive LCAT test is indicated by a titre of 1:2 (Trivedi et al., 2011). Irrespective of the antigen test result, lesional tissue samples, if available, should always be submitted for cytological and histological evaluations, and PCR.

LCAT titres are also an efficient way of monitoring the efficacy of therapy. Treatment is usually continued until a negative LCAT is obtained, but it has been reported that titres continue to decrease after stopping therapy in cats with clinical resolution and continued positive LCAT (O’Brien et al., 2006).

The latex agglutination test is a manual assay with operator-dependent visual interpretation of the test result. Automation of antigen detection is offered by enzyme immunosorbent assay (EIA), but the sensitivity of a commercial kit has been considered lower compared to a LCAT assay in human serum samples (Binnicker et al., 2012). This cryptococcal antigen EIA provided multiple negative results in a cat with confirmed nasopharyngeal cryptococcosis under therapy with fluconazole (Mc Ewan and Sykes, 2022). However, a positive discordant result was obtained with a LCAT and antigenaemia was confirmed with other tests afterwards (Mc Ewan and Sykes, 2022).

Lateral flow rapid tests are largely used in human medicine and their performance has been investigated in dogs, cats, and koalas. A first study evaluated two rapid tests in LCAT positive and negative canine and feline samples and reported adequate sensitivity and specificity values in both cases (Reagan et al., 2019). Conversely, two other studies suggest that the point-of-care antigen test studied was more suitable for ruling out cryptococcosis and that positive results need to be confirmed with the LCAT (Krockenberger et al., 2020).

Cytology

Cytology is an easy tool to diagnose cryptococcosis because the appearance of the organisms is characteristic and the number of yeasts in lesions is usually high, but a negative result does not exclude the diagnosis. Appropriate cytological samples can be obtained through impression smears from ulcerated skin lesions, fine needle aspirates of nodules and masses, impression smears of biopsy samples or broncho-alveolar lavage samples or CSF taps. In the case of renal involvement yeasts may be seen in the urinary sediment (Brandt and Blauvelt, 2010).

Smears or cytospin preparations stained with Romanowsky-type stain (Wright, Diff-Quick, Giemsa) may show pink to violet, round or budding extracellular yeasts that vary in size (4-15 μm) and shape and are typically surrounded by a clear more or less thick halo corresponding to the unstained capsule (Figs. 6, 7). Cytology of cytospin preparations of abdominal fluid showed clusters of mesothelial cells (Teh et al., 2024).

Fig. 6. Diff Quick stained smear of nasal exudate from a cat with C. neoformans infection. Note the prominent capsule (clear halo) and narrow-necked budding (arrow). Photomicrograph courtesy of Richard Malik.

Fig. 6. Diff Quick stained smear of nasal exudate from a cat with C. neoformans infection. Note the prominent capsule (clear halo) and narrow-necked budding (arrow). Photomicrograph courtesy of Richard Malik.

Fig. 7. Diff Quick stained smear of fine needle aspirate from a cryptococcal lesion. Note the enormous capsule surrounding the yeast cells. Photomicrograph courtesy of Mark Krockenberger

Fig. 7. Diff Quick stained smear of fine needle aspirate from a cryptococcal lesion. Note the enormous capsule surrounding the yeast cells. Photomicrograph courtesy of Mark Krockenberger

If a Gram stain is used, the organism appears as Gram positive with a Gram negative (pink) capsule. A pyogranulomatous inflammatory pattern on cytology is usually seen. Although filamentous forms are not commonly observed in tissues, these atypical filamentous morphologic forms of C. neoformans may be present in cats and yeast structures may lack a distinct capsule (Bemis et al., 2000; Lin, 2009; Evans et al., 2018).

Histology

Biopsy samples of the nasal mucosa, lymph nodes or skin nodules may be obtained for histology, but they may also be used to provide impression smears for cytology and material for culture and PCR. Haematoxylin-eosinstained sections show eosinophilic bodies surrounded by a clear halo and a pyogranulomatous reaction (Fig. 8). Atypically, rare capsule-deficient yeasts have been observed in granulomatous to pyogranulomatous and eosinophilic dermatitis in four cats in which Cryptococcus spp. infection was confirmed by immunohistochemistry, PCR and fungal culture (Myers et al., 2017). It has been postulated that capsule-deficient strains may cause a severe inflammatory response with only a few organisms present (Myers et al., 2017).

Mayer’s mucicarmine method specifically stains the capsule of Cryptococcus. Immunohistochemistry on tissue sections is used for species differentiation, using monoclonal antibodies (Fig. 9) (Krockenberger et al., 2001). 

Fig. 8. Early invasion of Cryptococcus gattii into the respiratory epithelium of a koala. Note the eosinophilic body surrounded by a clear halo. Photomicrograph courtesy of Mark Krockenberger.

Fig. 8. Early invasion of Cryptococcus gattii into the respiratory epithelium of a koala. Note the eosinophilic body surrounded by a clear halo. Photomicrograph courtesy of Mark Krockenberger.

Fig. 9 Use of immunohistology to demonstrate C. gattii in histological sections using monoclonal antibodies directed against different capsular epitopes. These show up as brown precipitates, highlighting both the yeast cell body and its capsule. Note also the narrow neck budding. Courtesy of Mark Krockenberg

Fig. 9 Use of immunohistology to demonstrate C. gattii in histological sections using monoclonal antibodies directed against different capsular epitopes. These show up as brown precipitates, highlighting both the yeast cell body and its capsule. Note also the narrow neck budding. Courtesy of Mark Krockenberg

Isolation/Culture

Isolation/culture is performed if the LCAT is negative, when titres are low or not available and is followed by PCR or matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF) testing of the cultured isolate (Barrs et al., 2024a). In cases of nasal discharge, samples from nasal biopsies should be submitted for culture, because the presence of Cryptococcus in nasal discharge cultures is not considered evidence of disease as asymptomatic carriage is not rare in cats (Danesi et al., 2014). Positive culture of biopsy samples and histological changes consistent with infection are considered diagnostic. Isolates are used for sensitivity testing of antifungal drugs, particularly useful when response to therapy has been poor and drug resistance is suspected.

Culture of biopsy samples is more sensitive than cytology in confirming infection. Cryptococcus is easily isolated in Sabouraud’s dextrose agar after incubation at 25°C and 37°C for 10 days but also on standard bacterial media. When samples are contaminated by bacteria, as usually occurs in biopsies obtained from the nasal mucosa, media containing antibiotics are useful to use for Cryptococcus culture. Mycology reference laboratories can discriminate isolated species complexes by subculture on canavine glycine bromothymol blue agar (Barrs et al., 2024a).

Polymerase chain reaction (PCR)

Polymerase chain reaction has been developed for genetic identification of Cryptococcus in CSF, urine, blood and biopsy samples and it is now routinely offered by laboratories (Kano et al., 2001; Meyer et al., 2003; Okabayashi et al., 2006; Evans et al., 2018; Vercelli et al., 2021; Reis et al., 2021). This method also allows identification of the pathogen species. A fungal broad-range PCR test has been developed (Bernhardt et al., 2015).

Indirect detection

Antibody detection is not a diagnostic tool because it cannot distinguish subclinical infection from active disease.

Treatment

Antimicrobial treatment

No prospective controlled studies exist on the treatment of feline cryptococcosis and all data are based on retrospective studies and case reports. Treatment guidelines have not been established and the choice of the appropriate antifungal drug depends on many factors. Owner compliance is crucial, because of the high costs in terms of both money and the long time required for treatment. Some retrospective studies on treatment outcomes of feline cryptococcosis have been reported with heterogeneous criteria used for evaluating the success of therapy (Medleau et al., 1995; Davies and Troy, 1996; Jacobs et al., 1997; Jacobson et al., 2023a). In the largest retrospective study, performed on 59 cats in 2006, 68% had a successful outcome (O’Brien et al., 2006). Most of them needed one single course of therapy of several months (1 to 24) duration and few cats received a second course of therapy because of clinical recurrence or raised LCAT titre. According to a retrospective study published in 2009, the clinical outcome may be favourable in approximately 2/3 of treated cats (McGill et al., 2009). Most recovered cats presented with sino-nasal or single lesions in the skin, subcutis or intestines, and the ones that did not recover had CNS or disseminated disease.

Amphotericin B (AMB), fluconazole and itraconazole are the most frequently used drugs. An in-depth review article of spectrum of activity, mechanisms of action, pharmacokinetic and pharmacodynamic properties, and adverse events of drugs used to treat invasive fungal infections in cats, including cryptococcosis, has been published by Barrs et al. (2024a). Data about different therapeutic protocols have been published, and there was no significant difference in outcome between cats treated with AMB-containing protocols and those treated with azole monotherapy using fluconazole or itraconazole (O’Brien et al., 2006). Because of renal toxicity of AMB, it was recommended in the past not to exceed a drug total dose of 4-8 mg/kg in the overall course of intravenous treatment (cumulative dose). However, the median cumulative dose of AMB for cats cured at the first attempt was higher (16 mg/kg; range 7-23 mg/kg) when using protocols based on subcutaneous administration compared to the intravenous route (Trivedi et al., 2011). Fluconazole is considered the first-line drug for mild forms of rhinitis and localized cutaneous forms that are ideally treated until clinical cure and negativization of LCAT is obtained (Barrs et al., 2024b). The median duration of treatment for fluconazole-treated cats was significantly shorter (4 months; range 1 to 8 months) than the median for the itraconazole group (9 months; range 3 to 24 months) (O’Brien et al., 2006).

In CNS, ocular and or systemic cases AMB, preferably in combination with flucytosine (or fluconazole when flucytosine is not available), is the first choice followed by a long treatment course of fluconazole or itraconazole (O’Brien et al., 2006; Barrs et al., 2024b). Cats with pre-existing renal disease should be treated with fluconazole or itraconazole only (i.e. no AMB). Liposomal formulations of AMB may be better tolerated but are very expensive and not easily available. Fluconazole seems to be more effective than itraconazole for infections in the CNS, eye and urinary tract and is also better tolerated (Trivedi et al., 2011; Hammond et al., 2011). In a case series report on 15 cats treated for CNS cryptococcosis, most of the animals (11/15) received more than one antifungal drug (AMB and fluconazole or itraconazole), sequentially or concurrently (Jacobson et al., 2023a). Fluconazole (14 cats) and AMB (11 cats) were the most used drugs. Amphotericin B deoxycolate was administered subcutaneously (five cats; median cumulative dose 4.0 mg/kg, interquartile range 1.5-8.5 mg/kg) and intravenously in case of liposomal formulation (five cats, median cumulative dose 11.5 mg/kg, interquartile range 11.4-12.0 mg/kg). Discontinuation of AMB at a dose < 10 mg/kg was due to death or euthanasia related to cryptococcosis (three cats), azotaemia (three cats), planned transition to fluconazole administration (one cat). Azotaemia was the most frequent adverse effect caused by AMB (8/11 cats). Most cats (10/15) received corticosteroids either as a single dose, as short term (up to seven days), or as long-term administrations (Jacobson et al., 2023a).

In general, continuation of treatment is recommended until the LCAT is negative. If the LCAT is negative at the time of diagnosis, and the disease was confirmed by other methods, or if LCAT is not available, treatment should be continued at least until 2 to 4 months after resolution of clinical signs. When high LCAT titres persist or fail to decline after six weeks of fluconazole administration, switching to another drug should be considered (Barrs et al., 2024b). Resistance to fluconazole was reported with some isolates that were susceptible to other azoles and itraconazole is a possible choice (Lester et al., 2011; Kano et al., 2015). However, drug susceptibility of isolates should be evaluated because isolates that show high MICs for fluconazole or itraconazole may be susceptible to other azoles (ketaconazole, voriconazole, posaconazole) (Barrs et al., 20224b). Some cases (any forms) not responding to different azoles can be treated in combination with terbinafine or with terbinafine alone (Barrs et al., 2024b). When clinical cure is obtained, LCAT declines four- to five-fold and then stays stable, response to therapy is considered adequate and treatment is stopped. Post-therapy LCAT monitoring is performed and treatment is re-instituted in case of LCAT titre increase. A three-month old kitten with seizures caused by cryptococcosis did not respond to fluconazole (10 mg/kg q12h PO) administration (Vercelli et al., 2021). However, clinical recovery and a negative blood PCR result occurred with three administrations of AMB (1 mg/kg q48h IV) (Vercelli et al., 2021).

An in vitro study found that fluconazole resistance is lost when the isolate grows in antifungal free media (heteroresistance) (Sykes et al., 2017).

Importantly, non-azole fungicide agrochemicals may affect antifungal susceptibility and the virulence of pathogenic fungi, including Cryptococcus (Bastos et al., 2019; Bastos et al., 2021).

For common antimicrobial treatments see table 1.

Table 1: Most commonly used drugs to treat cryptococcosis in cats

DrugDose and administrationNotes
Amphotericin B

(50 mg vial; stock solution 5 mg/ml)

0.25 mg/kg q48h IV (cumulative dose: 4-8 mg/kg)

0.5 mg/kg q48h SC in 350 ml/cat of hypotonic solution^ (cumulative dose: 7-23 mg/kg)

First choice in CNS, eye and systemic disease in combination with flucytosine (if available) or fluconazole.

Nephrotoxic drug: monitor renal function and avoid if renal disease present.

Flucytosine (250-500 mg oral formulations)25-50 mg/kg q8h POAlways in combination with amphotericin B (synergistic activity).

Lower dose in cases of renal disease.

Gastrointestinal adverse signs.

Fluconazole (50 mg capsule or 10 mg/ml oral suspension)50 mg/cat (or 10 mg/kg) q12-24h POTeratogenic drug.

Gastrointestinal adverse signs.

First choice monotherapy in localized forms of disease.

Itraconazole (10 mg/ml oral solution)5 mg/kg q24h POTeratogenic and hepatotoxic drug.
Terbinafine (250 mg tablet)30-40 mg/kg q24h POIn combination with azole drugs or as monotherapy in azole-refractory cases.

Gastrointestinal adverse effects.

^: 0.45% NaCl + 2.5% dextrose (obtained by combining 0.9% NaCl and 5% dextrose 1:1)

Symptomatic treatment

The clinical condition of cats with cerebral cryptococcosis may worsen soon after starting AMB therapy, presumably due to an inflammatory response and increased intracranial pressure. Short-acting corticosteroid (dexamethasone or prednisolone sodium succinate) therapy is reported to be of immediate benefit in such cases and associated with increased survival in the short term (O’Brien et al., 2006; Sykes et al., 2010).

Surgical excision of any nodules and masses located in affected tissues and organs must be considered as a valuable aid in cats under medical therapy (Hunt et al., 2002; Ferrari et al., 2023). In the case of masses causing severe dyspnoea, their excision is a priority (Mc Ewan and Sykes, 2022).

Prognosis

Prognosis is favourable in most cases, provided a diagnosis is obtained sufficiently early (before dissemination or before the development of invasive severe lesions) and patient and owner compliance is good to provide the needed long course of treatment (months) and follow-up (years). No data are currently available in cats about the prognostic importance of infecting species within the C. neoformans/C. gattii species complexes (Barrs et al., 2024b).

Although information on outcomes is quite limited, it seems that cats have a more favourable prognosis than dogs or horses which develop lower respiratory, disseminated and neurological disease more frequently and which are associated with a higher mortality (Duncan et al., 2006; O’Brien et al., 2006; McGill et al., 2009; Sykes et al., 2010).

In one retrospective study, disease severity did not influence outcome, although the presence of CNS involvement had a significantly adverse impact on the outcome of therapy (O’Brien et al., 2006). On the other hand, alteration of the mental status was the only negative prognostic factor in a retrospective study on cats with CNS form, and complete recovery was documented also in cats with a CNS form (Sykes et al., 2010; Hammond et al., 2011; Vercelli et al., 2021). Long survival time has been reported in 5/8 cats treated with CNS cryptococcosis (median 1,678 days) (Jacobson et al., 2023a). Compared with the broader study population, these cats had neurological signs at presentation less frequently and rarely abnormal mentation. After adjustment for treatment, CSF collection was associated with decreased survival time of the cats with CNS cryptococcosis that were studied by Jacobson et al. (2023a). When cryptococcosis is a differential diagnosis, CSF analysis should be reserved for the rare patients in which it is required to establish a diagnosis or to determine the infective biotype for animals in which this would alter treatment, ideally after cross-sectional imaging has evaluated herniation risk.

Vaccination

Vaccines are not available.

Prevention

Free-roaming cats in rural areas are potentially more exposed to Cryptococcus, even though urban cats can be contaminated through pigeon guano and the disease is diagnosed also in indoor cats. According to ecology, the presence of avian guano, particularly pigeon droppings, and some decaying vegetation substrates, such as Eucalyptus leaves, may be considered risk factors (Fortes et al., 2001). Knowledge of local fungal habitats that carry the largest risks of exposure and seasonal variations in the production of infectious propagules would be useful to develop preventive measures for both human and animal infections.

Zoonotic risk

Feline cryptococcosis, discovered over a century ago, is a non-contagious systemic fungal disease acquired from contaminated environment. For this reason, it is not considered a zoonotic disease. Animals may serve as sentinel hosts and the investigation of environmental sources based on feline cases of cryptococcosis is useful for intervention to minimize the risks of human and animal infections (Reis et al., 2021).

Acknowledgement

ABCD Europe gratefully acknowledges the support of Boehringer Ingelheim (the founding sponsor of the ABCD), MSD Animal Health, Vétoquinol and Virbac.

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