CHAPTER 39 YEAST INFECTIONS

Claudia Cafarchia

Department of Veterinary Public Health, University of Bari, Valenzano (Bari), Italy

INTRODUCTION

Yeasts are eukaryotic, unicellular, round or oval microor­ganisms that develop blastoconidia during asexual reproduction.

Whereas some yeasts can be retrieved from the environ­ment, others are considered to be normal biota of the gastrointestinal tract, skin and urinary tract in animals and humans.

As opportunistic pathogens, yeasts require impairment of host defence mechanisms to establish clinical infections. Infections are categorized as exogenous when derived from the environment, or endogenous when resulting from overgrowth of commensal organisms. Since the 1960s, several new yeast species have been reported as human and animal pathogens, owing to the increase in immunodefi­ciency syndromes and immunosuppressant chemotherapy in cancer treatment. Domestic and wild animals are known to act as carriers of human pathogenic fungi, with bird droppings considered the main source worldwide for such pathogenic yeasts as Candida spp. and Cryptococcus neofor­mans. Notable yeasts in animal diseases include Candida spp., Cryptococcus spp. and Malassezia spp. Others, such as Trichosporon spp., Rhodotorula spp., Sporobolomyces spp., Saccharomyces spp. and Geotrichum spp. rarely cause infections.

CANDIDA INFECTIONS

Currently over 200 ascomycetous yeasts make up the genus Candida, but only a few are opportunistic pathogens of humans and animals.

Candida yeasts are commensal organisms, inhabiting the skin, upper respiratory, alimentary and genital tracts of humans and warm-blooded animals. Candida counts of up to 10³ cfu/g in the mammalian intestinal tract are regarded as normal flora¹-¹). Candida spp. are also present in the environment, e.g. in soil or water, or on plants and fruit. Contaminated environments, including litter from poultry- and bird-rearing facilities, refuse-disposal areas, discharge sites for poultry operations and areas contami­nated with human waste have all been suggested as sources for Candida exposure in birds.

Infectious Diseases of Wild Mammals and Birds in Europe, First Edition. Edited by Dolores Gavier-Widen, J. Paul Duff, and Anna Meredith. © 2012 Blackwell Publishing Ltd. Published 2012 by Blackwell Publishing Ltd.

There are few reports of candidiasis causing disease in free- ranging wild animals and few investigations of its prevalence. Superficial infections have most frequently been described in birds, dogs and cats(¹), while systemic infections are described in captive wild mammals such as alpacas (Lama pacos)(²). Additionally, infected animals could be transmission vectors or reservoirs of strains causing human disease and may affect immunocompro­mised patients(³). Occurrence is generally not seasonal, and young animals are more susceptible to infection.

Candida spp. require impairment of host defence mech­anisms to establish clinical infections. Depending on the host’s immune status, infection is either restricted to the skin and mucous membranes as a superficial mycosis or it may spread haematogenously, resulting in systemic mycosis.

Disturbances of the cutaneous and mucosal microenvi­ronment are required for tissue invasion by the fungus, a process usually accompanied by phenotypic switching from yeast form to filamentous form in C.

There are no unique signs of disease. In superficial candidiasis, lesions are generally confined to the digestive or urogenital tracts in mammals. In birds, the mouth, oesophagus, and, primarily, the crop, may have greyish- white, loosely attached, plaque-like areas on their internal surfaces. Mycotic stomatitis has been reported in pups, kittens and foals, gastro-oesophageal ulceration has been reported in pigs and foals, and abortion and mastitis have been reported in cows(¹). No data on superficial candidiasis has been published in wild mammals, whereas systemic fungal infections have been reported in wild mammals and birds, especially Psittacidae(²). Multifocal erosive dermatitis associated with thoracic and abdominal serofibrinous effu­sions (following haematogenous spread), and multiple suppurative foci in lung, myocardium, kidney, pancreas and brain have been described in wild mammals⁽²⁾.

Diagnosis requires histopathological examination, con­firmed with fungal culture. Tissue sections, stained with periodic acid-Schiff (PAS) or methylamine silver methods may reveal budding yeast cells or hyphae. Candida spp. show oval, budding blastospores 3—8 μ m in diameter. Sup­purative to necrotizing inflammation with necrotizing vas­culitis might be present. Culture is carried out aerobically at 37° C for 2—5 days on Sabouraud dextrose agar with chloramphenicol (0.5 g/l). Colonies are white, round and convex with a diameter up to 4—5 mm. Identification requires biochemical profiles and chlamydospore and germ-tube formation. Germ tubes are visible after 2—4 hours incubation at 37°C in sheep, beef, rabbit or human serum and should help identify C. albicans.

Antifungal agents are employed in the treatment of human or pet candidiasis, but should be administered only after the results of antifungal susceptibility tests.

Cryptococcal infections

Cryptococcus is a fungal genus, of which two species, Cryp­tococcus neoformans and Cryptococcus gattii, cause nearly all human and animal cryptococcal infections. These species are characterized by a mucopolysaccharide capsule, whose antiphagocytic properties make it an important virulence factor. On the basis of capsular antigen, five different serotypes (i.e. C. neoformans var. neoformans (serotypes D and AD), C. neoformans var. grubii (serotype A) and C. gattii (serotypes B and C) are accepted.

Cryptococcus gattii is geographically restricted to tropical and subtropical areas, but has also recently been isolated in Southern Italy⁽³⁾. Cryptococcus gattii is associated with trees of the genus E ucalyptus, usually causing infection in immunocompetent animals. By contrast, C. neoformans vars. neoformans and grubii are commonly found world­wide in association with soil and the droppings of birds such as psittacidae, passeriformes, columbiformes and fal- coniformes⁽³⁾ and generally cause cryptococcosis in immu­nocompromised individuals. The association with bird excreta is because of the presence of creatine and uric acid, which are selectively metabolized by C. neoformans. Cryp­tococcosis occurs worldwide and is reported in various non- domestic animal species. Infection generally occurs exogenously by inhalation of aerosolized cells.

The primary reproduction site is the respiratory tract, where granulomas can form. Cryptococcus then reaches brain, eyes and other organs haematogenously, lymphog- enously or via direct extension through the nasal mucosa across the cribiform plate. Purulent haemorrhagic rhinosi­nusitis with nasal discharge and dyspnoea have frequently been described in cheetah (Acinonyx jubatus')⁴ⁱ⁴.

Diagnosis of infections mainly requires histopathologi­cal examination, which should be confirmed by fungal culture. Myelography, in mammals, indicating the pres­ence of an intramedullary compressive mass at various spinal cord levels, may help to diagnose meningoencepha- lomyelitis. Suitable specimens for laboratory examination include exudate, cerebrospinal fluid and biopsy/post mortem tissues. Budding yeasts with characteristically thick capsules can be demonstrated in fluid samples using India ink preparations. In tissue sections, yeast capsules are detected by Mayer’s mucicarmine stain. Fontana-Masson staining can be employed to detect melanin in Cryptococcus spp. cell walls. In all cases, Cryptococcus cells are round or oval with a diameter of 3.5—8μm. Culture is performed aerobically at 37°C for 1—2 weeks on Sabouraud dextrose agar with chloramphenicol (0.5 g/l). Niger seed agar as a selective and differential medium is useful.

I n domestic animals, itraconazole and fluconazole are probably the drugs of choice for systemic treatment of most cases of invasive cryptococcosis, both being suitable for oral administration. Fluconazole is less likely to produce significant side effects than itraconazole, and has the addi­tional advantage of reaching higher concentrations in the eye and central nervous system⁽⁸⁾. Treatment should extend several months beyond resolution of clinical disease. Topical or intralesional therapy with amphotericin B may also be helpful, e.g. using a high concentration as topical ointment or as intralesional infusion in cases of avian cryptococcosis involving the beak and nearby tissues⁽⁷⁾.

Malassezia infections

Malassezia spp. are lipophilic, non-mycelial, unipolar, budding yeasts belonging to the normal cutaneous micro­flora of most warm-blooded animals. These yeasts some­times act as opportunistic pathogens under the influence of predisposing factors, mainly related to changes in the cutaneous microenvironment and/or alterations in host defence mechanisms⁽⁹⁾. Currently, 13 species (M. furfur, M. obtusa, M. globosa, M. slooffias, M. sympodialis, M. restricta, M. dermatis, M. japonica, M. nana, M. yamatoen- sis, M. equina, M. caprae and M. cuniculi), are recognized as lipid-dependent, with only Malassezia pachydermatis not requiring lipid supplementation for in vitro growth⁴⁹). Malassezia yeasts are currently regarded as new emerging pathogens, as the incidence of their infections has increased during the last decade.

The lipid- dependent species are frequently associated with human skin disorders, whereas the non-lipid-depend- ent M. pachydermatis is considered an opportunistic path­ogen growing on animal skin surfaces and ear canals, even in wild animals⁽⁹⁾. In particular, M. sympodialis is the most frequent species isolated from the ear canal of large felids (56.9%) and of adult pigs (63.6%), which may also carry M. furfur (22.7%). By contrast, M. pachydermatis was the only species isolated from wild boar (12.9%), Cinta Senese swine (20.7%), juvenile pigs (13.6%) and small felids (43.1%)^l∙^10,11∙^l.

The pathogenic role of Malassezia yeasts is unknown and seems to be related to changes in the skin’s normal physico­chemical or immunological mechanisms, which may enhance or downregulate the molecular production of yeast virulence factors or antigens. In addition, Malassezia yeast proliferation is likely to be a preliminary step towards dermatitis and/or otitis. Superficial hyperplastic dermatitis with a predominance of lymphocytes and macrophages are described as histological features of Malassezia dermatitis. Epidermis and hair follicles show spongiosis, exocytosis and parakeratotic hyperkeratosis. Budding yeast cells ranging from 3 to 8 μm in diameter located within the keratin can be differentiated by PAS from other pyknotic nuclei of the stratum corneum.

Malassezia dermatitis may present with pruritus, inflam­mation and epidermal hyperplasia, whereas otitis is char­acterized by the presence of brown and malodorous cerumen(¹2).

Diagnosis of infection usually requires cytological and cultural techniques and/or histopathological procedures. For cytology, samples can be collected using sterile cotton swabs moistened with sterile saline solution (NaCl 0.9%), rolled on a clean glass slide and stained using May-Grun­wald Giemsa. Results can be considered positive if large populations of Malassezia cells are counted at 40? magni­fication (i.e. >5 Malassezia yeasts for skin sites and >10 for the ear canal, in dogs). Because of the lipophilic charac­teristic of these yeasts, many fungal media supplemented with a range of lipid sources have been used for culture. Dixon’s medium is now the commonest used. A large Malassezia yeast population in the medium could be indic­ative of Malassezia spp. infection. Malassezia yeast can be identified at species level by its morphology and physico­chemical characteristics as well as using molecular-biology tools.

Malassezia dermatitis or otitis is treated with antifungal drugs. Systemic therapy is often necessary, in particular when clinical signs are severe and widespread. Ketocona­zole and itraconazole are the most commonly used drugs. Topical therapy is an alternative in case of localized lesions and otitis. The commonest topical drugs are imidazole antifungals, chlorhexidine and lime sulphur.

Wild animals are known to act as carriers of human pathogenic fungi, with bird droppings considered the main source worldwide for such pathogenic yeasts as Candida spp. and Cryptococcus neoformans.

Among the yeasts species, herein presented, Cryptococcus spp. is the cause of systemic fungal infections in humans or in animals that have contact with avian roosts or nests.

By contrast, Candida spp. and Malassezia spp., even if potentially transmitted by infected animals via direct contact, do not pose a human health risk. However, animals known to have infections from these yeasts should be properly handled, and cages, equipment and other material in contact with infected animals should be disinfected.

REFERENCES

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