BT14 Fungi and Fungal Pathogens Lecture Notes PDF

Lecture title: BT14 Fungi and Fungal Pathogens Lecturer: Dr Gavin Paterson Introduction Fungi are eukaryotic organisms with classification often based on their morphology (including reproductive structures and spores, although this is not always possible) and pigmentation. The majority of fungi are...

Lecture title: BT14 Fungi and Fungal Pathogens Lecturer: Dr Gavin Paterson Introduction Fungi are eukaryotic organisms with classification often based on their morphology (including reproductive structures and spores, although this is not always possible) and pigmentation. The majority of fungi are harmless saprophytes with only a minority being able to cause opportunistic infections and even fewer posing a threat to healthy animals and humans (148,000 species known – only 400 pathogens). As eukaryotes, the number of anti-fungal reagents available is low compared to prokaryotes, and some of the anti-fungal agents used in humans are toxic to animals. Classification Traditionally four classes of fungi exist based on the absence or presence and type of sexual spore (ascospores, basidiospores, zygospores, chytridiomycota). Phylum Ascomycota Septate hyphae – examples of phylum : Microsporum spp., Trichophyton spp., Candida albicans, Aspergillus spp. Basidiomycota Septate hyphae – examples : Cryptococcus spp., Malassezia pachydermatis. Zygomycota Non-septate hyphae – examples : Mucor spp., Mortierella spp. Chytridiomycota Non-septate hyphae – example : Batrachochytrium dendrobatidis (major pathogen of amphibians causing massive declines in hundreds of species). Fungi are usually aerobes that nearly all have growth optimal at temperatures between 20oC and 30oC and one indication that a fungal species is potentially pathogenic is its ability to grow at 37oC (are exceptions). Fungi can be divided into yeasts and moulds (and dimorphs i.e. can grow either as yeast or mould) based on cell structure. Yeasts Unicellular. Larger than bacteria 3-10 µm. Oval / round. Gram-stain + ve (but much larger than typical bacteria). Reproduce by budding. Can also form spores. Yeast may form a projection = germ tube from parent yeast (e.g. Candida). Moulds Filamentous = hyphae (2-10 µm diameter). Grow parallel and close to surface. Branches mat to form an interconnected mycelium or vegetative mycelium. Mycelia can be centimetres long. Hyphae – septate (cross wall between cells) or non-septate (zygomycota). Septation often incomplete so have small central pore so organelles or nuclei can pass. Under certain conditions perpendicular hyphae are produced. At the tips of these conidiophores are formed containing conidia (or conidiospores). These asexual spores are important for the distribution of the organism and useful for identification. Dimorphism Many pathogenic fungi are dimorphic. Transition between yeast and mould forms dependent on environmental conditions. Yeast in tissue or at 37 oC, mould outside host or at lower temperature, e.g. soil. Cell Structure – eukaryotic Fungal cell wall contains chitin, mannoproteins (mannans) and glucans. Fungi have a plasma membrane (cell membrane) which contains large quantities of the sterol – ergosterol (distinct from cholesterol in animal and phytosterol in plant cell membranes). Ergosterol biosynthesis is the target for several antifungal drugs. Types of diseases caused by fungi of veterinary importance: 1. Contagious cutaneous diseases - commonly described as ringworm and caused by dermatophytes. 2. Cutaneous or mucosal infections. 3. Systemic or tissue infections - Sporothrix schenckii, Blastomyces dermatitidis, Histoplasma capsulatum, Coccidioides immitis. 4. Sensitization - allergic reactions to fungal products. 5. Mycotoxic disease - ingestion of preformed toxic fungal metabolic products. Dermatophytes (ringworm) Ringworm infections are transferable from one animal to another, usually by infection with conidiospores. The fungi are able to hydrolyse keratin for growth and are generally restricted to the cornified non-living keratin layer and appendages. The infections are potentially zoonotic. The infections are principally caused by species of two genera, Microsporum and Trichophyton. Some species show host specificity while others have a broad host range. The lesions formed result as much from the host’s reaction as from the action of virulence factors produced by the fungi. The response of the host causes spread of the dermatophyte away from the focus of the infection (towards normal skin/hide) and gives the ‘ringworm’ circular appearance of a healing centre and a spreading inflamed edge. Horse : T. equinum, T. verrucosum and M. equinum. Cattle : T. verrucosum, T. mentagrophytes. Dogs and cats : M. canis, M. distortum, T. mentagrophytes. Pigs : M. nanum. Minor trauma is important in the introduction of the infection. Conidia or hyphae fragments introduced into the stratum corneum germinate and proliferate. Hyphae thereafter invade the wall of the hair follicles eventually emerging into the follicular canal, penetration of the hair cortex ensues, hyphal proliferation and conidia formation takes place in the peripilar space and cortex. Keratinases principally contribute to spread and damage to hairs, along with collagenase and elastase. Most infections remain superficial but can be complicated by secondary infections from Staphylococci spp. and Streptococci spp. Diagnosis Most infections can be readily diagnosed by microscopy or culture. Culture may take 2-3 weeks. Traditionally many dermatophyte infections were diagnosed by fluorescence under UV light (Wood’s light), but this does not reliably detect all M. canis infections. Hair pluckings/scrapings from lesion sites can be examined microscopically for distinctive hyphae and/or conidia; usually a preparation is made in DMSO (36%) and KOH (20%) to digest protein debris and increase the clarity of the image. Scrapings are then plated onto selective agar (e.g. Sabouraud). YEAST and YEAST-LIKE INFECTIONS Candida albicans C. albicans is the only species of Candida that is commonly associated with disease. It grows as a budding yeast (approximately 6 µm in diameter) that can also produce pseudohyphae in animal tissue. C. albicans is present in small numbers on the skin and mucosal surfaces of animals. Most infections have predisposing causes, especially depression of normal flora through use of antibiotics. Other nutritional, immunological or management factors may be involved on occasion. C. albicans produces localised pyogenic lesions (thrush) in the mouth, intestine or genital tract. Production of extracellular toxin and enzymes such as neuraminidase and proteases contribute to virulence. Infections on mucous membranes are characterised by white to grey patches of pseudomembranous inflammation. Crop mycosis in young chickens and turkeys. Can cause mastitis in cows. Enteritis and mycotic stomatitis in young cats and dogs. Genital infections in horses and primates. Systemic infections can occur but rare. Diagnosis is by the presence of suggestive lesions and direct microscopy (Gram stain). Soft, creamy colonies on Sabouraud agar after 24-48 hrs and demonstration of large chlamydospores on appropriate media. Cryptococcus neoformans The only Cryptococcus species that is considered pathogenic for animals. Another budding yeast of variable size (3-20 µm in diameter). Does not produce pseudohyphae. Cryptococcosis occurs in dogs, cats, horses, foxes, guinea pigs, ferrets and some birds. It is also potentially pathogenic for humans. Infection is usually by inhalation with primary infections in the respiratory system, pharynx and paranasal sinuses. It can however disseminate to affect the CNS (cryptococcal meningitis). In this case, yeast-like cells can be detected in the cerebrospinal fluid, or in the pus of primary infection foci. C. neoformans produces a large capsule which is useful for diagnostic purposes (demonstrated using India ink). It produces shiny, slimy colonies on Sabouraud agar- initially white but develop a yellow/brown tinge. Malassezia pachydermatis A commensal on the oily areas of skin and ears of dogs. It is often isolated alone or in conjunction with certain bacterial species as a cause of canine otitis externa. The yeast can easily be recognised from lesions due to their flask shaped or ‘Russian doll’ appearance. This organism may also cause chronic dermatitis in dogs. SYSTEMIC or TISSUE MYCOSES The Dimorphic Fungi These fungi generally grow as mould with hyphae when growing in the external environment as saprophytes (or in culture under 30oC), but as yeast-like cells in animal tissues (or in rich culture medium at 37oC). However, the mould form appears more stable. Care should be taken as certain dimorphic fungi are highly contagious to humans via inhalation. The majority are not encountered in this country but are endemic to southern USA, Africa and South America. Many dimorphs target the lung: including humans (e.g. Blastomyces dermatitidis, Histoplasma capsulatum). Others exhibit more generalised spread (e.g. Spororothrix spp., Coccidioides immitis). UBIQUITOUS FUNGI - Mould – Aspergillus, Penicillin, Mucor, Rhizopus, Absidia. Aspergillus spp. Several hundred species of Aspergillus have been found but only a minority are associated with disease. Aspergillus fumigatus is the cause of over 90% of aspergillus infections, others include A. niger and A. nidulans. Aspergilli are ubiquitous soil saprophytes. Pathogenic strains can produce a range of extracellular toxins and enzymes that contribute to their virulence, including haemolysins and proteases. Aspergilli can cause disease in a variety of ways including induction of allergic reactions to released toxins (see below). A. fumigatus causes nasal aspergillosis in dogs, with a persistent turbinate or sinus infection. Fungal hyphae can be seen on the turbinate surface during rhinoscopy. The infection can disseminate giving widespread granulomas. In cattle, it can cause mastitis, mycotic abortion and mycotic pneumonia. Mycotic abortion is responsible for only 0.5% of bovine abortions, the other major cause of fungal abortion is Mortierella wolfii. In horses, A. fumigatus can cause guttural pouch mycosis, in which inhalation of spores can result in mycelia germinating and covering large areas of mucosa, the infection can involve the arteries and the nerves. It can also cause abortion in horses. Avian aspergillosis - Brooder pneumonia - Diffuse airway infection. As with many of the ‘mould’ infections, hyphae can often be visualised by microscopy in association with lesions. Most produce granulomatous reactions around the nasal area (after inhalation) or at the site of a wound. INHALATION SENSITIVITY Many air-borne fungal spores (conidia) may be involved in sensitisation and hypersensitivity responses including Mucor, Pencillium, Aspergillus. However, the best documented allergen is a thermophilic actinomycete - Micropolyspora faeni, which is the principle allergen identified in Farmer’s lung (humans), extrinsic allergic alveolitis of cattle and possibly chronic obstructive pulmonary disease (COPD) of horses. Removal of horses from contaminated hay usually results in recovery from COPD. The role of fungi in these and related conditions remains to be fully defined. MYCOTOXICOSIS These are acute or chronic intoxications caused by ingestion of toxins released by fungi growing on crops or stored feed. The toxins are metabolic by-products and are produced under defined conditions that require use of the specific metabolic pathway involved. There are over a hundred toxigenic fungi species known. Most of the toxins are low molecular weight compounds that can differ in their toxicological effects. Outbreaks of mycotoxicosis are often seasonal and sporadic and associated with batches of stored feed, or grazing on a certain pasture. The severity of the disease depends on the quantity and type of toxin consumed and prevention of re-exposure. Diagnosis is difficult and requires demonstration of the toxin in the food, or in the tissues or excretions of the animal. Ergotism: Ingestion of infected grasses and cereals (esp. rye) by domestic animals. The fungal agent responsible is usually Claviceps purpurea. It releases ergot alkaloids which are derivatives of lysergic acid. Gangrenous (chronic) or convulsive (acute) ergotism caused by ergotamine and ergometrine. Aflatoxicosis: aflatoxins produced by Aspergillus flavus or Aspergillus parasiticus growing on stored grain, maize and most nut crops. It effects cattle, pigs, poultry and trout. The toxins can have long tern affects as they are carcinogenic. ANTIFUNGAL AGENTS Limited number available due to because fungi are eukaryotes and the need for selective toxicity. Fungal cell wall an important target as not present in mammalian cells. Important antifungal include; polyenes, azoles, allylamines, echinocardins, pyrimidines and iodines. Development of antifungal resistance an growing concern and there is a need for antifungal stewardship as with other antimicrobials.