41 General features of fungi associated with disease in animals
Fungi are eukaryotic, non-photosynthetic heterotrophs which produce exoenzymes and obtain nutrients by absorption. Of the approximately 100,000 fungal species that have been described to date, only a few hundred are known to be pathogenic for animals and humans. The two main morphological fungal forms are moulds and yeasts. Moulds grow as branching filaments called hyphae, whereas the unicellular yeasts have an oval or spherical appearance. Although most fungi grow aerobically, some yeasts are facultatively anaerobic and fungi found in the rumen of cattle are strict anaerobes. Incubation temperatures and time required for development of distinctive colonial morphology are indicated in Table 41.1. Dimorphic fungi occur in both mould and yeast forms. Environmental temperature and other factors usually determine the form in which a dimorphic fungus occurs.
Table 41.1 Incubation conditions appropriate for fungal cultures.
| Incubation conditions | ||
| Fungal group | Temperature (°C) | Time |
| Dermatophytes | 25 | 2–4 weeks |
| Aspergillus species | 35–37 | 1–4 days |
| Yeasts (pathogenic) | 37 | 1–4 days |
| Dimorphic fungi | ||
| Mould phase | 25 | 1–4 weeks |
| Yeast phase | 37 | 1–4 weeks |
| Zygomycetes | 34 | 1–4 days |
Classification of fungi has traditionally relied heavily on morphology and sexual reproduction. The form of a fungal species during its sexually reproductive life cycle is termed its teleomorph, while its asexual form is referred to as its anamorph. Fungi that lack a meiotic stage are referred to as mitosporic fungi. Formerly, fungi with no known sexual stage were placed in a heterogeneous group called the Deuteromycota or Fungi Imperfecti. Molecular methods are increasingly being used to assign fungal species to their appropriate grouping. A dual naming system has been in use for many years with separate teleomorphic and anamorphic names used; as an example the teleomorphic name of the dermatophyte Microsporum canis is Arthroderma otae. Advances in diagnostic molecular methods should eventually replace this dual naming system. Seven phyla are recognized within the kingdom Fungi: Glomeromycota, Microsporidia, Blastocladiomycota, Chytridiomycota, Ascomycota, Basidiomycota and Neocallimastigomycota. Fungi of veterinary importance are found in three phyla: Ascomycota, Basidiomycota and Zygomycota. The status of the phylum Zygomycota is uncertain and it may be broken up, with species of veterinary importance assigned to the subphyla Mucoromycotina and Entomophthoromycotina currently listed as incertae sedis (Latin for ‘of uncertain placement’). However, the term ‘zygomycetes’ is still in use, encompassing species of interest previously recognized within the phylum.
Fungal species may be saprobes, symbionts, commensals or parasites. Saprobic fungi, which are widespread in the environment and are involved in the decomposition of organic matter, occasionally cause sporadic, opportunistic infections in animals. The parasitic dermatophytes cause ringworm in animals. Overgrowth of yeasts, which are often commensals on skin and mucous membranes, sometimes causes localized lesions.
Hyphal cell walls are composed mainly of carbohydrate components including chitin macromolecules with cellulose cross-linkages. In yeasts, cell walls contain protein complexed with polysaccharides. Both moulds and yeasts have nuclei with well-defined nuclear membranes, mitochondria and networks of microtubules.
Moulds tend to form large colonies with growth and extension of hyphae at their periphery. Several types of asexual spores are produced by fungi. Conidia are formed on conidiophores while sporangiospores are formed within a sac-like sporangium. In most yeasts, asexual division is by budding. Colonies of yeast-like fungi are soft, smooth and round. The pathogenic mechanisms whereby fungi produce disease are listed in Box 41.1. Factors which predispose to infection with fungi are outlined in Box 41.2.
Methods of differentiating fungal species include examination of sporing heads for conidial arrangement or the presence of a sporangium. Features of vegetative hyphae used for differentiation include the presence or absence of septa and pigment. The size, appearance and colour of fungal colonies are useful for species differentiation. Yeasts can be differentiated by colonial appearance and by the size and shape of individual cells.