Mycoses

Previous authors: J A PICARD AND H F VISMER

Current authors:
A Jonker - Faculty of Veterinary Sience, University of Pretoria, Private Bag X04, Onderstepoort, Gauteng, South Africa

Introduction

Fungi are ubiquitous in nature where they primarily function as decomposers, with some being pathogenic to insects and plants. Of the more than 100 000 identified species only 150 are known to cause disease in animals and humans. Included in this group are the toxin-producing fungi that cause mycotoxicoses²⁵² and the fungi that invade host tissue referred to as mycoses. With the exception of dermatophytes, animals seem to be ‘accidental’ hosts, with infections being acquired through exposure to a point source in nature of specific fungi that are able to survive in certain tissues of the animal due to an adapted ability to grow at body temperature (37 °C). Many non-invasive fungi, including those not known to be animal pathogens, can be involved in allergic reactions.

Of the aetiologic agents of mycoses, all except Pythium insidiosum belong to the kingdom Fungi, as they are nonmotile eukaryotic organisms that possess a cell wall, lack chlorophyll and reproduce by means of spores. The spore can be produced either sexually or asexually. Most fungi will also grow on artificial media. Two fungal forms exist, namely the mould or filamentous form, in which hyphae are present, and the unicellular budding yeast form. Some of the fungi, known as thermally dimorphic fungi that include some of the more pathogenic fungal genera, are able to exist in both forms, as yeasts at 37 °C or in host tissue, and as moulds at 22 °C or in the environment.

The fungi are divided into five phyla. Four of these are characterized by the sexual (telomorphic) production of spores, namely the Zygomycotina (containing agents of zygomycosis) the Ascomycotina, the Basidiomycotina and the Chytridiomycotina. Those that have been identified on the asexual (anamorphic) stage are classified as belonging to the phylum Deuteromycotina or Fungi Imperfecti. If, however, the telomorphic stages of these fungi are identified, they are classified as either an ascomycete or basidiomycete, and the name associated with this classification takes precedence. Most fungi that cause disease in animals are traditionally identified by their anamorphic stage, resulting in the situation that the same fungus may have two names.

The diagnostician is often faced with having to decide whether a fungus cultured from a lesion is the cause of the disease or only a contaminant. The presence of fungal elements in affected tissues, presence of an inflammatory reaction, tissue type affected, fungal species involved, number of fungal colonies cultured, and ability of the cultured fungus to grow at 37 °C may all be helpful in this regard. For example, a fungus identified from cerebrospinal fluid should be considered to be more clinically significant than fungi found on the skin or mucous membranes. As many different fungi in smears or sections of animal tissues are morphologically similar, their identification is usually made by either culturing or identifying them, or by applying immunological methods such as the immunofluorescence test.

The superficial, subcutaneous and internal mycoses that occur in livestock and horses are briefly dealt with in this chapter, with a greater emphasis being placed on those infections relevant to Africa.

Superficial and cutaneous mycoses

Dermatophytosis

Introduction

Dermatophytosis or ‘ringworm’ is a contagious fungal infection of the superficial, keratinized layers of the skin, and hair and nails of humans and animals caused by the genera, Microsporum, Trichophyton and Epidermophyton. This disease is considered to be of some importance as it is a zoonosis and a common occupational hazard.^{217, 267, 381} Studies have shown that in New Zealand 20,9 per cent³⁷⁴ and in Sweden 29 per cent¹⁹³ of human dermatophyte infections originated from animals, and in Switzerland 74 per cent of dairy farm workers experienced ringworm.¹⁹³ Economic losses are incurred by the down-grading of hides and the exclusion of affected animals from sporting events, shows or auctions.³⁷² Lesions in animals that have ‘healed’ before they are slaughtered often ‘reappear’ after the tanning process, resulting in the rejection of hides.¹⁹³ Animals that suffer from severe dermatophytosis can be stunted and show a weight loss of up to 20 per cent.³⁵¹ In livestock, confirmation of the diagnosis of ringworm is not generally undertaken, except perhaps in pure-bred animals, and then samples are rarely taken for laboratory confirmation. In South Africa, the disease, usually caused by Microsporum canis, is more commonly diagnosed in cats and dogs in urban areas.⁴⁸⁶

Aetiology

All dermatophytes with an asexual (anamorphic) phase of reproduction belong to the genera, Microsporum, Trichophyton and Epidermophyton, and those with a sexual (telomorphic) phase of reproduction to the family Arthrodermataceae, genus Arthroderma.¹⁸⁸ (Table 1). The telomorphic reproductive phase has not yet been identified in some dermatophytes of human and animal origin and it is thought that the ability to form this phase has been lost and that only clonal reproduction occurs.¹⁸⁸ In animals, infections caused by fungi of the genera Microsporum and Trichophyton are most common. Anamorphic dermatophytes are epidemiologically classified based on their origin as either zoophilic...