Special Veterinary Pathology

GENERAL PATHOLOGY OF MYCOTIC INFECTIONS

• Fungi are plant – like animal pathogens
• Fungal cells are large and thick cell walls contains ergosterol and polysaccharides
• Fungi produce a range of Pathology from acute Pyogenic to chronic granulomatous infections
• Pathogenic fungi produce mycotoxims and enzymes
• Some fungi induce strong hypersensitivity responses of type III and IV
• Immunosuppression and leucopenia favours fungal infections
• Corticosteroids and immunosuppressive drugs favour fungal infections
• Majority of the fungi are saprophytes and do not cause disease in animals.
• Fungal diseases can be classified into five groups

1.The only obligate parasites are dermatophyles

• They live in Keratin layers of the skin and cause “Ringorm”
• Such infections are called “Superficial mycoses”
• Dermatophytes do not live as saprophytes
• Spread is direct & Non invasive
• Do not produce granulomas

2. Some fungi establish themselves in normal individuals

• Called systemic or deep mycoses
• Example – histoplasmosis, coccidiomycosis, blastomycoes and sporotrichosis
• Live in mycelial forms or in yeast forms in the soil and plants

3. Debilitated and immunocompromised animals are most susceptible

• Produces classical “Granulomas”
• Subcutanous mycoses or “Mycetomas”
• Saprophytic fungi
• Enters through skin
• Grows in mycelial forms
• Localised lesions

4. Causes both localized and disseminated disease

• Examples – aspergillosis and zygomycosis
• More common in debilitated animals

5. Yeasts – single celled fungi

• Not highly Pathogenic
• Examples – Candidiasis, Cryptococcosis
• Produces superficial infection rarely systemic disease

Lesions

• All fungi can be easily seen in tissue sections stained with either haematoxylin or eosin or with special stains such as Gomori methanamine silver, periodic acid –Schiff (PAS) or Gridley stain
• Presence of a homogenous, brightly eosinophilic material known as “ Splendore-Hoeppli material surrounding individual organism or colonies of organisms
• It is believed that they are the product of the host, most likely an antigen-antibody complex
• The lesions produced by deep fungi are usually “classical granulomas” which is consisting of central core of necrotic debris usually containing pus. It is surrounded by a zone of epithelioid cells, macrophages and multinucleated giant cells. Variable number of lymphocytes, plasma cells and sometimes eosinophils may be present.
• Superficial mycoses (dermatophytoses) do not produce granulomas because the organisms are restricted to the superficial layers of the skin and are not invasive

Defence against fungal infections

• The first line of defence against invasive fungi such as Aspergillus or Candida, is the activation of the alternate pathway of the complement system
• Very small fungal fragments or spores may be ingested and destroyed by macrophages or by natural killer cells

Immunity

• Fungal infections can only be destroyed by the T cell mediated mechanisms
• Some T and NK cells cause a direct cytotoxic effect on yeast such as Candida albicans and Cryptococcus neoformans