Chapter 20 How do Fungi Cause Disease The

Chapter 20

How do Fungi Cause Disease? • The tissue damage associated with fungal infections results primarily from direct invasion of tissue, with subsequent displacement and destruction of vital structures, coupled with toxic effects of the inflammatory response. • Masses of fungal cells can cause obstruction of bronchi in the lungs and tubules and ureters in kidneys, leading to obstruction of the flow of bodily fluids. • Some fungi can grow in the walls of arteries and veins, leading to occlusion and tissue necrosis resulting from a lack of oxygen.

Classification of Fungal Diseases • Fungal infections (mycoses) can be classified into the following four categories: – Superficial mycoses—fungal infections of the outermost areas of the body (outer surfaces of hair shafts, the epidermis) – Cutaneous, hair, and nail mycoses (tinea or “ringworm” infections) – Subcutaneous mycoses – Systemic mycoses—the most serious types of fungal infections

Fungal Infections of the Skin • Dermatophytosis (Tinea [“Ringworm”] Infections, Dermatomycosis) – Caused by various species of filamentous fungi, including Microsporum, Epidermophyton, and Trichophyton spp. – These fungi are collectively referred to as dermatophytes. – Note that ringworm infections have nothing to do with worms.

Fungal Infections of the Lower Respiratory Tract Coccidioidomycosis (Valley Fever) – Coccidioides immitis, a dimorphic fungus; transmission occurs via inhalation of arthrospores • Cryptococcosis – Two subspecies of Cryptococcus neoformans, an encapsulated yeast; transmission occurs via inhalation of yeasts • Histoplasmosis – Histoplasma capsulatum var. capsulatum, a dimorphic fungus; transmission occurs via inhalation of conidia from soil •

Fungal Infections of the Lower Respiratory Tract (cont. ) • Pneumocystis Pneumonia (PCP; Interstitial Plasma-Cell Pneumonia) – Caused by Pneumocystis jiroveci, which has both protozoal and fungal properties – Was once classified as a protozoan, but is now classified as a fungus – Mode of transmission is unknown; perhaps direct contact, transfer of pulmonary secretions from infected to susceptible persons, or perhaps airborne

Fungal Infections of the Oral Cavity • Thrush – A yeast infection of the oral cavity – Common in infants, elderly patients, and immunosuppressed individuals – White, creamy patches occur on the tongue, mucous membranes, and the corners of the mouth – Caused by Candida albicans and related species

Yeast Vaginitis • Most cases are caused by the yeast, Candida albicans • Can be diagnosed by microscopic examination of a saline wet mount of vaginal discharge material, in which numerous yeasts and hyphae may be observed • The mere presence of yeast is not proof of yeast vaginitis, as the vaginal flora of up to 25% of healthy women can contain Candida spp.

Cryptococcal Meningitis • Can be caused by three subspecies of Cryptococcus neoformans, an encapsulated yeast • Cryptococcosis starts as a lung infection, but spreads via the bloodstream to the brain • Cryptococcal meningitis can be presumptively diagnosed by observing encapsulated, budding yeasts in CSF specimens examined by an India ink preparation