Selected Fungi and Fungal Diseases (Mycoses)



Selected Fungi and Fungal Diseases (Mycoses)

Zygomycetes : Rhizopus sp.,

rapid growers; opportunistic pathogens; non-septate hyphae

respiratory & sinus infections, brain abscess: risk factors - immune suppression, especially cancer or diabetes

Ascomycetes: Penicillium sp. - usually not pathogenic; source of antibiotics

Aspergillus fumigatus - opportunistic respiratory infections - "fungus ball" in lungs , “Farmer’s Lung”

Basidiomycetes - The only member of this division that causes human infection is Cryptococcus neoformans – see discussion under Systemic infections

Deuteromycetes:- Contains most of the fungi pathogenic for humans – example: the dermatophytes that cause various types of ringworm belong to this division – see discussion below.

Fungal Infections -- Classified by he Extent of Tissue Involvement – See Table 5.2 p 139

I. Superficial infections – Infect the top layerof the epidermis

Tinea versicolor -- caused by Malasezzia furfor – extremely contagious;

II. Cutaneous Infections – can penetrate deeper than the the top layer of the epidermis ; can hydrolyze keratin

Dermatophytes: - p565

Infect upper layers of skin, hair, nails; cause ringworms

Tinea pedis (athlete's foot), Tinea cruris ("jock itch"), Tinea capitis (scalp) Tinea corporis (ringworm of the Body) ; easily spread from person to person

Treated with topical sprays or ointments; keep area clean, dry, exposed

III. Subcutaneous Infections – Fungal spore is usually introduced deep into the subcutaneous layer by a puncture wound

Sporothrix schenckii - see p669 - "Rose Gardener's Disease"; spores are found on rose thorns, sphagnum moss, rough timbers; can carry fungus; introduced beneath the skin by small trauma ; travel through the connective tissue of the subcutaneous layer, causing cellulites; can spread along the lymphatic vessels. ( a dimorphic fungus)

IV. Systemic Infections – Spores are usually inhaled into the lungs; Usually start as respiratory infections. Can spread through the blood into deep tissues and infect any major organ of the body. Some can move into the Central Nervous System and cause meningitis. Sometimes fatal. Usually caused by dimorphic fungi.

Dimorphic Fungi - grow as molds at normal environmental temperatures (18-250C) and as yeasts at 370C ( human body temperature)

1. Coccidiodes immitis - see p584, p669

• Pulmonary disease - Coccidiodomycosis, San Joaquin Valley Fever

• U.S. - Southwest, Southern California

• Grows as mold near base of desert plants ; spores released into the soil - inhaled with dust during dry periods, windstorms, earthquakes etc.

• Large Spherules formed in body tissues ( 370C) – release single endospores

• Not an opportunistic infection – causes infection even in healthy hosts ; high mortality

2. Cryptococcus neoformans p. 583

• Dimorphic fungus; large yeast with capsule; demonstrate capsule with India Ink (negative stain)

• Diagnosis - usually latex agglutination test for Cryptococcus antigens in serum or CSF

• Cryptococcosis (respiratory) or Cryptococcal meningitis; can disseminate into skin, bones, viscera

• Found in soil; pigeon droppings, starling droppings; spores inhaled from contaminated droppings or soil

• Major problem in AIDS patients; other people at risk: steroid users, cancer, diabetes

3. Histoplasma capsulatum - see p.679 p669

• Histoplasmosis - respiratory disease resembling tuberculosis ; can spread to skin, brain, intestines, heart, liver , spleen, and bone marrow

• In U.S. - found in Mississippi and Ohio River Valleys

• Grows in bird or bat droppings ( these are high in nitrogen) - spores inhaled from soil or droppings in woods, caves

4. Blastomyces dermatitidis - see p669

• Blastomycosis ( Chicago disease) - skin or respiratory disease - can spread to affect any major organ

• In U.S. – Common in Midwest, including Missisippi River Valley

• Spores found in wooded settings; inhaled – causes skin lesions; respiratory infections which can disseminate

• Frequently acquired through occupational or recreational exposure to spores in forests ( hunters, campers, hikers, rangers, etc) ; also infect dogs.

Opportunistic Infections

1. Pneumocystis carinii - see p681

Pneumocystis pneumonia - usually only a problem in immunocompromised patients

Is it a fungus or a protozoan parasite? New rRNA typing suggests fungus ( related to yeasts)

Common indicator of AIDS - very uncommon in other patients

2. Yeast Infections

Candida albicans - see p742 ( majority of human yeast infections are due to this species)

• Opportunistic infections of mucous membranes and skin: oral candidiasis (thrush), vaginal

candidiasis, diaper rash, etc.

Saccharomyces cerevesiae - not a pathogen - Baker's yeast; Brewer's yeast – important in food production and also in genetic engineering.. Foreign genes can be inserted to produce desired protein products.

Mycotoxin Producers

Aspergillus flavus - aflatoxin - assoc. with peanut crops; liver damage; carcinogen -

Amanita phalloides - poisonous mushroom; phalloidin; powerful neurotoxin-

Claviceps purpurea - ergot; gangrene, hallucinations, bizarre behavior; ingestion of contaminated grains or

cereals

Plant Pathogens - potato blight, Dutch Elm disease, Chestnut tree blight; rots, smuts, blights, rusts

Allergic reactions – Many people have allergic reactions when exposed to fungal elements, especially spores. Inhaled spores can trigger asthmatic reactions, other respiratory problems such as bronchitis, skin rashes, inflammation, etc.

The spores act as stimulators of the immune system. The immune system produces an exaggerated protective response.

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