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Microbiology, Fifth Edition
Microbiology, 5/e
Lansing M Prescott, Augustana College
Donald A Klein, Colorado State University
John P Harley, Eastern Kentucky University
Human Diseases Caused by Fungi and Protozoa

Study Outline


  1. Fungal Diseases
    1. Medical mycology-discipline that deals with fungi that cause human disease; fungal diseases are called mycoses
    2. Superficial mycoses
      1. Most occur in the tropics
      2. The fungi that cause the disease are limited to the outer surface of the hair and the skin
        1. Piedras are infections of hair shaft that result in formation of a hard nodule
        2. Tineas are infections of the outer layer of skin, nails, and hair
      3. Treatment involves removal of skin scales and infected hairs; prevention is by good personal hygiene
    3. Cutaneous mycoses-dermatomycoses, ringworms, tineas
      1. Occur worldwide; most common fungal diseases
      2. Three genera, Epidermophyton, Microsporum, and Trichophyton, are involved
      3. Diagnosed by microscopic examination of skin biopsies and by culture on Sabouraud=s glucose agar
      4. Treatment-topical ointments, oral griseofulvin, or oral itraconazole (sporanox)
      5. Different diseases are distinguished according to the causative agent and the area of the body affected (tinea barbae-beard hair, tinea capitis-scalp hair, tinea corporis-smooth or bare parts of skin, tinea cruris-groin, tinea pedis-athlete’s foot, tinea mannum-hands, tinea unguium-nail bed)
    4. Subcutaneous mycoses
      1. The fungi that cause these diseases are saprophytes in soil; they gain entry by puncture wounds; disease develops slowly over a period of years, during which time nodule develops and then ulcerates; organisms spread along lymphatic channels, producing more nodules at other locations; treatment is with 5-fluorocytosine, iodides, amphotericin B, and surgical excision; diagnosis is by culture of the infected tissue
      2. Examples include chromoblastomycosis, maduromycosis, sporotrichosis
    5. Systemic mycoses
      1. Caused by dimorphic fungi (except for Cryptococcus neoformans, which has only a yeast form); usually acquired by inhalation of spores from soil; infection begins as lung lesions, becomes chronic, and disseminates through the bloodstream to other organs
      2. Blastomycosis-Blastomyces dermatitidis
        1. Occurs in three clinical forms: cutaneous, pulmonary, and disseminated
        2. Diagnosis is aided by serological tests; antifungal agents are effective; surgery may be necessary to drain large abscesses; no prevention or control measures
      3. Coccidiomycosis-Coccidioides immitis
        1. Acquired by inhalation of spores
        2. Usually an asymptomatic or mild respiratory infection that spontaneously resolves in a few weeks; occasionally progresses to chronic pulmonary disease
        3. Diagnosis is by culturing; serological tests are also available; treatment with several antifungal agents; prevention involves reduction of exposure to dust in endemic areas
      4. Cryptococcosis-Cryptococcus neoformans
        1. Aged, dried pigeon droppings are a source of infection; fungus enters by the respiratory tract
        2. Is a minor transitory pulmonary infection that can disseminate and cause meningitis
        3. Diagnosis is by microscopic examination of specimens and immunological procedures; treatment includes amphotericin B or intraconazole; no prevention or control measures
      5. Histoplasmosis-Histoplasma capsulatum
        1. A facultative fungus that grows intracellularly
        2. Found world wide in soils; spores are easily spread by air currents and inhaled; the spores are most prevalent where bird droppings have accumulated
        3. A disease of the reticuloendothelial system; symptoms are usually those of mild respiratory involvement; it rarely disseminates
        4. Diagnosis by immunological tests and culture; most effective treatment is amphotericin
      B, ketoconazole, or intraconazole; prevention and control by using protective clothing and masks and by soil decontamination where feasible
    6. Opportunistic mycoses
      1. Opportunistic organisms are normally harmless but can cause disease in a compromised host
      2. Aspergillosis-Aspergillus fumigatus or A. flavus
        1. Portal of entry is respiratory tract; inhalation can lead to several types of pulmonary aspergillosis; the fungus can spread to other tissues and organs; in immunocompromised patients, invasive aspergillosis (mycelia in lungs) may occur
        2. Diagnosis depends on examination of specimens or isolation and characterization of fungus; treated with intraconazole
      3. Candidiasis-Candida albicans
        1. C. albicans is part of normal microbiota and can be transmitted sexually
        2. Exhibits a diverse spectrum of disease: 1) Oral candidiasis (thrush)-mouth; common in newborns 2) Paronychia-subcutaneous tissues of the digits 3) Onychomycosis-subcutaneous tissues of the nails 4) Intertriginous candidiasis-warm, moist areas such as axillae, groin, and skin folds (
        3. g., diaper candidiasis, candidal vaginitis, and balanitis)
        4. Diagnosis is difficult; no satisfactory treatment; cutaneous lesions can be treated with topical agents; oral antibiotics are used for systemic candidiasis
      4. Pneumocystis carinii pneumonia
        1. It was once considered a protozoan parasite but recent comparisons of rRNA genes and other genes have shown it to be more closely related to fungi
        2. Disease occurs almost exclusively in immunocompromised hosts including more than 80% of AIDS patients; the fungus remains localized in the lungs, even in fatal cases
        3. Definitive diagnosis involves demonstrating the presence of the organism in infected lung material or PCR analysis; treatment is by oxygen therapy and combination drug therapy; prevention and control is through prophylaxis with drugs in susceptible persons
  2. Protozoan Diseases
    1. Amoebiasis (amebic dysentery)-Entamoeba histolytica
      1. Ingested cysts excyst in the intestine and proteolytically destroy the epithelial lining of the large intestine
      2. Disease severity ranges from asymptomatic to fulminating dysentery, exhaustive diarrhea, and abscesses of the liver, lungs, and brain
      3. Diagnosis is based on finding trophozoites in fresh warm stools and cysts in ordinary stools; serological testing also should be done; treatment with several drugs is possible; prevention and control involves avoiding contaminated water; hyperchlorination or iodination can destroy waterborne cysts
    2. Cryptosporidiosis-Cryptosporidium parvum
      1. Found in the intestines of many birds and mammals, which shed oocysts into the environment in fecal material; when oocysts are ingested, they excyst in the small intestine; the released sporozoites parasitize intestinal epithelial cells
      2. Major symptom of infection is diarrhea; diagnosis is by microscopic examination of feces; immunological tests are also available; treatment is supportive; patients will usually recover, but the disease can be fatal in late stage AIDS patients
    3. Freshwater amoebae-Naegleria and Acanthamoebae; facultative parasites that cause primary amebic meningoencephalitis and keratitis (particularly among wearers of soft contact lenses); found in fresh water and soil; diagnosis is by microscopic examination of clinical specimens
    4. Giardiasis-Giardia lamblia
      1. Most common cause of waterborne epidemic diarrheal disease
      2. Transmission is usually by cyst-contaminated water supplies, and disease is common in wilderness areas where animal carriers shed cysts into otherwise “clean” water
      3. Disease varies in severity; asymptomatic carriers are common; may be chronic or acute
      4. Diagnosis is by identification of trophozoites; immunological tests are also available; treatment is usually metronidazole (Flagyl); prevention involves avoiding contaminated water and the use of slow sand filters in the processing of drinking water
    5. Malaria-Plasmodium species
      1. Transmitted by bite of an infected female Anopheles mosquito; reproduces in the liver and also penetrates erythrocytes
      2. Periodic sudden release of merozoites, toxins, cell debris from the infected erythrocytes and TNF-a and interleukin-1 from macrophages triggers the characteristic attack of chills and fever; anemia can result, and the spleen and liver often hypertrophy
      3. Diagnosis is by microscopic examination of blood smears; serological tests are also available; treatment is by chloroquine or related drugs
    6. Hemoflagellate diseases-caused by flagellated protozoa that infect blood
      1. Leishmaniasis-transmitted by sandflies from canines and rodents; can be mucocutaneous, cutaneous, or visceral; symptoms vary with the particular etiological organism involved; treated with pentavalent antimonial compounds; recovery usually confers permanent immunity; vector and reservoir control and epidemiological surveillance are the best options for control
      2. Trypanosomiasis
        1. T. brucei causes African trypanosomiasis; transmitted by tsetse flies; causes interstitial inflammation and necrosis of the lymph nodes, brain, and heart; causes sleeping sickness (uncontrollable lethargy)
        2. T. cruzi causes American trypanosomiasis (Chagas’ disease); transmitted when bite of tiatomid bug is contaminated with insect feces; symptoms are similar to those caused by T. brucei
        3. Trypanosomiasis is diagnosed by microscopic examination of blood and by serological tests; drugs are available only for treatment of African trypanosomiasis; vaccines are not useful because the parasite can change its coat to avoid the immune response
    7. Toxoplasmosis-Toxoplasma gondii
      1. Fecal-oral transmission from infected animals; also transmitted by ingestion of undercooked meat and by congenital transfer, blood transfusion, or tissue transplant
      2. Most cases are asymptomatic or resemble mononucleosis; can be fatal in immunocompromised individuals
      3. Acute disease is characterized by lymphadenopathy, enlargement of reticular cells, pulmonary necrosis, myocarditis, hepatitis, and retinitis; a major cause of death in AIDS patients
      4. Diagnosis is by serological tests; chemotherapeutic agents are available for treatment; prevention and control requires minimizing exposure by not eating raw meat and eggs, washing hands after working in soil, cleaning cat litter boxes daily, keeping cats indoors, and feeding cats commercial food
    8. Trichomoniasis-Trichomonas vaginalis; a sexually transmitted disease; host accumulates leukocytes at the site of infection; in females, this leads to a yellow purulent discharge and itching; in males, most infections are asymptomatic; treatment is with metronidazole








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