Brock Biology of Microorganisms: Eukaryotic Pathogens

Brock Biology of Microorganisms: Eukaryotic Pathogens - Fungi, Protozoa, and Helminths

I. Fungal Infections

A. Pathogenic Fungi and Classes of Infection

  • Pathogenic Fungi Statistics:

    • Approximately 50 fungal species are known to cause disease in humans.

    • Types of Fungi:

      • Can be single-celled yeasts or hyphae-forming molds.

      • Hyphea can intertwine to form mycelia, which are visible masses.

      • Some pathogenic fungi exhibit dimorphism, existing in both yeast and mold forms.

B. Mechanisms of Fungal Disease

  • Fungi cause disease through three primary mechanisms:

    1. Inappropriate Immune Responses:

    • Fungi may produce potent allergens, which can trigger asthma attacks or other hypersensitivity reactions.

    1. Mycotoxins:

    • Example: Aflatoxins

    1. Infections (Mycoses):

    • Types of mycoses include:

      • Superficial

      • Subcutaneous

      • Systemic

II. Fungal Diseases: Mycoses

A. Superficial Mycoses

  • Definition:

    • Initiated by dermatophytes, fungi that colonize hair, skin, or nails, and infect only the surface layers.

  • Terminology:

    • Dermato = skin; phyte = plant (historically, fungi were viewed as degenerate plants).

  • Example:

    • Trichophyton species cause infections such as athlete’s foot.

B. Subcutaneous Mycoses

  • Definition:

    • Occurs when fungi infiltrate deeper layers of skin, presenting a more serious condition than superficial infections.

  • Examples of pathogens:

    • Sporothrix

    • Fonsecaea

    • Cladosporium

C. Systemic Mycoses

  • Definition:

    • Typically initiated by inhaling spores, leading to fungal proliferation in internal organs.

  • Examples of diseases caused by specific fungi:

    • Histoplasmosis caused by Histoplasma capsulatum.

    • Coccidioidomycosis caused by Coccidioides immitis.

    • Blastomycosis caused by Blastomyces dermatitidis.

    • Paracoccidioidomycosis caused by Paracoccidioides brasiliensis.

    • Cryptococcosis caused by Cryptococcus neoformans.

  • Important Note: Fungal infections are particularly severe in individuals who are immune-compromised or during pregnancy, with antibiotic use, or in older populations.

III. Visceral Parasitic Infections

A. Amoebae and Ciliates: Entamoeba, Naegleria, and Balantidium

  1. Entamoeba histolytica

    • Pathogenic protist that is predominantly transmitted through contaminated water.

    • Can be asymptomatic or lead to diarrhea and dysentery, with symptoms including intestinal inflammation, fever, and potential anemia due to blood loss.

    • Complications can include invasion into the liver and possibly lungs or brain if untreated.

    • Treatment involves amoebicidal drugs.

  2. Naegleria fowleri

    • Free-living amoeba found in soil and natural water sources.

    • Infection typically results from swimming in warm, contaminated water and progresses to primary amebic meningoencephalitis (PAM) as it enters through the nose and burrows into the brain, causing extensive hemorrhage and damage.

    • Effective drug treatment is possible if infections are detected early.

  3. Balantidium coli

    • Ciliated intestinal parasite of humans and pigs; it alternates between the trophozoite and cyst stages.

    • Infection occurs through fecally contaminated water and yields symptoms similar to amebiasis.

B. Other Visceral Parasites

  1. Giardia intestinalis

    • A flagellated anaerobic parasite with mitosomes that produces resilient cysts.

    • Associated with giardiasis, which causes explosive foul-smelling diarrhea.

    • Many carriers can remain asymptomatic.

  2. Trichomonas vaginalis

    • Flagellated anaerobic parasite, utilizing hydrogenosomes instead of mitochondria.

    • Transmitted person-to-person primarily through sexual contact and can survive on moist surfaces (e.g., toilet seats or sauna benches).

    • Symptoms in women include vaginal discharge, itching, and burning, while many men remain asymptomatic.

    • Commonly treated with the antiprotozoal drug metronidazole.

  3. Cryptosporidium, Toxoplasma, and Cyclospora

    • All are coccidian parasites transmitted through fecally contaminated food or water.

  4. Cryptosporidium parvum

    • Lives as a parasite in warm-blooded animals and sheds thick-walled oocysts in feces that contaminate water supplies.

    • Notably associated with waterborne disease outbreaks, including the largest in the U.S. in 1993.

    • Oocysts are resilient to chlorine and UV radiation.

  5. Cyclospora cayetanensis

    • Produces oocysts shed in animal feces, with most cases linked to contaminated fruits or vegetables.

    • Major outbreaks have occurred, with one resulting in 2,299 cases of cyclosporiasis.

  6. Toxoplasma gondii

    • A parasite associated with warm-blooded animals, where oocysts are shed in feces, commonly transmitted via cats or undercooked meat.

    • Toxoplasmosis is primarily asymptomatic except for complications in immunocompromised individuals, leading to potential organ damage and birth defects.

IV. Blood and Tissue Parasitic Infections

A. Plasmodium and Malaria

  1. Overview of Malaria

    • Caused by the protists of the genus Plasmodium.

    • Has a complex life cycle involving Anopheles mosquitoes as vectors.

    • Affects 350 million people worldwide, resulting in over 600,000 deaths annually, mostly in tropical and subtropical regions.

  2. Life Cycle:

    • Involves the maturation of gametes within the mosquito, transmission via mosquito bite, and a series of life stages including the erythrocytic stage, where merozoites infect red blood cells.

    • Symptoms of malaria arise during the infection process in the human host.

  3. Diagnosis and Treatment:

    • Diagnosis often conducted via blood smears.

    • Common drugs include chloroquine and primaquine, with the potential for disease recurrence years after initial infection.

    • Strategies to control malaria include eliminating mosquito breeding grounds (e.g., swamps).

B. Leishmaniasis, Trypanosomiasis, and Chagas Disease

  1. Leishmania

    • Leishmania tropica or L. mexicana:

      • Causes cutaneous leishmaniasis, transmitted by sandfly bites, leading to nodules and ulcers on the skin.

      • Treatment involves antimony compounds.

    • Leishmania donovani:

      • Causes visceral leishmaniasis, affecting internal organs and requiring prompt treatment to avoid fatalities.

  2. Trypanosoma

    • Trypanosoma brucei:

      • Causes African sleeping sickness, transmitted by tsetse fly bites, resulting in infection of the central nervous system. Treatment requires anti-trypanosomal drugs.

    • Trypanosoma cruzi:

      • Causes Chagas disease, transmitted by the "kissing bug", affecting the heart and digestive system, predominantly in Latin America.

C. Parasitic Helminths: Schistosomiasis and Filariasis

  1. Schistosomiasis

    • Also known as snail fever, caused by the trematode Schistosoma.

    • The life cycle involves snails and humans, with eggs released into freshwater, transforming into miricidia, which infect snails. Cercaria then burrow into human skin, potentially causing surface lesions.

    • Treatment typically involves praziquantel.

  2. Filariasis

    • Transmitted by mosquitoes, Bancroft's filariasis (elephantiasis) is caused by the nematode Wuchereria bancrofti, leading to chronic lymphatic system infections and severe leg enlargement.

  3. Onchocerciasis

    • Also called