Visceral and Tissue Parasites — Helminths, Protozoa, and Mycoses Explained | Chapter 34 from Brock Biology of Microorganisms

What are the major parasitic infections affecting human tissues and organs, and how do they impact health worldwide? Chapter 34 of Brock Biology of Microorganisms provides a comprehensive overview of helminthic, protozoal, and fungal infections that invade visceral tissues. This chapter explores transmission, life cycles, pathogenesis, immune evasion, diagnosis, and treatments of globally significant parasites.

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Helminthic Infections

• Trichinosis (Trichinella spiralis): Contracted from undercooked meat, this roundworm causes facial swelling, fever, muscle pain, and potentially heart or brain involvement. Diagnosed via muscle biopsy and serology; treated with benzimidazole drugs.
• Schistosomiasis (Schistosoma spp.): Blood flukes transmitted by freshwater snails; eggs cause significant immune-mediated tissue damage, especially in the liver, spleen, and bladder. Chronic infection leads to organ damage and hematuria. Treated with praziquantel.
• Onchocerciasis (Onchocerca volvulus): Spread by blackflies, causing river blindness as microfilariae migrate through skin and eyes. Treated with ivermectin.

Protozoal Tissue Infections

• Leishmaniasis (Leishmania spp.): Spread by sandflies, these protozoa infect macrophages and present as cutaneous ulcers, mucocutaneous lesions, or visceral disease (kala-azar). Treated with antimony compounds or miltefosine.
• Trypanosomiasis (Trypanosoma spp.): African sleeping sickness (tsetse fly) leads to CNS involvement and sleep disturbances; American Chagas disease (kissing bug) affects the heart and GI tract. Notable for immune evasion via antigenic variation. Treatments include nifurtimox, benznidazole (Chagas), and melarsoprol (late-stage sleeping sickness).
• Malaria (Plasmodium spp.): Transmitted by Anopheles mosquitoes, malaria involves a liver stage followed by cyclic red blood cell lysis, causing fever, chills, and anemia. Treated with artemisinin-based therapies; prevention includes bed nets and prophylactic drugs.
• Primary amebic meningoencephalitis (Naegleria fowleri): A rare but often fatal CNS infection acquired from warm freshwater, entering via the nose and rapidly destroying brain tissue.

Fungal (Mycotic) Infections – Mycoses

• Superficial mycoses: Affect hair and skin (e.g., Malassezia, Trichophyton), causing tinea (ringworm, athlete’s foot). Treated with topical antifungals.
• Subcutaneous mycoses: Penetrate skin and underlying tissue, often following trauma or exposure to soil (e.g., Sporothrix). Typically require oral antifungals.
• Systemic mycoses: Result from inhaling spores, causing lung and multi-organ involvement. Examples: Histoplasma capsulatum (bat/bird droppings), Coccidioides (desert soil), Cryptococcus neoformans (pigeon droppings, immunocompromised patients). Treated with amphotericin B or azole antifungals.

Glossary: Key Terms from Chapter 34

• Mycosis: Any fungal infection (superficial, subcutaneous, systemic).
• Helminth: Parasitic worm (e.g., flukes, nematodes).
• Antigenic variation: Strategy for immune evasion via changing surface proteins (notably in trypanosomes).
• Benzimidazole: Drug class for treating helminthic infections.
• Macrophage: Immune cell targeted by Leishmania.
• Vector: Organism (fly, mosquito) that transmits pathogens.
• Zoonosis: Disease naturally transmitted from animals to humans.

Conclusion: The Global Burden of Parasitic Diseases

Chapter 34 emphasizes the diversity and clinical significance of parasitic and mycotic infections worldwide. Many of these diseases involve complex life cycles, immune evasion, and chronic tissue invasion, posing challenges for treatment and control—especially in resource-limited settings.

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