Parasitic Infections

Definition

Parasitic infections are diseases caused by protozoa, helminths, or ectoparasites that infect humans, often transmitted through contaminated food, water, soil, or vectors. They range from asymptomatic colonization to severe systemic illness depending on the parasite and host factors.

Epidemiology

• Highly prevalent in tropical and subtropical regions with poor sanitation and limited access to clean water.
• Over 3 billion people globally are infected with intestinal parasites.
• Protozoan infections (e.g., malaria, amoebiasis, giardiasis) and helminth infections (e.g., ascariasis, hookworm, schistosomiasis) are most common.
• Children, travelers, immunocompromised individuals, and those in endemic areas are at higher risk.
• Vector-borne parasitic diseases (e.g., malaria, leishmaniasis) are common in Africa, Asia, and Latin America.

Etiology

• Protozoa: single-celled organisms (e.g., Plasmodium spp., Entamoeba histolytica, Giardia lamblia, Toxoplasma gondii).
• Helminths: multicellular worms (e.g., Ascaris lumbricoides, hookworm, Taenia spp., Schistosoma spp.).
• Ectoparasites: live on the host’s surface (e.g., lice, mites, fleas).
• Transmission routes include fecal-oral (contaminated food/water), vector-borne (mosquito, sandfly, tsetse fly), skin penetration (schistosomiasis, hookworm), and ingestion of undercooked meat.
• Poor sanitation, open defecation, contaminated drinking water, and inadequate hygiene are major contributing factors.

Pathophysiology

• Parasites enter the body via ingestion, inhalation, vector bites, or skin penetration.
• They invade or colonize specific organs (intestinal tract, blood, liver, brain, skin).
• Elicit immune responses leading to inflammation, tissue injury, and chronic disease.
• Some parasites (e.g., Plasmodium) cause cyclic infection and immune evasion.
• Nutritional deficiencies, anemia, and growth retardation are common sequelae of chronic helminth infections.
• Tissue-dwelling parasites may cause granuloma formation and fibrosis.