helminth
Summary
Helminths are multicellular parasitic worms, classified into three main groups: nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). They cause significant global disease burden, often via fecal-oral transmission, skin penetration, or vector/intermediate host involvement, and classically induce eosinophilia and elevated IgE due to Th2-mediated immune responses.
Detail
Helminths are eukaryotic, multicellular parasites that infect humans and animals, causing a spectrum of diseases ranging from asymptomatic infection to severe organ damage. They are divided into three phyla: Nematoda (roundworms, e.g., Ascaris lumbricoides, hookworms [Necator americanus, Ancylostoma duodenale], Strongyloides stercoralis, Enterobius vermicularis, Trichinella spiralis, filarial worms like Wuchereria bancrofti and Onchocerca volvulus), Cestoda (tapeworms, e.g., Taenia solium, Taenia saginata, Echinococcus granulosus, Diphyllobothrium latum), and Trematoda (flukes, e.g., Schistosoma species, Clonorchis sinensis, Paragonimus westermani, Fasciola hepatica).
Transmission routes vary: fecal-oral ingestion of eggs (Ascaris, Enterobius, Taenia), skin penetration by larvae (hookworms, Strongyloides, Schistosoma cercariae), ingestion of undercooked meat/fish containing larvae (Taenia, Trichinella, Diphyllobothrium), or vector-borne transmission (filarial worms via mosquitoes/blackflies).
Immunologically, helminth infections classically trigger a Th2-predominant immune response, characterized by IL-4, IL-5, and IL-13 production, leading to eosinophilia, elevated serum IgE, and mast cell activation. This is in contrast to the Th1 response typical of intracellular pathogens.
Clinical manifestations are diverse: GI symptoms (abdominal pain, malabsorption, obstruction with heavy Ascaris burden), anemia (hookworm-induced blood loss), neurocysticercosis (Taenia solium larvae in CNS, a leading cause of adult-onset seizures worldwide), hydatid cysts (Echinococcus), elephantiasis (Wuchereria bancrofti lymphatic obstruction), river blindness (Onchocerca volvulus), and hepatosplenic disease with portal hypertension (Schistosoma mansoni/japonicum) or bladder cancer risk (Schistosoma haematobium, squamous cell carcinoma).
Diagnosis typically involves stool ova and parasite exams, blood smears (for filariasis), serology, or imaging (CT/MRI for cysts). Treatment includes anthelmintics such as albendazole/mebendazole (broad-spectrum, inhibit microtubule formation), ivermectin (for Strongyloides, Onchocerca), praziquantel (for cestodes and trematodes, increases calcium permeability causing paralysis), and diethylcarbamazine (for filariasis).
High-yield board associations include: Strongyloides hyperinfection in immunosuppressed patients (steroids), Trichinella from undercooked pork causing myositis and periorbital edema, Enterobius causing perianal pruritus diagnosed via tape test, and Schistosoma haematobium's link to bladder squamous cell carcinoma.
Sources
- First Aid for the USMLE Step 1
- Murray's Medical Microbiology
- CDC DPDx Parasitology Database
- Harrison's Principles of Internal Medicine
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