Parasitic worms belonging to groups such as flatworms, roundworms and thorny‑headed worms that live in or on hosts and may cause chronic infections in humans and other animals.
Explanation
Helminths are multicellular eukaryotic parasites that occupy internal organs or tissues of vertebrate and invertebrate hosts. They include flatworms (Platyhelminthes), which encompass trematodes (flukes) and cestodes (tapeworms), roundworms (Nematoda), and less commonly thorny‑headed worms (Acanthocephala). Many have elongated bodies with protective cuticles or teguments and lack complete digestive systems, absorbing nutrients directly from host tissues or gut contents. Helminths often require multiple hosts to complete their life cycles. Trematodes and cestodes frequently depend on snails, fish or arthropods as intermediate hosts before reaching vertebrates. Soil‑transmitted nematodes such as Ascaris lumbricoides and hookworms have direct cycles involving eggs or larvae in contaminated soil. Infection typically occurs through ingestion of contaminated food or water, penetration of larvae through skin, or transmission by blood‑feeding vectors.
These parasites can persist in hosts for years. They cause morbidity by mechanical damage, nutrient theft and modulation of immune responses that may lead to chronic inflammation or immunosuppression. Global helminth infections affect over a billion people, particularly in tropical and subtropical regions with poor sanitation. Diseases caused by helminths include schistosomiasis, ascariasis, trichuriasis, hookworm disease, lymphatic filariasis and onchocerciasis. Diagnosis relies on detection of eggs, larvae or adult worms in stool, blood or tissue samples, or on serologic tests. Control strategies involve improved sanitation, vector control, health education and mass drug administration. Anthelmintic drugs such as albendazole, mebendazole, ivermectin and praziquantel are effective against many species. Research into helminth biology has also highlighted their ability to modulate host immunity, leading to investigations of helminth‑derived molecules for treatment of autoimmune disorders.
Representative Species and Diseases
Schistosoma mansoni, S. haematobium and S. japonicum are blood flukes that cause schistosomiasis when their cercariae penetrate the skin of people wading in infested freshwater. Taenia solium is a tapeworm transmitted via undercooked pork; ingestion of its eggs can lead to neurocysticercosis, a major cause of epilepsy in endemic regions. Ascaris lumbricoides is a large roundworm that infects the small intestine via ingestion of eggs contaminating food or soil. Hookworms (Ancylostoma duodenale and Necator americanus) larvae penetrate the skin and migrate to the gut, causing anemia. Filarial worms such as Wuchereria bancrofti and Brugia malayi are transmitted by mosquitoes and can block lymphatic vessels, resulting in lymphatic filariasis and elephantiasis. Onchocerca volvulus, transmitted by blackflies, causes river blindness. These examples illustrate the diversity of helminths and the varied routes through which they infect hosts.
Helminths remain major causes of disease in many parts of the world. Integrated control measures and access to effective anthelmintic drugs can reduce their burden, while studies of their immune-modulating capabilities may provide insights into novel therapies.
Related Terms: Parasite, Trematode, Nematode, Cestode, Schistosomiasis