Strongylida

Strongylida is an order of parasitic nematodes in the phylum Nematoda, comprising many species that infect the digestive tracts and respiratory systems of vertebrates, especially mammals. Collectively known as strongyles, members of Strongylida display considerable diversity in morphology and life cycle but share certain features, such as a robust buccal capsule with teeth or cutting plates and a male copulatory bursa.

Most Strongylida have direct life cycles or involve one or more intermediate hosts. Eggs shed in the host’s feces hatch into free-living larvae, which molt to the infective third-stage larva (L3). Infection occurs by ingestion of L3 or, in some species, by skin penetration. After establishing in the host, they develop into adults that inhabit the intestine, cecum, or, in cases of lungworms, the respiratory tract. Some species have complex life cycles that include gastropods or arthropods as intermediate hosts.

Economic and medical importance: Strongylids are among the most significant parasites of livestock, causing anemia, weight loss, impaired growth, and gastrointestinal disease. Notable groups include gastrointestinal nematodes such as Haemonchus, Ostertagia, and Trichostrongylus in ruminants, and hookworms such as Ancylostoma and Necator in humans and carnivores. Pulmonary strongyles and other metastrongyloid worms infect the lungs of various hosts.

Taxonomy and systematics: The order Strongylida comprises several superfamilies and families, with taxonomic placement having been refined by morphological and molecular data. It includes both large-stout nematodes and smaller forms, occupying niches in the gut and lungs. Ongoing research continues to clarify relationships within this diverse lineage.

See also: Nematoda, parasitology, veterinary parasitology.