The Superfamily Strongyloidea
Members of the superfamily have large, complex buccal capsules, often with a corona radiata (a series of leaf-like structures on the border of the labial region), and are mainly gut parasites although a few species occur in the respiratory or urinary systems (Lichtenfels, 1980b). The superfamily is divided into the families Strongylidae (including the large strongyles of equines), Chabertiidae (including the nodular worms, Oesophagostomum spp.), Syngamidae (including the gapeworms of birds and a single species in swine, Stephanurus dentatus) and Deletrocephalidae (of Rhea americana). Their oval, smooth-shelled, poorly developed eggs pass out in waste products of the host and, like those of hookworms, embryonate rapidly in the external environment under suitable conditions of moisture and temperature. In the Strongylidae, Chabertiidae and Stephanurinae, the first-stage larva hatches and develops rapidly to the infective stage. Prior to, and during, moulting, larvae are lethargic. The small first-stage rhabditiform larva has a long filamentous tail, and a narrow, elongate buccal cavity. The second stage is slightly larger than the first but morphologically similar to it. During development to the strongyliform third stage the tail becomes blunt and conical. The stoma is closed and the larva retains the shed cuticle of the second stage with its attenuated tail.
In the gapeworms (Syngaminae), development to the third stage takes place in the egg. The oesophagus in the first two larval stages lacks the valved bulb and the attenuated tail used in feeding and movement by larvae of the strongylids, the chabertiids and Stephanurus dentatus (Stephanurinae).
The usual route by which strongyles infect the final host is oral; members of the superfamily are predominantly parasites of herbivorous hosts such as horses, ruminants, ratite birds and certain Australian marsupials (Lichtenfels, 1980b). The infective larvae of some species, such as the gapeworms and the swine kidney worm (Syngamidae), sequester themselves in earthworm and gastropod paratenic hosts, where they are available to hosts such as swine, anseriforme and passeriforme birds and small non-herbivorous mammals.
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