Order Strongylida the Bursate Nematodes
The order includes the bursate nematodes, divided into five well-defined superfamilies. The Diaphanocephaloidea is a peculiar minor superfamily with two genera in lizards and snakes. The cephalic extremity is in the form of two lateral jaw-like structures. The hookworms (Ancylostomatoidea) and the strongyles (Strongyloidea) have large, globular buccal capsules which enable them to attach to the intestinal mucosa and suck the blood of the host. Buccal capsules are absent in trichostrongyles (Trichostrongyloidea) and lungworms (Metastrongyloidea) and the buccal cavity is markedly reduced in size. The trichostrongyles are mainly gut parasites; a few species occur in other parts of the body. The lungworms are usually found within the lungs but many species are associated with the vascular system distant from the lungs.
The diaphanocephaloids, hookworms, strongyles and trichostrongyles are monoxenous, although paratenic hosts are frequently used in the transmission of some species. The lungworms are almost entirely heteroxenous and most make use of earthworms and gastropods as intermediate hosts in which development from the first to the third larval stage takes place.
Transmission in the monoxenous superfamilies usually involves the development in the external environment of eggs to the first stage which hatches, and moults to the second stage. The first and second stages may feed on bacteria in the environment (e.g. hookworms and strongyles) or they may have sufficient nutrient reserves in their cells to develop and persist for considerable periods without feeding (e.g. Dictyocaulus). Second- and third-stage larvae that feed in the environment have an oesophageal bulb with a prominent valve which is absent or much reduced in species that do not feed. In some species (e.g. Dictyocaulus) development to the first stage and hatching take place in the definitive host. In other species embryonated eggs are passed by the host and the entire development to the third stage occurs in the egg (e.g. Amidostomatidae, Nematodirus, Syngaminae). In one peculiar genus (Ollulanus) the entire development to the third stage takes place in utero. The third and infective stage, which generally retains the cuticle of the second stage (often with a long attenuated tail), does not feed and typically enters the host through the skin (many hookworms) or per os (most strongyles and trichostrongyles). The third stage is homologous to the dauer larva of the free-living rhabditids from which the Strongylida presumably evolved. It is the infective and dispersal stage and generally resistant to adverse environmental conditions. Larvae which penetrate skin exsheath before or during their invasion of the skin. Larvae which are ingested by the definitive host exsheath in some region of the gut (the rumen, abomasum or intestine, depending on the species). It is believed that larvae respond to factors in the gut and release a fluid which acts on a specific region of the cuticle, causing a break which allows larvae to escape (Sommerville, 1957).
Most of the Strongylida have a tissue phase in the definitive host. In the diaphanocephaloids, hookworms and strongyles which have large buccal capsules in the adult form, a provisional or temporary buccal capsule is formed during development to the fourth stage. During development of the fourth stage the adult buccal capsule is formed beside and beneath the provisional buccal capsule, which is shed at the final moult. A reduced provisional buccal capsule appears rarely in the fourth stage of some trichostrongyloids, the adults of which lack buccal capsules. Buccal capsules are absent in the metastrongyloids.
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