Subfamily Dictyocaulinae
The subfamily contains Bronchonema and the well-studied and important genus Dictyocaulus of ruminants and horses.
Dictyocaulus
Three well-known species of the genus have been investigated - two in ungulates and one in equines. Members of the genus are medium-sized and occur in the bronchi and trachea, where they are associated with bronchitis, giving rise to a clinical syndrome (including coughing) known in cattle and sheep in the UK as husk (for a brief history of husk see Allan and Johnson, 1960). Eggs are oval and thin-shelled, and embryonate to the first-stage in utero. After oviposition most eggs hatch in the air passages but some remain unhatched, at least until they reach the large intestine. First-stage larvae passed in faeces of the host are rather stout with slightly tapered tails with rounded ends. The cells of the larvae (except in D. arnfieldi) are packed with nutrient granules which obscure anatomical details but there is no evidence that the oesophagus has a valved bulb or that the larva feeds (Daubney, 1920). Under suitable conditions of temperature and moisture the first-stage larva enters a lethargus and moults to the second stage in a few hours. The second stage is similar in morphology to the first-stage larva and its cells are also packed with granules. The larva moults to the third stage in a few more hours and the third-stage larva retains for a short time both shed cuticles but that ofthe first stage is eventually cast. The third stage is typically strongyliform and the tail is rather short and sharply conical. The granules which obscured the internal anatomy of the first two larval stages largely disappear. The sheath fits closely to the body and barely extends beyond the tail end of the enclosed larva. Third-stage larvae are negatively geotactic and fairly resistant to cold and freezing conditions and moderate drying.
The definitive host becomes infected by ingesting infective third-stage larvae contaminating its food. Larvae migrate on to vegetation but many apparently remain in faeces. The dispersal of larvae from faeces by means of exploding sporangia of fungus (Pilobolus spp.) may be important in the transmission of larvae to cattle which avoid feeding near faecal material (Robinson, 1962; Robinson et al., 1962; Bizzell and Ciordia, 1965b; Taranik et al., 1978; Doncaster, 1981; Rodriguez Diego et al., 1981, 1983; Jorgensen et al., 1982; Boon et al., 1983; Roque et al., 1983). In the definitive host, third-stage larvae invade the small intestine, enter the lymphatics and travel to the mesenteric lymph nodes, where they remain for 6—7 days and develop to the fourth and fifth stages without significant growth (Hobmaier and Hobmaier, 1929d; Kauzal, 1933; Anderson and Verster, 1971; Verster et al., 1971). The small fifth-stage worms as well as some fourth-stage larvae then travel via the thoracic duct to the anterior vena cava and from there to the heart and the pulmonary arteries. In lungs the worms grow markedly, mature and produce larvated eggs in 3—4 weeks postinfection. The patent period in infections is variously reported and may be greatly extended by the presence of arrested immature worms (late fourth and early fifth stages) in the lungs (Taylor and Michel, 1952; Michel, 1974; Gupta and Gibbs, 1975).
An effective live vaccine produced by attenuating infective larvae with ionizing radiation has been developed. The attenuated larvae invade lymph nodes and stimulate an immune response but usually fail to reach the lungs, or are expelled rapidly if they do so (Jarrett et al., 1959; Poynter et al, 1960; Jarrett and Sharp, 1963; Sharma et al, 1988).
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