Oesophagostomum
Members of the genus, known as nodular worms, are common parasites of the large intestine of pigs, ruminants, primates and rodents. Those in domestic animals are considered significant pathogens. The buccal capsule is relatively reduced and thin-walled. The cuticle of the cephalic end is inflated and a transverse groove is usually present near the excretory pore; this groove tends to offset the cephalic end from the rest of the body of the parasite. The genus is rich in species and is divided into subgenera (Lichtenfels, 1980b). For a review of sporadic reports in humans, see Polderman and Blotkamp (1995).
Marotel (1908) recognized that the three morphological stages found in nodules in cattle in France belonged to Oesophagostomum radiatum. Veglia (1924, 1928) provided the first sound account of the development and transmission of a species of the genus Oesophagostomum. Since then about seven other species parasitic in livestock have been studied. Eggs are oval with thin shells and are laid usually in the 16- to 32-cell stage (Fig. 3.3A). In faeces, eggs develop rapidly into rhabditiform first-stage larvae (Fig. 3.3B) which hatch as early as 24 h at optimum temperatures. First-stage larvae escape from one pole of the egg. Larvae feed on bacteria in the environment and moult to the second-stage (Fig. 3.3C) about 24 h after hatching. Both first- and second-stage larvae have long attenuated tails, rhabditiform oesophagi and a limited number of intestinal cells. Second-stage larvae moult to infective larvae (Fig. 3.3D) as early as 3—5 days from the time of hatching. The third-stage larva has a club-shaped oesophagus (strongyliform), a delicate tubular buccal cavity and a sharp, conical tail. The larva, however, retains the cuticle of the second stage with its long filamentous tail. Infective larvae can live for long periods and, in some species, are capable of surviving winter.
The definitive host usually becomes infected by ingesting third-stage larvae, although calves have apparently been infected experimentally through the skin (Mayhew, 1939). In the small intestine larvae exsheath and within about 3 days invade the mucosa, where they become surrounded by the transparent capsule in which they are coiled (Fig. 3.3G). In the capsules the larvae develop to the fourth stage (Fig. 3.3E-I) and then enter the lumen, where they shed the cuticle of the third stage. The fourth-stage larva has an oval, thin-walled provisional buccal capsule. Cervical papillae appear and the oesophagus takes on the form of that in adults. Sexes can be differentiated by the shape of the tail, which in the male is directed dorsally and is shorter than that in the female. Fourth-stage larvae move from the small intestine posteriorly to the caecum and colon, where they moult and reach adulthood. The final moult occurs about 2 weeks postinfection but the females do not produce and deposit eggs until about 1 month postinfection, when eggs appear in the faeces of the host.
Fig. 3.3. Developmental stages of Oesophagostomum venulosum: (A) unembryonated egg; (B) first-stage larva; (C) second-stage larva; (D) infective third-stage larva; (E) late fourth-stage larva, anterior end, dorsal view; (F) late fourth-stage larva, posterior end; (G) encapsulated late third-stage parasitic larva (4 days postinfection); (H) late fourth-stage larva, anterior end; (I) late female fourth-stage larva, posterior end. (After A. Goldberg, 1951 - courtesy Proceedings of the Helminthological Society of Washington.)
Fig. 3.3. Developmental stages of Oesophagostomum venulosum: (A) unembryonated egg; (B) first-stage larva; (C) second-stage larva; (D) infective third-stage larva; (E) late fourth-stage larva, anterior end, dorsal view; (F) late fourth-stage larva, posterior end; (G) encapsulated late third-stage parasitic larva (4 days postinfection); (H) late fourth-stage larva, anterior end; (I) late female fourth-stage larva, posterior end. (After A. Goldberg, 1951 - courtesy Proceedings of the Helminthological Society of Washington.)
O. asperum (Railliet and Henry, 1913)
O. asperum is a parasite of the caecum and colon of goats. According to Rao and Venkataratnam (1977a) eggs were 95-112 X 60-68 mm in size. When cultured in aerated water at room temperature (13.6-37.4°C) first-stage larvae hatched in about 60 h. First-stage larvae were 390-461 mm in length and the intestine was composed of eight to ten cells. The first moult commenced in 70 h when second-stage larvae were 687-722 mm in length. The second moult occurred in 122-150 h in faecal culture. Infective larvae were 895-920 mm in length; the tail of the sheath was 117-128 mm in length. Infective larvae were active in water for 20 days and survived for 45 days.
Rao and Venkataratnam (1977b) gave five kids infective larvae orally. Fourth-stage larvae were collected from the large intestine of two kids 22 and 29 days postinfection. Immature and mature fifth-stage worms were found in one kid 8 weeks postinfection. The prepatent period was said to be 48 days.
O. columbianum Curtice, 1890
O. columbianum is a common parasite of the large intestine of sheep, goats, alpaca and certain wild African antelope. According to Veglia (1924, 1928) eggs embryonated into first-stage larvae in 15-20 h and a day later moulted to the second stage. Infective larvae appeared in about 3 days after the second moult. Larvae migrated on to vegetation. When ingested by the host, larvae exsheathed and entered the mucosa and submucosa of the small and large intestine, where they became encapsulated. In 4 days larvae moulted to the fourth stage, which entered the lumen of the large intestine 5-8 days postinfection. Fourth-stage larvae moulted to adults 27 days postinfection and eggs appeared in faeces at 40 days. Some adults lived for up to 21 months.
Agrawal (1966) showed that eggs would develop and hatch, and larvae develop, at temperatures from 15 to 37°C but the optimum was 30°C. At room temperature infective larvae lived for 103 days.
Fotedar and Wali (1982) gave the size of eggs as 80-90 X 50-60 mm and reported that they were laid in the four- to eight-cell stage. First-stage larvae were 300-350 mm, second-stage larvae 650-700 mm and third-stage larvae 700-800 mm in length. The optimum temperature for growth was 28-30°C. Infective larvae remained alive for 3 weeks.
Attempts to infect lambs cutaneously with O. columbianum were unsuccessful and fourth-stage larvae given orally to lambs failed to result in infection (Shelton and Griffiths, 1968a). Shelton and Griffiths (1968b) reported that subcutaneous inoculation of infective larvae seemed to sensitize lambs and subsequent oral infections were more severe because larvae were retained longer than normal in the gut wall.
Dash (1973) reported two histotropic phases in infections of O. columbianum in lambs. In first infections, third-stage larvae became encapsulated mainly in the wall of the anterior small intestine. The third moult occurred 5 and 10 days postinfection, after which larvae returned to the lumen and migrated to the large intestine. Some larvae entered the wall of the large intestine and were arrested at the mid-fourth stage whereas other larvae grew to adulthood in the lumen of the large intestine. Few larvae developed to adulthood after second infections. Some were arrested in the first histotropic phase in the small intestine and others in the second histotropic phase in the large intestine. Dash (1973) suggested that the second histotropic phase is abnormal and indicative of a poorly adapted host-parasite relationship, as also suggested by Taylor and Michel
(1953), Dobson (1966) and Shelton and Griffiths (1968a,b). Perhaps O. columbianum is derived from species in wild hosts such as African antelopes.
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