The significance of inhibition in the parasitic development of abomasal nematodes in New Zealand sheep

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Abstract
Extract Infective larvae of Haemonchus contortus when ingested by sheep normally undergo development through a series of moults to become sexually mature, adults, a process that takes 14 to 15 days (Veglia, 1915 Veglia, F. 1915. The anatomy and life history of the Haemonchus contortus (Rud). 3rd and 4th Rep. Direct. Vet. Res., Union of South Africa, : 348–500. [Google Scholar] ). In some circumstances, however, such larvae may become arrested in their development at an early fourth larval stage 3 to 4 days following ingestion (Blitz and Gibbs, 1971 Blitz, N. M. and Gibbs, H. C. 1971. Morphological characterization of the stage of arrested development of Haemonchus contortus in sheep. Can. J. Zool., 49: 991–995. [Google Scholar] ). A similar phenomenon of arrested or inhibited development has also been recorded in Ostertagia circumcincta and Trichostrongylus axei infections in sheep; in the former again occurring at an early fourth stage, (Sommerville, 1953 Sommerville, R. I. 1953. Development of Ostertagia circumcincta in the absomasal mucosa of the sheep. Nature. Lond., 171: 482–483. [Google Scholar] , 1954 Sommerville, R. I. 1954. The histotropic phase of the nematode parasite Ostertagia circumcincta. Aust. J. agric. Res., 5: 130–140. [Google Scholar] ) and in the latter at a loosely defined “fourth larval stage” (Reid and Armour, 1972 Reid, J. F. S. and Armour, J. 1972. Seasonal fluctuations and inhibited development of gastro-intestinal nematodes of sheep. Res. vet. Sci., 13: 225–229. [Google Scholar] ).