On the specific distinctness of the ovine and bovine 'Strains' of Haemonchus contortus (Rudolphi) Cobb (Nematoda: Trichostronglidae).

Abstract

The investigations reported in this paper were undertaken to determine whether Haemonchus contortus from sheep and H. contortus from cattle are identical or different species. The length of the spicules and the distances from the hooks to the tips of the spicules were found to be significantly greater in males from cattle. Differences were also seen in the shape and dimensions of the appendage of the genital cone in males from the two hosts, but these were not constant. A study of adult females showed at least 14 different types of vulva1 process. In the majority (88.9 per cent.) of females from sheep there was a long, linguiform process, whereas in the majority (93.5 per cent.) of females from cattle the process was short and rounded. In experimental infestations, despite a change of host, these two types maintained their characteristics in accordance with the host of origin. The progeny of single females from sheep and cattle, respectively, gave a distribution of types similar to that of natural infestations in these hosts. The infective larvae from the two hosts were found to be visually distinct. Measurements showed significant differences in total length, tail length, and in the ratio of total length to tail length, larvae from cattle having the greater measurements and smaller ratio. A change of host did not alter these difference and no association was apparent between the type of larva and the type of vulva1 process in the parent female. In an experiment to test the ability of the two populations to interbreed, mixed sheep and cattle larvae were given to the same host. The subsequent generation of larvae could be classified into sheep and cattle types without any difficulty and there was no evidence of any intermediate types. Evidence of this reluctance to interbreed was also seen under conditions favouring cross-infestation in the field, when both types of larvae maintain their respective characteristics. In accordance with the above findings and as there is also evidence of some degree of host specificity, it is concluded that the populations in sheep and cattle represent distinct species. The name Haemonchus contortus (Rud. 1803) is retained for the species defined in this paper as occurring in sheep and the name Haemonchus placei (Place 1893) is proposed for the species occurring in cattle.