Phylogenetic analysis of 25 morphological characters among the 12 species of Haemonchus resulted in 1 most parsimonious tree (60 steps; consistency index = 0.67, retention index = 0.80). Monophyly for Haemonchus was diagnosed by 3 unequivocal synapomorphies, including the asymmetric origin of the dorsal ray, relative size of the ventral rays, and the presence of a barb on each spicule tip. Species of Haemonchus have complex histories with respect to host and geographic associations: (1) origins in Africa with basal diversification in antelopes (H. krugeri, H. lawrencei, H. dinniki, H. horaki), (2) independent events of colonization for those species in Caprini and Bovinae (H. contortus, H. placei, H. bedfordi, H. similis), (3) colonization and development of core host associations within Camelidae (H. longistipes) and among Antilopinae, Tragelaphini, and Giraffidae (H. mitchelli, H. okapiae, H. vegliai), and (4) geographically widespread species that are represented only by those that have been translocated with domestic stock. The North American fauna is characterized by 3 introduced and exotic species, H. placei, H. contortus, H. similis, which emphasizes the importance of continued documentation of faunal diversity in the context of predictive foundations derived from phylogenetic studies. Satellite associations for species of Haemonchus, particularly among Cervidae and Camelidae in the Neotropics and Cervidae, Antilocapridae, and possibly wild Caprinae in the Nearctic, have been a consequence of introductions and exchange of parasites at historical interfaces for managed and natural ecosystems. Such distributions are emblematic of the overriding significance of anthropogenic factors as determinants of the global distributions for pathogenic parasites in domestic and wild ruminants.

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