The systematics of nematodes that cause ostertagiasis in domestic and wild ruminants in North America: an update and a key to species.

Studies of medium stomach worms (Subfamily Ostertagiinae) of domestic and wild ruminants in North America have refined our understanding of the systematics, host distribution and biogeography of these nematodes. Strong support for recognition of monophyly for the Ostertagiinae has resulted from preliminary phylogenetic analyses of morphological characters among these and other trichostrongylids. Evaluation of morphological and biochemical characters in our laboratory and elsewhere has not refuted the polymorphism hypothesis where paired major and minor morphotypes are postulated to represent polymorphic species within the genera Ostertagia (Ostertagia ostertagi/Ostertagia lyrata; Ostertagia leptospicularis/Ostertagia kolchida; Ostertagia gruehneri/Ostertagia arctica; Ostertagia mossi/Ostertagia dikmansi), Teladorsagia (Teladorsagia circumcincta/Teladorsagia trifurcata (= Teladorsagia davtiani), and Marshallagia (Marshallagia marshalli/Marshallagia occidentalis). The minor morphotype of Ostertagia bisonis, recognized in Eurasia (considered to be Teladorsagia kazakhstanica), has yet to be discovered in North America. The taxonomy of these species has remained stable since an earlier overview of Ostertagia spp. and Teladorsagia spp. in North American domestic stock at the 1986 Ostertagia Workshop. Investigations since 1986 have focused on comparative morphology and the clear differentiation of Ostertagiinae, primarily from wild ruminants, that have been reported or may potentially occur in domestic bovids. Among these are species which infect wild and domestic sheep (Marshallagia marshalli/Marshallagia occidentalis), bovids (Ostertagia bisonis), and cervids (Ostertagia mossi/Ostertagia dikmansi, Ostertagia leptospicularis/Ostertagia kolchida, Ostertagia gruehneri/Ostertagia arctica, Mazamastrongylus odocoilei and Mazamastrongylus pursglovei) in North America. A key is presented for identifying 15 species of the Subfamily Ostertagiinae that are parasitic in domestic and wild ruminants of North America. Four exotic species (Camelostrongylus mentulatus, Spiculopteragia spiculoptera (= Spiculopteragia boehmi), Spiculopteragia asymmetrica and Longistrongylus curvispiculum) have been reported in North America, but only in game farms and are not included in the key. Hyostrongylus rubidus, a member of the Ostertagiinae normally parasitic in pigs and only rarely reported from domestic ruminants, is not included in the keys. Additional data from comparative morphology and molecular analyses will promote a reassessment of the generic-level systematics of the Ostertagiinae and allow an evaluation of host-parasite evolution and historical biogeography.