SYSTEMATICS OF GASTROINTESTINAL NEMATODES OF DOMESTIC RUMINANTS - ADVANCES BETWEEN 1992 AND 1995 AND PROPOSALS FOR FUTURE-RESEARCH

The systematics of trichostrongyloid nematodes of ruminants provides a foundation for diagnostics and responds to the need to identify eggs in feces, free-living larvae from pastures or fecal cultures and larva l or adult nematodes collected from hosts. These needs are associated with diagnostic problems or research projects. Difficulties. in identi fying all developmental stages of trichostrongyloid nematodes of domes tic ruminants still severely limit the effective diagnosis and control of these parasites. Phylogenetic hypotheses as the basis for predicti ve classifications have been developed only for the subfamilies of the Trichostrongylidae. This report briefly describes recent progress in the development of improved tools for identification, phylogenetic ana lyses and predictive classifications. It also describes future researc h needed on the identification and classification of trichostrongyloid nematode parasites of domestic ruminants. Nematodes included are spec ies of the superfamily Trichostrongyloidea known to be important patho gens of domestic ruminants. The information summarized is presented by nematode developmental stage and by taxonomic groups. Eggs: While egg s of some trichostrongyloid nematode parasites of ruminants can be rea dily identified to their genus (Nematodirus), and some to species (e.g . Nematodirus battus), most of the important pathogens (including the Ostertagiinae and Haemonchinae) cannot be identified morphologically o r morphometrically even to family level. However, DNA technology has b een developed for determining not only the presence of specific pathog ens in eggs from fecal samples, but also for estimating the percentage of the total eggs that each pathogen comprises. This new method will make possible a rapid determination of which individual animals in a h erd should be treated. Larvae: The most commonly-used method for ident ifying infective larvae is time-consuming (several weeks), unreliable for estimating intensities of individual species as components of mixe d populations and requires highly trained specialists. Available ident ification keys for larvae are not well illustrated and need to be augm ented. Adults: Recent advances in the identification of adult trichost rongyloids and their systematics are organized by taxonomic group. Gen era included are Ostertagia, Haemonchus, Cooperia, Trichostrongylus an d Nematodirus. Recently, the first phylogenetic analysis of the Tricho strongylidae family established monophyly for the family. A similar an alysis of the Molineidae is needed. Ostertagia: Several studies of pol ymorphism summarized the phenomenon and listed 19 polymorphic species in five genera. Two studies of DNA differences within and among polymo rphic species of Ostertagiinae supported earlier hypotheses that the s pecies pairs represent polymorphic species. A phylogenetic analysis of the Ostertagiinae and generic concepts are needed, Haemonochus: A key to three species of Haemonchus provides, for the first time, morpholo gical characteristics for the microscopical identification to species of individual adult nematodes of either sex. The Food and Drug Adminis tration is now requiring that results of drug trials include identific ation of Haemonchus to species. Cooperia: Studies using random amplifi ed polymorphic DNA methods showed a high degree of variation within an d among C. oncophora/C. surnabada, but supported a polymorphic relatio nship for the species pair. A phylogenetic analysis of the Cooperiinae is needed. Trichostrongylus: Restriction Fragment Length Polymorphism s (RFLPs) of genomic DNA of two strains of T. colubriformis indicated a high degree of intra- and inter-strain DNA polymorphism, However, ot her studies demonstrated expected species level differences between T. colubriformis and T. vitrinus using Random Amplified Polymorphic DNA (RAPD) methods. Sequences of the second Internal Transcribed Spacer Re gion (ITS-2) ribosomal repeat showed sequence differences of 1.3-7.6% among five well-defined species of Trichostrongylus. This provides a s tandard for species level differences within the Trichostrongylidae. N ematodirus: The origin of iii. battus in the British isles is still a mystery. Recently, DNA studies have provided evidence that populations on both coasts of the United States originated from Canada. A phyloge netic study of Nematodirus is in progress. Modern systematic methods h ave not yet been applied to the development of classifications for all subfamilies and most genera of the Trichostrongyloidea. Additional fa ctors complicating these problems are a lack of knowledge of the paras ites of wild bovids and cervids, the international transport of wild a nd domestic hosts and environmental changes that may alter the parasit e fauna in a modern farm setting. Published by Elsevier Science B.V.