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Background  The ability of infective larvae of cattle gastrointestinal nematode (GIN) species to overwinter on pastures in northerly climatic zones with very cold dry winters is poorly understood. This is an important knowledge gap with critical implications for parasite risk assessment and control.    Methods  Infective third-stage larvae (L3) were quantified in samples of fecal pats, together with adjacent grass and soil, before and after winter on three farms in southern, central and northern Alberta. Nemabiome ITS2 metabarcoding was then performed on the harvested L3 populations to determine the species composition. Finally, parasite-free tracer calves were used to investigate if the L3 surviving the winter could infect calves and develop to adult worms in spring.    Results  Farm level monitoring, using solar powered weather stations, revealed that ground temperatures were consistently higher, and less variable, than the air temperatures; minimum winter air and ground temperatures were − 32.5 °C and − 24.7 °C respectively. In spite of the extremely low minimum temperatures reached, L3 were recovered from fecal pats and grass before and after winter with only a 38% and 61% overall reduction over the winter, respectively. Nemabiome ITS2 metabarcoding assay revealed that the proportion of L3 surviving the winter was high for both  Cooperia oncophora  and  Ostertagia ostertagi  although survival of the former species was statistically significantly higher than the latter.  Nematodirus helvetinaus  and  Trichostrongylus axei  could be detected after winter whereas  Haemonchus placei  L3 could not overwinter at all. Adult  C. oncophora ,  O. ostertagi  and  N. helvetianus  could be recovered from tracer calves grazing after the winter.    Conclusions  The largest proportion of L3 were recovered from fecal pats suggesting this is important refuge for L3 survival. Results also show that L3 of several GIN parasite species can survive relatively efficiently on pastures even in the extreme winter conditions in western Canada. Tracer calf experiments confirmed that overwintered L3 of both  C. oncophora  and  O. ostertagi  were capable of establishing a patent infection in the following spring. These results have important implications for the epidemiology, risk of production impact and the design of effective control strategies. The work also illustrates the value of applying ITS2 nemabiome metabarcoding to environmental samples.

parasites and vectors

High levels of third-stage larvae (L3) overwinter survival for multiple cattle gastrointestinal nematode species on western Canadian pastures as revealed by ITS2 rDNA metabarcoding