Differences between female and castrated male lambs in susceptibility to natural, predominantly Teladorsagia circumcincta infection

•Castrated Scottish Blackface male lambs had significantly higher faecal egg counts than female lambs grazing the same field.•Male lambs were heavier and had higher numbers of adult nematodes at necropsy. There were no differences in the length of adult females Teladorsagia circumcincta.•The results suggest that higher egg counts in castrated males may be a consequence of greater infection due to heavier grazing. The difference between castrated male lambs and entire female lambs in susceptibility to gastrointestinal nematode infection was investigated in a flock of Scottish Blackface sheep. Castrated male lambs had higher faecal egg counts, higher body weights, lower parasite-specific plasma IgA activity and more adult nematodes. There were no detectable differences in the length of adult Teladorsagia circumcincta. Heavier egg shedding by males was associated with greater worm burdens. The difference in plasma IgA activity may have been a consequence of larger numbers of adult nematodes inhibiting the transfer of mucosal IgA to the plasma. At least part of the sex difference in egg production could be a consequence of males ingesting larger amounts of grass and hence numbers of infective larvae because of their larger size. The differences between castrated males and females contribute to the observed variation in nematode egg production in naturally infected lambs.