The effects of immunization of sheep with Trichostrongylus colubriformis larvae on worm burdens acquired during grazing

Abstract

The effects of immunization of sheep with Trichostrongylus colubriformis larvae on worm burdens acquired during grazing. International Journal for Parasitology 19: 177–181. Romney sheep, reared helminth-free in pens to 5 months of age, were immunized against Trichostrongylus colubriformis by giving two doses of 200,000 T. colubriformis infective larvae at 15 day intervals to assess protection from natural challenge during grazing. Five immunized sheep and five unimmunized sheep were grazed on infested pasture for 4 weeks, and were then returned to the pens for 4 weeks before slaughter. Worm burdens, gastrointestinal histology and mucus antiparasite activity were examined at slaughter. Faecal egg counts and haematological examinations were carried out at regular intervals throughout the trial. Significant protection ( P < 0.05) was afforded immunized sheep against adult T. colubriformis (87%), T. axei (67%), Nematodirus spathiger (91%) and Osteragia spp. (42%). Greater numbers of immature Nematodirus spp. and Ostenagia spp. were present in immunized sheep ( P < 0.05). Overall, a significant ( P < 0.05) 42% reduction in total nematode burdens was afforded by immunization of the sheep with T. colubriformis larvae. Immunized sheep had significantly ( P < 0.05) more globule leukocytes, mast cells and eosinophils in gastrointestinal tissue and significantly ( P < 0.05) higher levels of mucous antiparasite activity than unimmunized sheep. Haematological observations showed some sheep had transient eosinophilia during immunization or grazing. Both immunized and unimmunized sheep showed depressed ( P < 0.05) total leukocyte counts during grazing which returned to pre-grazing levels within 1 week of return of the sheep to the pens. Overall, haematological parameters reflected parasite challenge and were unrelated to worm burdens acquired.