Attempts to control haemonchosis in grazing ewes by vaccination with gut membrane proteins of the parasite

A vaccination trial was conducted to evaluate the potential benefit of Haem onchus contortus gut membrane proteins as vaccine antigens under field cond itions in Louisiana. The trial was conducted in the summer of 1996 in a flo ck of ewes grazing pasture naturally infected with H. contortus. Ewes were randomly assigned to three treatment groups (vaccine, adjuvant only, and sa line) and fecal egg counts (FEC, measured as eggs per gram of feces), packe d cell volumes (PCV), and antibody levels were monitored fortnightly for 12 weeks. It was shown by FEC that there were large individual variations in susceptibility to H. contortus in both vaccinated and non-vaccinated sheep, a finding which could have masked differences between treatments when anal yzed by conventional statistical methods. Based on their egg counts before the period when the vaccination could have had an effect, all ewes were cat egorized as 'susceptible' or 'relatively resistant'. The significance of di fferences between FEC, PCV and antibody responses of vaccinated and control sheep were tested separately for the 'susceptible' and 'relatively resista nt' category. The 'susceptible' vaccinates shed 65% fewer worm eggs during the period when the vaccine could have had an effect, but the difference wa s only significant on Week 6 post-vaccination. In these experiments, it was difficult to completely exclude the confounding effect of having 'relative ly resistant' sheep in the control group. More studies are needed to furthe r evaluate H11 and H-gal-GP antigens under field conditions. (C) 2000 Elsev ier Science B.V. All rights reserved.