A COMPARISON OF THE RESPONSES TO REPEATED EXPERIMENTAL INFECTIONS WITH HAEMONCHUS-CONTORTUS AMONG SCOTTISH BLACKFACE LAMBS

Twenty helminth-naive Scottish Blackface lambs were given three infect ions with 10000 infective larvae of Haemonchus contortus at 8 week int ervals. An additional six lambs served as uninfected controls and eigh t lambs were infectivity controls. The lambs were 7 months old at the start of the infection. Four of the 20 lambs developed severe haemonch osis and were put down during the experiment, The remaining 16 lambs p lus uninfected controls were necropsied 8 weeks after the third infect ion, The mean faecal egg count peaked 6-8 weeks after the first infect ion, gave a second smaller peak 6-8 weeks after the second infection b ut produced no peak after the third infection, Mean red blood cell cou nts fell rapidly during the first infection, then rose gradually durin g the second and third infections. The mean values suggested that two infections were sufficient to produce effective immunity in the sheep population but they masked considerable individual variation, Eleven a nimals appeared relatively resistant following the first infection, wh ile two animals were relatively susceptible to even the third infectio n. The repeatability of mean faecal egg counts or mean red blood cell counts for each animal during the replicate infections were very high, because the rankings of the individual sheep remained remarkably stab le, Faecal egg counts were very strongly correlated with red blood cel l counts. Multiple regression analysis showed that four factors-faecal egg counts, red blood cell counts, weight and sex-accounted for essen tially all of the observed variation in worm burdens among the lambs. Therefore, under these controlled experimental conditions, additional markers appear unnecessary for the detection of resistance status.