Inhibition of Ostertagia ostertagi at the Early Fourth Larval Stage: II. The Influence of Environment on Host or Parasite

Abstract

SUMMARY Groups of 4 susceptible worm-free calves were grazed in late autumn on paddocks of equal area which had been contaminated at different times of the year with a laboratory culture of Ostertagia ostertagi or with O. ostertagi derived from a natural field infection. Further susceptible calves were grazed on a worm-free paddock and inoculated orally with O. ostertagi larvae cultured in the laboratory. In the calves inoculated orally only negligible numbers of inhibited O. ostertagi were present, but inhibited fourth stage larvae were a feature of the worm burdens of all the calves which ingested larvae developed at pasture. A significantly higher proportion of inhibited fourth stage larvae was found in the worm burdens of calves which had grazed on the paddock contaminated with the natural field injection. It was concluded that inhibition of larval development in naturally occurring bovine ostertagiasis was primarily dependent on 2 factors: an innate susceptibility of a particular strain of larvae to inhibition and the environmental circumstances of late autumn which, acting on larvae on pasture, produce an optimal stimulus for the subsequent inhibition of ingested larvae.