Biologic Approaches for the New Millennium Bulletin 883-00 Basic Biology Pastures upon which sheep were grazed the previous season will have varying amounts of surviving, overwintered worm larvae. When sheep consume these larvae in the spring, the larvae become egg-laying adults in a few days. These eggs result in a new generation of larvae on the pasture which can infect grazing lambs and ewes. Continuous worm-production cycles magnify the pasture s larval burden, which may result in very dangerous pastures late in the summer. Although extreme heat and dryness will kill new generation larvae on pasture, experiences in eastern Ohio during the summer of 1999 indicate that the larvae can survive quite well with just a little forage for shade.
Sheep acquire some amount of resistance to internal parasites with age and exposure.
Lambs are totally non-immune until they have been exposed during grazing. Until resistance has developed, worm infections can develop more easily in lambs, and worm egg output is generally much higher than in ewes. Nevertheless, resistance is never complete and can be overcome if the exposure is high enough or if the animals resistance is weakened by poor nutrition, disease, or other stress. The ultimate goals are to keep larval contamination on pastures low and to minimize the use of dewormers to avoid selection for drug resistance in the parasites. Figure 1 - The typical life cycle takes 21 days to complete.