Traditional parasite control programs involving rotational treatment with anthelmintics at regular intervals are commonly recommended by veterinarians. However, this approach is based on concepts and strategies developed more than 50 years ago when Strongylus vulgaris (large strongyle bloodworm) was the most important parasitic pathogen of horses (Drudge and Lyons, 1966). The rationale for this parasite control scheme was rather simple: to kill S. vulgaris worms before they could mature and lay eggs that would contaminate the environment. Since it took about two months for strongyle eggs to reappear after treatment, treatment every two months prevented S. vulgaris eggs from being shed on pastures. This approach was very successful in controlling S. vulgaris infections, and disease from S. vulgaris is now very rare in managed horse populations.