IN BRIEF: PARASITE RESISTANCE

Frequent deworming treatments, scheduled using only the calendar, lead to the growth of parasite populations that are resistant to anthelmintics, the only chemicals available to fight them. Here’s how that happens: Individual worms carrying rare, random, genetic mutations that provide resistance to a particular chemical class of dewormer occur naturally in the parasite population. Too frequent or indiscriminate treatments with that dewormer kill off the susceptible worms, leaving behind resistant worms, which then reproduce. After each deworming treatment, resistant worms become a larger proportion of the population, and they pass their resistant genes to the next generation. Soon most of the parasite population on the farm is resistant to that class of dewormers. The more frequently a resistance-prone chemical is used on a property, the more rapidly this growth in resistance can happen.