Should I deworm my sheep on a schedule?

Answer: No. Internal parasites, especially Haemonchus contortus, have developed drug resistance. Drug treatment gets rid of the worms that are susceptible to that particular drug; resistant parasites survive and pass on “resistant” genes. No dewormer is 100% effective, and we know that worms that survive a dose of dewormer are resistant to that dewormer. Therefore, each time you deworm, the proportion of resistant worms increases, and consequently, frequent deworming greatly increases the rate at which resistance develops.Each time animals are dewormed, the susceptible worms are killed. The resistant ones survive and will reproduce, thus leading to a population of very resistant worms. Meanwhile, under-dosing causes larger numbers of the intermediate-strength worms to survive. The weakest, most susceptible worms are killed. But because of the weak dose, more of the stronger worms will be able to survive and reproduce, creating a population of stronger worms in the next generation. Once an animal has been treated (if dosed properly), only resistant worms remain. If the animals are moved to a clean pasture they deposit only resistant worms on the pasture, and there are no susceptible worms to dilute the worm population.Because of this drug resistance, deworming on a schedule may lead to trouble down the road. Instead, pay close attention to pasture management and nutrition, and to your animals’ condition. Take fecal samples and use the FAMACHA technique to monitor your animals. Only treat the ones that need it, and only when they actually need it.Learn more about techniques to manage parasites and to prolong the efficacy of dewormers in the ATTRA publication Managing Internal Parasites in Sheep and Goats. It includes a section on Dewormer Assessment, and also discusses the FAMACHA technique.