Anthelmintic resistance is a genetic change (mutation) in individual worms within a parasite population that allows resistant worms to survive treatment. Because it is genetically based, resistance to the same chemical group is passed on to the next generation.
There are a number of ways that resistance is expressed. For example, some worms have genetic changes that increase the rate of excretion of the drench from their cells. Others carry genes that enable the worms to detoxify the drench, making it ineffective, or that alter or by-pass the biochemical pathway targeted by the drench.
Resistance is heritable, meaning that resistance is passed on from one generation to the next. Repeated use of the same chemical group can cause ongoing selection of the more resistant parasites, as susceptible types are killed. If the relative proportion of resistant worms in the population increases compared to susceptible worms, and the resistant worms come to dominate, then the effectiveness of the chemical to control the parasite will become compromised.
Importantly, as resistance to different chemical groups occurs through different mutations within the worms, resistance to one chemical group does not usually equate to resistance to other chemical groups. However, resistance against an active within a chemical group, will confer resistance (though not necessarily to the same extent) to all other actives within the same chemical group. Avoid continuous use of actives within the same chemical group to reduce the build-up of resistant individuals within populations.