Worms are considered to be resistant if they survive exposure to a
standard recommended dose of anthelmintic.
This ability to survive is then passed on to its offspring and the proportion of resistant worms increases over time. Anthelmintic resistance is not reversible, so farmers should use a post drench efficacy test to check the effectiveness of the wormers they use.
The percentage of resistant worms, and the number that survive treatment, increases with time as illustrated.
The speed which resistance develops depends on how carefully and sustainably
anthelmintics are used and how effectively sustainable practices are employed to address the three main selection pressures for anthelmintic resistance on worm populations in sheep:
- treating sheep when the refugia population is small.
Research from Wales Against Anthelmintic Resistance Development (WAARD) showed the majority of farms surveyed had a degree of resistance to all three older wormer classes2