Resistance of worms to drenches is a genetic trait. The number and type of resistance genes and the way they are inherited (eg dominant or recessive etc) varies depending on the drench.
These resistance genes arise by mutation, which are spontaneous or random changes in genes. Having resistance genes to a particular drench gives worms a competitive advantage over their peers if the worm population is exposed to the drench in question. The resistant individuals have a much better chance of surviving – there is selection for resistance – and passing on their favourable genes to the next generation. If exposure to the particular drench is on-going, resistant worms become more and more common until the drench becomes almost ineffective.
Some factors that favour selection for resistance include:
Things to avoid if possible in order keeping drenches alive for longer:
How do I test the effectiveness of drenches on my farm?
If a drench is effective, the number of worm eggs (eggs per gram of faeces) will drop by at least 95% after 2-3 days. If the sheep are exposed to worms after the drench, which is likely, then worm eggs from the newly ingested worms will not appear in the faeces for about 3weeks.
A worm egg count on around 10 animals 7-14 days after a drench will give you an indication of its effectiveness. To make the assessment more precise, also do a worm egg count on the day of drenching, so you can more precisely gauge whether the reduction in worm egg counts has exceeded the 95% benchmark.