ANTHELMINTIC resistance is one of the biggest challenges facing the sheep industry, but what is it and what can we do about it? If a wormer is less than 95 percent effective, and other causes of treatment failure have been ruled out, then it is likely that anthelmintic resistance is present. This means that when the correct dose of wormer is given, some of the worms present are carrying genes which allow them to survive the treatment that should have killed them. The issue is that if this goes unrecognised, the susceptible worms are killed by the wormer but the resistant worms survive and become more concentrated within the worm population. As these worms are harder to kill, this means that the problem can become a lot worse in a short space of time if no other measures are taken to slow down the development of anthelmintic resistance.By the time anthelmintic resistance is suspected, usually when a wormer is given and little or no improvement is seen in the condition of the lambs, it is likely too late for that class of wormer to be useful in your flock in the future as the resistance will be too widespread. That’s said it is important to rule out other causes of treatment failure before assuming resistance is present. Sheep must be not be underdosed as this will also allow worms to survive and continue to cause damage. Common causes of underdosing include underestimation of weight and not calibrating equipment, both of which are easily sorted. Also ensuring that the correct product, which is in date and has been stored correctly is essential.To confirm if anthelmintic resistance is present, a faecal egg count reduction test is carried out which compares faecal egg counts pre and post treatment to confirm if the wormer has been at least 95 percent effective or not. Summer is the ideal time to carry these out as the first egg count needs to be at least 300 epg for the results to be reliable and it is likely that untreated growing lambs would have counts of at least this amount. Ten sheep are needed for each wormer to be tested, and an addition 10 for a control group which will be sampled but left untreated. Sheep should be marked so that they can be identified for the second sample as it must be the same sheep which are sampled both pre and post treatment for the comparison to be reliable. Individual dung samples should be collected and submitted for faecal egg counts. On the same day as collection, each group should be treated with the appropriate wormer ensuring the correct dose is given. Then second samples should be taken a specific number of days after treatment, how many depends on which wormer has been used but the range is between 7-14 days.Whether anthelmintic resistance is confirmed, or your status is unknown, measures should be put in place to slow its development as unless you are carrying out FECRTs regularly, it could be present without your knowledge. One of the main areas to focus on is not bringing resistant worms onto your farm, so quarantine treatments are essential for all bought in sheep.