4. Amino-acetonitrile derivative -- Monepantel (trade name Zolvix).
Teagasc studies show there is widespread stomach worm resistance to the first group (Benzimidazole). Therefore, where these products are used after the middle of May, one would want to be sure that they are still working on the farm in question. A simple faecal egg count reduction test will demonstrate this -- contact your vet or agricultural advisor for more details.
It is important to note, however, that this product is still very effective against Nematodirus -- ie the worm that causes infection in late spring/early summer.
The second family (Levamisole) has, in the same Teagasc studies, been shown to be effective on about half of the sheep farms. That means using this product after the middle of May is again risky if you are unsure as to the efficacy on your farm. Do the faecal egg count reduction test.
It is important to point out that the resistance issues with this wormer do not apply to Nematodirus. Therefore, both this and the previous wormer group can be safely used to control Nematodirus, even where it has been established that there is a resistant stomach worm population on the same farm.
There does not appear at this stage to be any resistance issues to any of the other two family groups, so these should be used sparingly and only when necessary so as to try and slow down the speed at which a worm populations develop resistance to these.
Over the years, there has been a lot of debate about the merits of treating ewes for stomach worms. When ewes are treated for stomach worms, the main benefit has been a reduction in eggs that they pass on to the pasture.
Given the fact that certain sheep farms already have resistance to two or possibly three of the anthelmintic families, the benefit of treating ewes with anthelmintics must be weighed up against the risk of speeding up the rate at which resistance to the remaining effective anthelmintics will happen.
Therefore, because most sheep farmers will treat their lambs at set times regardless of the pasture burden (ie 6 weeks, 10 weeks and weaning) and because it is impossible tell with any degree of certainty when the post lambing egg rise will happen in the ewe flock, I would not recommend routine worm treatments for the entire adult ewe flock.
However, treating individual thin ewes, hogget ewes rearing lambs or ewes that are under pressure rearing three lambs does, of course, make sense.