THE FUTURE success of current worm control treatments depends on slowing the development of resistance to products as much as possible.
Following the Sustainable Control of Parasites in Sheep (SCOPS) recommendations, as devised by industry experts and endorsed by DEFRA, will be one way of reducing the spread of resistant parasites.
The basic SCOPS recommendations involve:
Decisions about judicious use of anthelmintics in worm control programmes are complex and will require ongoing consultation with advisers.
Quarantine should be applied to sheep purchased from other flocks and sheep which have spent time grazing on other farms or common land. Sheep should be kept separate and treated with two anthelmintics – a levamisole product and a macrolcyclic lactone product are recommended.
Sheep should be weighed before dosing, with all sheep in a group dosed to the weight of the heaviest sheep. Groups can also be split where there is wide variation in weights. Drench gun calibration is also important and poor gun calibration can lead to underdosing and increase the chance of resistance developing (see FW Livestock Jan 14).
When deciding when to use anthelmintics the SCOPS guidelines suggest avoiding dosing ewes at tupping, apart from dosing sheep that appear to require treatment due to low condition score. However, ewes should still be dosed post-lambing when they are most likely to excrete worm eggs.
Unnecessary exposure of worms to an anthelmintic can lead to increased selection pressure for anthelmintic resistance. Narrow spectrum anthelmintics should be used where possible, particularly where one worm species is the target.
This will help reduce the spread of resistance as the worm gene pool is maintained with both resistant and non-resistant worms, limiting the number of resistant worms on farm.
The objective of management practices is to minimise the reliance on and use of anthelmintics by avoiding exposure to parasite burdens that would lead to clinical disease. Breeding is also able to reduce reliance on anthelmintics as rams can be selected for worm resistance. This will be particularly useful in flocks breeding their own replacements.
While there is no strong evidence that rotating anthelmintic classes is an effective strategy to delay the appearance of resistance, it is possible that rotation could delay the appearance of macrocyclic lactone resistance on farms where the gene for resistance is either absent or present at low levels.