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The growing problem of anthelmintic (wormer) resistance on sheep farms was made apparent last week when the UK’s first case of resistance to the group four anthelmintic Zolvix was reported.
Resistance issues with group one (white), two (yellow) and three (clear) wormers are relatively common. Advice published by Sustainable Control of Parasites in Sheep (SCOPS) last week urged sheep farmers not to use these products exclusively until resistance has built up on farms and the products are ineffective.
Worm populations on sheep farms are unavoidable so the aim with wormer programmes is not to remove all worms from sheep and pasture. Instead, wormers should be strategically used to maintain populations of worms that are not resistant to as many anthelmintic products as possible.
This is where group four (orange) and five (purple) wormers come in. Zolvix is the only available group four product and it is used for the control of roundworms. The only group five product available is Startect Dual Active which controls both roundworms and lungworms and has no known resistance issues.
The SCOPS advice is to use Zolvix and Startect in wormer programmes as a quarantine treatment for newly purchased sheep and as a mid- to late-season treatment for lambs. Group four and five products have a key role in removing worms on farms that are resistant to anthelmintics in group one, two and three.
Quarantine
A quarantine procedure for bought-in sheep is required to ensure that resistant worms are not brought on to the farm. It is recommended that newly purchased stock are kept separate from the rest of the flock for at least three weeks and treated with a group four or five product to remove populations of resistant worms.
SCOPS suggests that farmers should alternate between Zolvix and Startect when using them for their purpose as a quarantine treatment and an annual treatment for lambs.
Group four and five wormers were previously only available under veterinary prescription in the UK. However, last year both products were approved for sale in agricultural merchants that have a member of staff trained as a suitably qualified person (SQP).
This was to encourage an increased use of Zolvix and Startect on UK sheep farms for quarantine and mid-/late-season treatment in lambs. However, there is a risk that overuse of these products could lead to further resistance issues.
For example, on the sheep farm where group four resistance was reported last week, the issue stemmed from worms on the farm being resistant to anthelmintic products in group one, two and three and the farm was using Zolvix almost exclusively to control worms in all sheep.
Eventually worms on the farm mutated and became resistant to Zolvix. Sheep were also understood to be put on to clean pasture with a low worm burden, so the only worms passed out on to the pasture were Zolvix resistant.
Uptake
To encourage worm populations that are not resistant to anthelmintics, sheep should be put back on to dirty pasture with a high worm burden for four or five days after treatment, or else around 10% of sheep should not be treated to allow them to infect pasture with worms that are not resistant.
Taking a fresh sample of faeces from a representative group of sheep to the vet for a faecal egg count will help avoid overuse of anthelmintic products. This allows the parasites present in sheep to be identified and the most effective wormer product to be used.
Narrow spectrum products should be used if possible as resistance can build up if broad-spectrum products are used unnecessarily. Farmers are advised to alternate between wormers to avoid a build-up of resistance. However, it is important that products have different active ingredients and not just different product names.
The simplest and cheapest way of testing the effectiveness of an anthelmintic product used on a sheep farm is to conduct a faecal egg count after the treatment.
SCOPS recommends that dung samples are taken seven days after treatment with group two wormers and 14 days after products in group one or three have been used. The accuracy of the test is improved if samples are also taken on the day of treatment to give an estimate of the reduction in faecal egg count.
Under-dosing sheep is a key factor which can lead to anthelmintic resistance developing on farms and the general rule is to dose for the heaviest sheep in the group.
However, liveweights can be easily underestimated so a few of the largest sheep should be weighed to get an accurate understanding of the heaviest weights. If there is a significant range in liveweights, then the group should be subdivided and the heaviest sheep in each sub-group should be weighed to set dose rates.
Dosing equipment should be calibrated by discharging a single dose into a measuring cylinder or 10ml syringe with the plunger removed. There should be no air bubbles and the gun should be adjusted until the actual volume discharged is the correct dose rate.
Equipment should be cleaned with warm soapy water after use and checked for kinks or worn seals. If the product is administered by injection, only clean sterile needles should be used.
Restrained
Sheep need to be properly restrained when drenching to ensure they swallow the proper amount of wormer. SCOPS recommends placing a hand under the sheep’s head and tilting it slightly to the side. The gun should be inserted into the side of the mouth and placed over the tongue.
Wormers should be stored in a secure place, away from direct sunlight between 4°C and 25°C.
Use-by dates on packaging should be checked before use and, once a wormer is opened, it should be used within the time frame shown on the packaging.
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