Anthelmintic Resistance in Roundworms and Improved Drenching Techniques
Summary written by Bob Coop BSc PhD and Frank Jackson BSc PhD
Taken from Moredun Foundation Newsheet Volume 3 No 2

Gastrointestinal roundworms still are a major contributor to reduce efficacy of production in many intensive sheep rearing enterprises, particularly through sub-clinical infections. Control is heavily dependent on the regular use of anthelmintics (wormers/drenches), combined where practicable with pasture management. The long-term sustainability of frequent drenching with anthelmintics has been questioned in view of the increasing prevalence of strains of roundworms which have developed resistance to one or more classes of drugs. Anthelmintic resistance has become a major problem over the last decade in many sheep rearing areas of the Southern Hemisphere. For example in parts of South Africa, South America and Australia there are farms with roundworms which are resistant to all three classes of broadspectrum anthelmintics (benzimidazoles, levamisole, avermectins) and also some narrow spectrum wormers such as the salicylanilides (closantel). Fortunately, the situation in the UK is less serious, the development of anthelmintic resistance has been slower.

This may in part be due to less frequent treatment and the species of worms but also to the fact that only a very small proportion of the overall population of worms are within the animal at any point in time, the majority of the population being present on the pasture as free-living infective larvae. As a consequence only a small fraction of the overall population will be screened with the drug ensuring that the selection pressure from a single treatment remains low.

It is important that farmers adopt strategic worming strategies and techniques to delay the development and transmission of resistant roundworms and this Newsheet highlights practical advice on use of wormers. An appendix of selected wormers, categorised under the 3 classes of anthelmintic, is provided for guidance.

• The incidence of anthelmintic resistance is increasing in the UK. Once the parasite population on a farm becomes resistant then for all practical purposes this resistance is permanent.
• Careful management of parasite control on the farm may slow down the rate of development of resistance and help to conserve drench efficacy. This is important simply because parasites have the ability to develop resistance faster than new products can be developed.
• The following strategies may be of use in helping to tackle the problem.
1. Adopt an annual (slow) rotation of anthelmintics between each of the following drug families.

Benzimidazoles
Levamisole
Ivermectin

2. Ensure that you use the correct dose rates: Always follow the manufacturers instructions and make sure that your drenching equipment is accurate.
3. Check stock bodyweights and dose for the largest animal within the group being treated. Follow the manufacturers instructions concerning storage and handling of anthelmintic
4. Where possible use management practices such as cropping, using adult non susceptible stock to reduce the parasite population on pasture and hence the requirement for frequent treatment.
5. Treat all purchased stock on arrival on your farm with anthelmintics from two different families to avoid importing resistance.
6. Do not drench unnecessarily. Consult your practitioner for advice to ensure that you use the optimum number of drenches per season.
7. Do not alternate drench families within a season
8. Do not use one drench family continuously for several seasons.
9. Do not mix drenches.

And when drenching

1. Treatment frequency and underdosing are factors that most influence the rate of selection of resistance.
2. In order to minimise the rate of selection it is important to make each treatment as effective as possible.
3. The effectiveness of any treatment depends not only upon the level of exposure that the parasite receives but also on the duration of exposure.
4. The rumen normally acts as a depot for many of the drugs, which become mixed with particulate contents and pass down the gut allowing the drug to be absorbed.
5. Poor administration of oral drenches (into the mouth rather than over the back of the tongue) may stimulate oesophageal groove closure which can lead to the drug bypassing the rumen and thus reduce drug uptake and efficacy.
6. The rate of movement of digesta and drug through the gastrointestinal tract can also influence drug uptake and efficacy. If possible withhold animals from feedstuffs for 24 hours prior to treatment and for 3 hours post treatment, particularly when using drenches in the white drench (benzimadazole) family
7. Studies by scientists in Australia and at Moredun have shown that simply reducing rumen fill prior to treatment with a white drench can extend bioavailability and increase efficacy against resistant parasites by as much as 40%.