The different uses of anthelmintics has possibly led to many sheep parasites emerging resistance to control treatments
Rob Smith, B.Vet.Med. MRCVS. director of Farm First Vets and member of COWS (Control of Worms Sustainably) stakeholder group, a UK-based initiative to promote sustainable control of cattle parasites.
Overuse, inappropriate use, and sometimes incorrect use, of anthelmintics has led to many sheep parasites developing resistance to parasite control treatments, making it hard for some livestock farmers to control parasitic disease and reduce production losses. The situation in beef cattle is currently less critical but the cattle sector should take this opportunity to slow the build up of resistance to wormers by learning from the mistakes of the sheep industry and following best practice.
Though cattle and sheep have few economically important parasites in common, the same fundamentals of parasite control apply. Overuse of any treatment will eventually result in resistance and the treatment will become less and less effective. The goal is to treat and prevent clinical disease, reduce production losses and preserve active ingredients, while at the same time minimising selection for resistance.
Liver fluke is one example where the impact of sheep parasite control can impact cattle. Liver fluke affects both species, but while cattle are less likely to show clinical signs of infection, at certain times of the year liver fluke infection can be fatal for sheep. Acute fluke disease in sheep is caused by juvenile fluke and is often responsible for considerable losses. Triclabendazole is one of the best treatments against acute fluke as it even kills the very early immature stages of fluke in the liver, but there are reports of resistance developing.
There are other flukicides available and these should be used at times of the year when only older juvenile fluke and adult fluke are present in cattle, for example, later in the winter and early spring. In cattle this could include closantel, clorsulon (included in IVOMEC® Super), nitroxynil (TRODAX®), and others, avoiding the use of triclabendazole. However, care should be taken, as closantel and nitroxynil can also be used in sheep, so the same potential for resistance could occur if use is not managed correctly.
Mixed farms are more likely to experience difficulties with resistant fluke, especially where triclabendazole has been extensively used in the sheep flock, and where cattle graze the same pasture.
Pasture management is an effective method of parasite control. In sheep, the provision of clean or low-risk pasture was overlooked while wormers were readily used and effective. This reliance on treatments over other control methods has contributed to the build up of resistance. By identifying and using low risk pastures, cattle farmers can reduce the amount of wormer used in their stock, thus reducing selection for resistance while still managing parasite burdens in the herd.
Young, grazing cattle are the most vulnerable to parasitic disease and most likely to experience production loss. So, any clean grazing, such as silage aftermath, should be prioritised for these classes of cattle, reducing the selection pressure. Older classes of cattle can follow the youngstock group on to this pasture.
The age of an animal is an important deciding factor in whether to treat or not. Adult beef cattle rarely suffer from clinical signs of worms and, though they may shed eggs onto the pasture, the benefits in not treating are usually greater than the potential reduction in egg output. Leaving healthy adult cattle un-wormed ensures a sufficient number of parasite eggs, which have not been exposed to selection pressure (and are known as ‘refugia’) reach the pasture, which helps to slow the development of resistance.
Weaned young cattle at grass will benefit the most from targeted treatments and the provision of clean or low-risk pastures.
Deciding whether to treat youngstock should ideally be informed through regular growth measurements and the results of diagnostic tests.
Where cattle are not making their expected 0.7-0.8kg of daily weight gain, and there is sufficient forage available, parasites may be cause for concern. The presence of worms or fluke can be determined using a variety of diagnostic tests, the most simple of these being the faecal egg count (FEC) test, usually performed on dung collected from a group of animals (minimum 10). Other diagnostic options include blood tests. The choice of diagnostic method will depend on the herd and farm situation.
If a parasite burden is identified, the need to treat, and any product recommendations, should take into account the history of the farm, including: recent problems, the grazing and worming history of the animals, their age, and local weather conditions.
COWS guidelines (1) suggest five basic R’s as principles for sustainable parasite control. If followed, these principles will help cattle farmers reduce selection for resistance and increase the time that anthelmintic treatments remain effective. These include: selecting the right product for the type of worm to be treated, using treatments on the right animals at the right time, using the right dose rate, and administering the product in the right way.
Ivomec Super Injection for cattle contains ivermectin and clorsulon. Trodax 34% w/v solution for injection contains nitroxynil.UK: POM-VPS. Further information available in the SPC or from Boehringer Ingelheim Animal Health UK Ltd, RG12 8YS, UK. UK Tel: 01344 746960 (sales) or 01344 746957 (technical), IE Tel: 01 291 3985 (all queries). Ivomec and Trodax are registered trademarks of the Boehringer Ingelheim Group. ©2019 Boehringer Ingelheim Animal Health UK Ltd. All rights reserved. Date of preparation: Apr 2019. AHD12101. Use Medicines Responsibly.
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