Avermectins, also known as the Group 3 Clear wormers or ML’s have become a cornerstone of parasite treatment over the last 40 years. Discovered in the 1970’s and formulated for veterinary use in the early 1980’s they became the biggest selling global veterinary products by the late 1980’s.

They revolutionised the management and treatment of many parasites. Highly effective at killing gut-worms and lungworm, species of mites including mange and sheep scab, potent insecticides they are also used to control lice, bots and warbles.

Different routes of administration including oral, injectable and pour-on made them very easy to use. Ivermectin was the original avermectins to come to the market but with the development of longer acting formulations and marketing of much cheaper generic products has made them even more convenient and affordable to use. This has resulted in avermectins being the most popular parasite control for many, from worm control to sheep scab treatment.

Combination products containing avermectins and flukicides, marketed for convenience, are not always the most responsible or effective use of these drugs. Nor the cheapest option.

It is concerning how we have now come to rely on this class of parasite treatment and often forgotten about alternative measures for parasite control including biosecurity, grazing management, vaccination against cattle lungworm and sheep dip.

Overuse of these drugs has already resulted worryingly in resistance. Common reports of sheep gut worm resistance together with ineffective sheep scab treatments and cattle lungworm resistance has set alarm bells ringing.

Equally worrying is the effect avermectins have on the health of our soils, pastures and environment. Ivermectin is excreted in the animal gut, is present in their dung and can persist on the pasture for up to six months. Highly toxic to aquatic animals, Ivermectin binds tightly to the organic material in the soil and stays in the soil killing the beneficial nematodes and insects, including dung beetles on the pasture. Dung beetles play a vital role in the pasture health. Unhealthy pastures have much less biodiversity and insect life, having knock effects on bird and wildlife populations.

In cattle, avermectins are now the most common worming products use for youngstock grazing parasite control. Dosing every five weeks with Ivermectin has been superseded with longer-acting pour-on’s lasting up to eight weeks or even longer-acting high dose injectables lasting all grazing season.

Eprinomectin with a zero-milk withdrawal has been heavily marketed to be used in dairy cattle at grazing based on bulk milk tank antibody testing. But please take advice from your local vet before using these products.

Parasite control in grazing cattle is very important. Arguably, cattle lungworm is the most important, as clinical outbreaks can result in severe disease and mortality. Low levels of lungworm exposure results in strong age-related immunity, over worming in youngstock and short grazing seasons in dairy cattle can result in groups of cattle with little or no immunity making outbreaks potentially more severe.

Effective lungworm vaccination was developed more than 50 years ago, given pre-turnout it allows strong immunity to develop against the disease without the need to use any wormers. Its use has become less popular due to the reliance on avermectins but it should be seriously considered as part of sustainable parasite control on the modern grazing farm.

Pulse release oral boluses, delivering doses of short acting benzimidazole throughout the grazing season should also be considered, controlling the risk of clinical disease and still allowing immunity to develop with much less environmental impact.

It is therefore possible to manage grazing cattle, without the use of avermectins at pasture. A housing dose is commonly practised preventing Ostertagia Type II infections and to kill any residual lungworm. Ivermectin use at housing is a cheap effective method and will have less impact on pasture health, but other alternatives are available.

The use of avermectins in sheep has also become routine, due to their efficacy and ease of use. Not only for their worming action but also the mainstay of sheep scab treatment and biosecurity. This results in their use throughout the year, often at a whole flock level.

For sustainable parasite control, we must start and use these products more responsibly. Resistance has already been widely reported, entirely down to their misuse.

Exposing whole parasite populations to Avermectins is the driver for resistance. The SCOPS website is an excellent resource, giving advice on the responsible use of anthelmintics. www.scops.org.uk.

Only use anthelmintics when necessary, the treatment of ewes at lambing time and pre-tupping has become the norm. We must never use avermectins more than once in a season and never treat the whole flock.

It is very important to leave some animals untreated. Lean ewes with triplets and twins are likely to have the largest spring-rise. Therefore, there’s a strong case for leaving fit ewes and singles untreated. At tupping, fit adult ewes will have excellent immunity to any worm burden. It is hard to justify worming at this time of year.

Fluke dosing in the autumn may be essential, depending on weather and pasture conditions. But targeted treatments designed to treat immature fluke are important. Combination fluke and worm products are rarely the best choice. Albendazole at the fluke dose, is only effective against adult liver fluke and not recommended in the autumn. Also, combination moxidectin/triclabendazole products used in the autumn is an irresponsible use of moxidectin.

Routine dosing of whole flocks throughout the summer is also hard to justify. Dosing for nematodirus in the spring with a white wormer based on the forecast is the most responsible use. Further dosing throughout the summer should be targeted to just the lambs that need it and use routine worm egg counts and live weight gains will identify groups that require treatment. Faecal worm egg reductions tests should also be used after dosing to ensure any treatment has been effective. Also using a Group 4 or 5 wormer, mid/late summer should also be considered to slow down resistance.

Pasture management should be part of sustainable parasite control, lowering worm burdens for the most susceptible youngstock. Re-seeds, aftermath and pasture that had grazed a different species the previous year will have the lowest burden. Pastures that have grazed youngstock that season will have the highest burden.

We cannot continue with our current culture of avermectins reliance, think twice before reaching for the Clear option. Nor are combination products the responsible or cheap alternative. Some of the old ways are still the best, vaccination for lungworm in cattle and dipping sheep might be a hassle but is much more sustainable.