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Anthelmintic Resistance – Have you been worming your sheep unnecessarily?

Sylvaine Lacrosse BVetMed MRCVS, Molecare Veterinary Services

Anthelmintic resistance (AR) is a very topical subject in sheep farming, and for those unsure of the definition, an anthelmintic is a wormer and AR is just the technical way of saying ‘worms are becoming resistant to wormers’.

The development of resistance in any organism, whether it be bacteria, fungi or parasites, is a normal process of evolution, which happens over time, with organisms evolving more suitable traits to survive. However, as an industry, wormers have been widely available for years and their overuse and misuse has sped up the development of AR through the artificial selection of worms with ‘survivable’ AR genes. If we allow AR to get out of control, soon enough we won’t have any functional wormers left, which would be disastrous to farming!

What wormers do we use?

There is an extensive choice of anthelmintic (wormers) available, and these all fall within five groups. The first three groups include the white, yellow and clear drenches, which have been overused and misused in the past, resulting in farms having a range of resistance to these anthelmintic groups.

 Groups 4 and 5 are separate as there has been no observed resistance to these wormers in the UK…yet! Hence, their use needs to be protected and can only be used under veterinary guidance, for instances where resistance to the other wormer groups has been observed or in the case of a well thought out quarantine strategy for bought-in sheep.

What is the situation on your farm?

There will be many farms in the fortunate position of having minimal or no AR. This does not mean that these farms should continue irresponsible worming practices as it is only a matter of time before AR becomes a big problem!

Whilst there are many principles of responsible use of anthelmintics, the focus of this article will be the use of faecal egg counts (FECs) in monitoring the AR situation on farms.

What is a faecal egg count?

As the name suggests, a FEC is a count of the number of worm eggs in faeces, indicating the burden of adult worms in the gut of the sheep. A FEC is done by mixing faeces with a salt solution to allow the eggs to float. The mixture is then prepared so we can see the eggs under the microscope, count them and report this as an ‘eggs per gram’ value (epg). Below, one of our vets is doing just that.

What is the benefit of FECs?

A FEC provides us with an epg value which will fall above or below a threshold that warrants worming. The threshold we use is 300 epg. If it is found that sheep’s faeces is below the threshold, there is no indication to worm them. This prevents unnecessary worming and is a step towards preventing AR. There is also a financial benefit because of the money saved not spent on wormer!

Faecal Egg Count Reduction Test

The most reliable and structured way to use FECs to test for AR is by doing a ‘Faecal Egg Count Reduction Test’ (FECRT). Resistance can be tested for in one of the anthelmintic groups or ideally in groups 1, 2 and 3 to get the best representation of resistance on farm.

A select number of sheep are randomly allocated to either a control or treatment group. Before any treatment, a FEC is taken from each group to be used as a baseline. The difference in FEC pre and post treatment, compared to the control group is what is used to determine AR. If resistance is observed, this is when you and your vet may discuss the responsible use of groups 4 and 5.

There are many more principles regarding the responsible use of anthelmintics, which are very well described on the SCOPS website, www.scops.org.uk, and well worth a read through. However, if there is one thing that can be taken away from this article, it is the importance of monitoring the level of worm burden on farm and whether or not you actually need to treat them. This in itself would be a big step forward in our fight against anthelmintic resistance.

For more information phone Molecare Vets on 01392 872934.

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