The potential severity of anthelmintic resistance is often overlooked in presentations which highlight facts and figures on the issue. This is possibly due to farmers thinking it is an issue that will not affect their system. Sometimes we need to hear from farmers who have experienced a problem before we sit up and take notice.

At last week’s Northern Ireland Sheep Programme webinar, Trevor Nixon left viewers in no doubt as to the potential impact that anthelmintic resistance can have on a flock.

Trevor and Ruth Nixon and their daughter Amy. \ Houston Green

The problem escalated in 2020 when lambs were dosed at the start of July and, despite being prioritised in grazing top-quality grass, they failed to thrive during the month, with some lambs actually losing weight.

Problems intensify

Trevor said: “I wasn’t happy in the few years before [2020] with lamb condition during the summer months but I passed it off that maybe my grass management wasn’t up to scratch or that weather was wet and the feeding value was just not in the grass.

I thought lambs were maybe not just as good as I had hoped

“Lambs would get to 38kg to 40kg but it would be a frame rather than flesh cover driving this and I found it hard to get the final finish on them. I generally overcame it by introducing meal feeding and would eventually get lambs drafted a long time later than I would have hoped.

“I started doing more with EID tagging and performance recording in 2020 and weighed lambs when dosing at the start of July. I thought lambs were maybe not just as good as I had hoped, with some a little shy on flesh, but again I passed it off as there were also some niggling issues with lameness.

Trevor and Ruth Nixon and their daughter Amy. \ Houston Green

“During July lambs looked as if they weren’t thriving well. Some were going dry in the wool and they just weren’t right. It was only when I put lambs in to weigh at the end of July that I saw the extent of the problem.

“A minority of lambs had recorded a low level of liveweight gain but these were very low on flesh cover, while others had even lost weight.”

Veterinary assistance

Trevor immediately contacted his vet, who raised anthelmintic resistance as the potential source of the problem. Lambs had been treated with an avermectin-based product up to that and were subsequently treated with a wormer from a different anthelmintic class. Tests were carried out to see if resistance was the underlying issue.

Trevor said he could see an immediate response, with lambs turning a corner in a week to 10 days. The recovery was slow due to a month’s lost performance.

Looking back, I should have acted quicker on my suspicions

It resulted in fewer lambs being drafted for slaughter, increased meal feeding and a higher percentage of lambs sold as stores.

“Looking back, I should have acted quicker on my suspicions. It cost me money and affected my business as replacement ewe lambs were not as strong as they should have been and this contributed to a decision to run them over dry. Having seen how lambs have performed this year I can safely say that problems were building for a few years.”

Further diagnosis

Trevor was keen not to see a repeat of the issues which blighted performance in 2020 and opted to carry out a faecal egg reduction test this summer to cover white (benzimidazoles) and yellow (levamisole) drenches and also further investigate the resistance issue to clear wormers.

The heading macrocyclic lactones covers a number of derivatives, with two different groups – avermectins, which includes ivermectin, and abamectin, which includes moxidectin-based products. The latter is the third product which was investigated.

A plan for testing was put in place along with Trevor’s CAFRE adviser Ruth Moore and programme adviser Senan White. This unearthed some useful learnings for the farm.

A worming programme has now been put in place that will see products alternated and worming only taking place after an identified need. \ Houston Green

The plan was to carry out the reduction test in May but faecal egg testing was carried out by Ruth via one of a number of mobile FECPAK units purchased by CAFRE to demonstrate the benefits of faecal egg testing to Business Development Group farmers.

Useful learnings

The first FEC counts were low, ranging from fewer than 100 eggs per gram (EPG) to 200 EPG. A number of tests were carried out in the following weeks but higher than normal temperatures and dry conditions meant there was no increase in the worm count. Trevor said in the past he would likely have routinely dosed lambs within this window from May through to June, with this dose essentially an investment that would deliver no return while also contributing to the rate at which resistance develops.

The faecal egg count increased in line with rainfall at the end and a further round of faecal egg sampling was carried out in early July. Three batches of lambs numbering 10 in each group were selected at random and faecal samples were collected from these. The faecal egg count for the three batches is detailed in Table 1.

A reading of 500 epg would generally trigger a recommendation to treat lambs. Senan pointed out on the night that one group of lambs was below this level and under normal circumstances would not require treatment.

However, to fit in with the initiative for the webinar a decision was taken to treat this batch. The three batches of sheep were treated with a levamisole-based, a benzimidazole-based and a moxidectin-based product.

Test results

Senan said resistance is deemed to be an issue where treatment does not kill at least 95% of worms present. He said the protocol for yellow wormers is to carry out a repeat faecal egg count after seven days following treatment, between 10 and 14 days later for the white drench and between 17 and 21 days later for the moxidectin-based wormer.

As detailed in Table 1, Ruth carried out repeat tests across all groups at seven and 14 days later when visiting the farm. The yellow wormer had a worm count of 51 epg, which equates to a treatment rate in excess of 95%.

The white wormer had a zero worm count while the clear wormer also recorded a treatment rate in excess of 95% at 14 days, with Senan predicting this would likely be lower at the recommended time frame of 17 to 21 days.

Trevor said the batch of lambs with the highest worm count were the cleanest lambs with the least sign of scouring which backs up the research that lamb cleanliness and scouring are not reliable tools in determining a worm burden.

Greater care needed with anthelmintic selection

Professor Bob Hanna from the Agri-Food and Biosciences Institute (AFBI) warned that the sheep sector will have to change its current practices in terms of use of anthelmintics if we are to have any hope of safeguarding the efficacy of the various anthelmintic classes.

Prof Hanna presented stats which showed that in a trial, resistance was identified on 81% of the farms to white wormers, 50% for ivermectins, 62% for moxidectin and 14% for levamisole.

Clear wormers are also used more due to their use for treatment of external parasites

With regards to levamisole, he said only a small number of farmers in the group were using this product. Clear wormers are also used more due to their use for treatment of external parasites.

He said the issue could be even worse on farms now and said farmers must take more heed of using the most appropriate product for the time of year.

This, he said, is currently not being carried out, with other survey findings showing that farmers were using clear wormers for nematodirus when many do not have a claim to treat the condition.

Farmers were advised to alternate between anthelmintic classes and to make far greater use of faecal egg counts

He also stressed the importance of farms that have identified triple resistance to the common anthelmintic classes using an orange wormer as a last resort in routine treatment of worms. He said the orange and purple wormer (where available) should be used as a quarantine treatment and potentially strategically at a time of year when it is seen to deliver the greatest benefit.

Farmers were advised to alternate between anthelmintic classes and to make far greater use of faecal egg counts, both in identifying the need to treat animals and also in identifying if resistance is an issue.

Finally, Senan White detailed that inappropriate selection and overuse of the same product is only one part of the problem, with inadequate volumes being administered, incorrect dosing techniques and treatment of animals which do not require treatment such as healthy ewes also contributing to the rate of resistance developing.


There was good interaction on the night and a panel including Siobhan Corry, AFBI, Ruth Moore, CAFRE, and Senan White, CAFRE programme adviser, received a high number of questions. The following is a summary of the main questions.

What work has been undertaken to establish finishing systems for lambs on farms with no remaining effective anthelmintic doses?

This is an area where there has not been much research carried out yet in the UK or Ireland. There is a greater focus on it in countries such as Australia and New Zealand where there is resistance to all five anthelmintic classes. Thankfully, this does not appear to be a major issue on farms here but it could well come if farmers do not protect the new anthelmintic classes (amino-acetonitrile derivative (orange wormer) and spiroindole (purple wormer)).

These wormers should be protected while a programme can be developed to use the other anthelmintic classes in combination to try to achieve greater control. This will be influenced by the level of resistance identified to any one anthelmintic class and should be developed in conjunction with your vet. – Siobhan Corry and Darren Carty

How often should a faecal egg count be carried out?

The period between faecal egg counts will be influenced by a number of variables including the time of year, weather, predicted risk etc.

During the main worm season it will typically suffice to carry out a faecal egg count every two to three weeks.

The time frame may be extended during prolonged dry spells or drought conditions which are becoming more typical while alternatively you may want to carry out one quicker where there was an medium reading in the previous test and you want to be cautious. – Ruth Moore

Is barber’s pole worm a problem in Northern Ireland?

I haven’t seen it yet personally as I work in the Omagh region and it is generally a wetter area. There have, however, been a couple of cases of Barber’s pole worm, or Haemonchus contortus, recorded in the Stormont Veterinary Lab (AFBI) and a number of cases in the south in the east of the country.

It likes dry and warm conditions and is a nasty worm that can hit hard very quickly and cause massive damage.

The parasite can produce a lot of eggs quickly and as a sheep won’t typically get diarrhoea it is harder to identify issues quickly. It is not a worm you want on your farm and it is important you are carrying out a good quarantine protocol to ensure you do not bring it in with purchased sheep. – Siobhan Corry

Professor Bob Hanna mentioned in his presentation that nematodirus has now been identified in autumn and winter. Is it becoming a bigger issue at this time of the year and why?

We have seen peaks of nematodirus in autumn and even winter that are lower than the spring peaks but still significant.

It is likely related to a change in climatic conditions providing a suitable environment while there are usually also farm-specific factors such as lambs possibly being repeatedly dosed and not getting a chance to acquire natural resistance or lambs being born after the main risk period for nematodirus or not exposed to the parasite for some reason in spring.

It is something we are monitoring closely and that farmers may need to be more wary of in the future – Siobhan Corry