Balancing worm control and resistance in sheep flocks
As we turn the corner from听spring to听summer, worm burdens become an increasing concern for sheep farmers across the country.听Also, climate change is changing the epidemiology of worms meaning we听can鈥檛听rely on calendar-based treatments.听听
A sustainable parasite and worm control plan should be part of the farm health plan designed alongside the farm practice vet.听Internal parasites are a challenge to flock health,听welfare听and productivity.听While wormer products (drenches and injections) remain an essential tool for parasite control, many farmers are finding that these treatments are no longer as effective as they once were.听听
Why is this听happening? The answer听lies听in听Anthelmintic听(wormer products)听Resistance (AR). As resistance increases, managing parasite burdens becomes more difficult, making AR an increasingly听important issue听for sheep farmers.听听
What does resistance mean?听听
Worms that are not killed by the correct dose of wormer product.听听
Resistance has been further accelerated by factors such as over-reliance on a single wormer class, underdosing due to inaccurate weight estimates and routine blanket dosing.听AR听can be slow to develop and once听established听is听very hard听or impossible to reverse.听
Why worry about resistance?听
AR听is a major concern because it can reduce flock productivity and increase costs for farmers. When wormers stop working properly, sheep may continue to carry high worm burdens, which can lead to poor growth rates, weight loss, diarrhoea, lower body condition, and reduced overall performance.听听
Lambs may take longer to reach market weight, while breeding sheep may have poorer fertility and reduced milk production. In severe cases, high parasite burdens can cause illness or death.听
These productivity losses can have financial implications also,听as farmers may face lower profits due to slower growth and lighter animals being sold, alongside increased spending on听appropriate treatments.听If AR becomes widespread, fewer effective wormers听remain听available听which makes parasite control more difficult and expensive in the future. Therefore,听sustainable worm control plans are essential to protect听both animal health and business profitability.听
How to slow the development of resistance?听
A parasite control plan听should be听specific to your farm听鈥撎齪lease听speak to your vet.听
Correct use of products听听
It is important to use the right product at the right time at the听correct听dosage rate, making sure dosing equipment is checked/calibrated.听The focus should be on the correct听selection听of wormer.听Careful use and rotation of wormer classes can help slow the development of AR within a flock. Repeated use of the same drench group places selection pressure on the worm population, increasing the likelihood that resistant worms survive and reproduce.听
The three older broad-spectrum wormers 鈥� white,听yellow听and clear - have been used widely for years with more resistance developing to these classes. Newer classes are available (orange and purple) with significantly lower levels of resistance reported.听听
Currently there are no recorded resistance to purple (group 5) yet in UK.听听
Only treat when needed听-听Faecal Egg Counts (FEC)听
FEC testing is becoming more important in modern worm control management. FECs allow farmers and vets to assess the parasite burden on their sheep before deciding whether treatment is necessary 鈥� they also can help evaluate whether wormers are still working effectively on farm.听
FECs work by counting the number of worm eggs in faeces 鈥� the results are presented as eggs per gram听(epg) of the dung with the听epg听providing听an indication听of the number of adult worms in the gut of the sheep.听However,听there are some limitations to be mindful of including听different species听affecting interpretation of results and faecal consistency can make a difference.听
To undertake a FEC, most local vets offer testing, or you can use commercial testing kits 鈥� such as FECPAK - or undertake a DIY approach!听听
For advice on how to collect a good quality faecal sample, watch
Instead of relying on an assumption about worm burdens FEC testing can provide evidence-based information to aid worm control decision making.听
1. Dosing strategies 听
When controlling worms in sheep, it is important not to treat every animal in the flock. Treating all sheep with wormers too often can lead to worms becoming resistant to treatment, meaning wormers may stop working听overtime. Instead, farmers aim to maintain听refugia, which is the group of听worms that have not been exposed to wormers. Ideally, around 10% of animals are left untreated. These untreated worms听remain听sensitive to wormers and breed with any resistant worms that survive treatment, helping to reduce resistant genes in the worm population and slow down听AR听in the flock.听
One way to do this is听through听Targeted Selective Treatment (TST).听TST means only treating sheep that are struggling to cope with worms, while leaving healthier animals untreated. Sheep should not be chosen at random, as this can reduce the benefits of refugia.听听
Instead, farmers should use signs or markers to decide which animals need treatment. These include听visible听diarrhoea, faecal egg counts (FECs), body condition score, liveweight gain, and overall performance. TST can help reduce medicine costs and save labour by avoiding unnecessary treatments, but it does require farmers to be confident in checking sheep health and recognising which animals need help.听
2. Grazing strategies 听
Worm control isn鈥檛 solely about making sure you use the right drench; grazing management also plays a major role in reducing parasitic pressures. The risk levels for pastures with various grazing histories table on 听highlights the importance of focusing on pasture risk assessment (high/medium/low) depending on grazing history and stock class. 听
Table 1. Risk levels for pastures with various grazing histories
3. Quarantine protocol 听
Quarantine protocols play听an important role听in parasite control by reducing the risk of bringing resistance听worms into the flock.听New sheep or animals returning from grazing elsewhere may carry parasites that are resistant to wormers. If these sheep join the flock听immediately, resistant worms can spread and make parasite control more difficult. To prevent this, new animals should be kept separate from the flock and treated before mixing.听听
Further information on quarantine treatments and procedures can be found听on
How do you know if you have resistance?听
Wormer resistance may be present if sheep continue to show signs of worms after treatment or if wormers no longer seem to work effectively.听The best way to check for resistance is to听do a FEC reduction test.听This involves taking听a FEC听sample from animals before treatment and testing the exact same animals again after dosing to check whether worm egg numbers have reduced. If egg counts stay high, it may听indicate听resistance to the wormer.听
Protecting the听future:听
There is no single solution to听AR.听Sustainable听worm control requires a combined approach involving monitoring, targeted听treatment听and good grazing management. It is no longer as simple as killing all worms in the gut of the sheep,听it is about protecting animal performance whilst also ensuring treatments can remain effective and available for years to come.听
5听top听tips听
- Worm control is a vital part of flock health planning. Alongside your farm vet, ensure that a sustainable worm control plan is included as part of the flock鈥檚 annual health plan.听
- Ensure that any anthelmintic treatment given is fully effective. Calibrate equipment, ensure that medicine has been correctly stored and听is correctly听administered.听
- Aim to reduce reliance on anthelmintics through management practices and pasture risk management e.g. rotational grazing and grazing alongside cattle.听听
- 听A faecal egg count听can be used to help听determine听the need to treat, test the effectiveness of a听treatment听and provide information on pasture contamination.听听
- Ensure a robust quarantine protocol is in place to reduce the risk of bringing resistant worms and other parasites on to the farm.听听
Katrina Coutts, Consultant,听Katrina.coutts@sac.co.uk听
Posted by SAC Consulting on 16/06/2026
