Gastrointestinal worms in sheep

There are more than 20 different species of nematode parasites in sheep, many of which have little or no impact on the health and welfare of the sheep. However, some do have a significant impact on health and productivity and a careful approach to worm control is needed.

: The four gastrointestinal worms of most concern are:
Nematodirus battus - this is usually the first of the gastrointestinal (GI) worms to affect lambs at the beginning of the grazing season.
Haemonchus contortus, Teladorsagia (Ostertagia) circumcincta and Trichostrongylus spp - these are the most pathogenic gastrointestinal worms that affect lambs and adult ewes or rams. Usually, this is a mixed infection that causes a drop in growth rates, scouring and weight loss as the worm burden increases.
: Adult female worms present in the sheep gut lay eggs that pass out of the sheep in the dung. While in the dung, each egg hatches into a first-stage larva (L1), before developing and moulting to second-stage larvae (L2). The L2 larvae then moult again into third-stage larvae (L3), known as the 'infective' larvae, which migrate onto grass, inside water droplets. Once ingested, the infective larvae travel into the wall of the stomach or intestines and develop into fourth-stage larvae (L4), and then about 14 days later into adult worms. The time between the L3 larvae being eaten by the sheep and the eggs being passed out in the dung is known as the prepatent period and is normally 16-21 days.
The lifecycle of sheep roundworms is summarised here:

These worms are likely to have some level of anthelmintic resistance to one or more of the three older wormer groups¹.
: The timing of these worms appearing on pasture, and the level of challenge they pose to stock, depends entirely on previous grazing management and weather conditions. These worms will usually complete several lifecycles during a grazing season, with pasture worm burdens increasing rapidly when lambs and ewes continually graze the same pastures.
: Anthelmintics (wormers) are vital to control worm infections and protect against productivity losses.
There are five groups of wormers available to sheep farmers in the UK:

Group 1 (BZ) wormers are white drenches containing albendazole, ricobendazole (eg Rycoben™), oxfendazole or fenbendazole. These are the treatment of choice for nematodirus.
Group 2 (LV) wormers are yellow drenches containing levamisole.
Group 3 (ML) wormers are clear drenches that contain macrocyclic lactones, such as doramectin that can be used to treat both worms and sheep scab, and is effective against benzimidazole-resistant worms.
Group 4 (AD) wormers are orange drenches containing monepantel. Zolvix™ is an orange drench that is a broad spectrum anthelmintic. It kills all economically significant gut worms, even worms resistant to 1-BZ, 2-LV and 3-ML wormers. Every flock, every year should use Zolvix™. Replacing one current worming dose with Zolvix™ will help preserve the efficacy of the older wormers (1-BZ, 2-LV, 3-ML).
Group 5 (SI) wormers are purple drenches containing derquantel and abamectin.
: Use Zolvix™ as a 'break dose' in lambs in the mid-late season as a break dose in lambs (any time from July onwards depending on worm burden; individual farm needs will vary). By the latter part of the grazing season, most lambs will have received wormer treatments involving one or more of the group 1, 2 or 3 products. Any worms resistant to these groups will have survived these treatments and, because they can live inside a sheep for months, they accumulate over the season. This population of worms in the lambs is therefore made up of a higher proportion of anthelmintic resistant worms than the population in refugia when the levels of resistance are low.

Treatment with an effective product, such as Zolvix, will remove these worms and stop them from reproducing and adding their genes to the overall population. To get the best result, Zolvix should be incorporated into a worm control programme when the level of resistant worms is low².
Gregor explains how incorporating Zolvix™ into his worm control strategy has managed worm burdens so effectively that his lambs achieve the target growth rates:
“Executing an effective worming programme is vital for us, as it allows us to protect investment already made in our flock, while protecting stock due to be sold the following year. To clear worm burdens around September time, we’ve opted to use an annual late season break dose using Zolvix as part of our worming protocol. It gives us the result that we need to get – which is getting stock thriving and growing again.”
: Use Zolvix™ as a farm protection quarantine dose for any stock moving on to the farm. All sheep coming on to the farm are a risk and should be yarded and treated on arrival. This includes both incoming stock and stock coming back on to the farm from a different holding.
There are five elements to quarantine treatments:
Yard or house animals on arrival to stop them dropping any worm eggs on pasture.
Choose at least one of the newest groups of wormer to treat - eg Zolvix™ (4-AD).
Weigh sheep, calibrate equipment and make sure sheep are drenched correctly. A scab treatment can also be used at this time.
Continue to hold off from pasture for 24-48 hours to allow the treatment to work.
Turn out on to dirty pasture – pasture that has carried sheep this season. In addition, a treatment for scab is also advised and SCOPS advises two wormers to be used. For instance if using Dectomax™ for scab control this would also act as a second wormer dose in conjunction with Zolvix™.
When a treatment is needed for fluke at the same time as worms, a combination product can be used such as
Combinex™ Sheep: containing levamisole (Group 2-LV, yellow drench) and triclabendazole, for the treatment of roundworm, lungworm and liver fluke, including fluke over 2 days of age.
Fasimec™ Duo: containing ivermectin (Group 3-ML, clear drench) and triclabendazole, for the treatment of roundworm, lungworm and liver fluke, including fluke down to less than 1 week.
Neil explains the benefits he's found by using Zolvix in his worming strategy for both a quarantine dose and a mid-late season break dose:
“Having an effective worming programme helps us ensure our lambs continue to grow and meet their target weights, as quickly and efficiently as possible. We’re doing all we can to slow down resistance to all wormer groups, by only drenching when required, breaking the resistance cycle by adding Zolvix into our strategy, and undertaking post-drench efficacy tests to monitor wormer performance. By using Zolvix, our lambs are hitting their target weights quicker and spending less time on-farm, due to them gaining weight more efficiently, making our production costs lower.”
: At present, resistant and multi-resistant worms are threatening the efficacy of existing anthelmintics. Worm control practices that were previously recommended have been shown to increase the rate at which anthelmintic resistance develops. As such, it is important we adopt a more sustainable approach to worm control. The principles apply to every farm, but how you apply these principles depends on your own farm situation. This will always include integrating the newer groups of anthelmintics (eg Zolvix™) into your worm control plans:
It is important to treat with the right product, for the right worms, at the right time. Further information on sustainable worm control, including which products to use, and when, can be found on the SCOPS website.