Stomach Worm Control Plan for Sheep Flock in 2024 and Slowing Down Development of Anthelmintic Resistance

By Glen Corbett, B&T Drystock Adviser, Teagasc Galway/Clare

With the lambing season 2024 under way with some and on the doorstep of others, it’s timely now to plan ahead in terms of a dosing plan on sheep farms in 2024. Grazing sheep are continually exposed to gastrointestinal nematodes (stomach worms). In the case of lambs, heavy infection can result in a depression in appetite and increased protein loss from the gut, which results in ill-thrift and in severe cases even death.

Two major types of stomach worms infect lambs, Nematodirus and Strongyles. Each worm type has its own particular life cycle and different worm types predominate depending on the time of year, geographic location and local weather conditions. Nematodirus is a lamb crop to lamb crop infection. This is due to the fact that eggs passed by lambs one year hatch the following spring and are available to infect the next year’s crop of lambs. Therefore, Nematodirus can be a major cause of parasitic gastroenteritis in young spring lambs.

The Department of Agriculture, Food and the Marine, in conjunction with Met Eireann predict when Nematodirus eggs will hatch and every year produce a forecast predicting the peak hatch and advise when farmers should treat to prevent disease due to this parasite. Assuming exposure, lambs develop immunity to Nematodirus relatively quickly, usually from three months of age. Watch out for this warning, it might come before the end of March in 2024. Any lambs on the ground that are over 2-3 weeks old will need a white dose at that stage. A dose to cover coccidiosis may or may not be needed next, watch out for the symptoms (blood scour) and get in quick if deemed necessary.

Later in the season, i.e. from June onwards, other Strongyle worms predominate. Immunity to these worms is slower to develop, although sheep generally have good immunity from one year of age. However, older sheep can be susceptible to these worms if immunocompromised or under stress.

Control and Treatment:

Good stomach worm control is highly dependent on the availability of effective wormers. Despite the large number of products on the market, there are currently only five classes of wormer licenced in Ireland for the control of stomach worms in sheep and all products fall into one of these classes. Of the 5 classes there are 3 main/traditional ones, these classes are 1) Benzimidazole (white wormer - 1-BZ), 2) levamisole (yellow wormer - 2-LV) 3) macrocyclic lactones (clear wormer - 3-ML).

Anthelmintic resistance refers to the ability of worms to survive a dose that should kill them. Wormers from different classes have different modes of action. However, within the same class all products share the same mode of action and therefore when resistance develops to one product within a class generally other products in the same class are also ineffective.

Anthelmintic Resistance in Ireland:

Anthelmintic resistant worms were first identified in Ireland in the 1990s. Since then, the prevalence of anthelmintic resistance has increased and this now represents a major threat to the sustainability of our sheep production system. A recent study tested 18 farms in Ireland for resistance to the three commonly used wormer classes, white, yellow and clear. Of concern was the high percentage of farms with resistance to the macrocyclic lactones, a class that contains Ivermectin. The prevalence of resistance to this class in particular has increased substantially in the last 10 years.

Four Key Actions to Slow the Development of Anthelmintic Resistance:

Given the evidence for widespread anthelmintic resistance and the urgent need to implement strategies to slow the further development of resistance, four key actions have been identified that can be implemented on the majority of sheep farms in Ireland. These four actions are outlined below:

1 Dosing Ewes:

Do not dose mature ewes for stomach worms unless there is a demonstrated need. Mature ewes should have good immunity to stomach worms and should not need to be treated. Refraining from treating ewes will reduce unnecessary dosing. However, it is important to bear in mind that there are some exceptions in which ewes may need to be treated for stomach worms. For example, thin, immunocompromised or otherwise sick ewes may benefit from treatment. In this case, the treatment can be targeted only to those ewes that need it.

2 Use only white wormers to control Nematodirus:

As outlined and mentioned already above, Nematodirus can be a problem in young lambs. Nematodirus eggs hatch en masse in spring, and if this coincides with when lambs start eating significant quantities of grass it can lead to severe disease. Because of the life cycle of Nematodirus, where it generally hatches once per year in spring, anthelmintic resistance is much slower to develop in this worm. To-date anthelmintic resistance has not been recorded in Nematodirus in the Republic of Ireland. Therefore, white wormers can be used to control this worm. Resistance to white wormers is common in the Strongyle worms that predominate later in the season; therefore, this wormer will not be effective on many farms later in the season. Using white wormers to control Nematodirus will reduce use of the other wormers and represents appropriate use of an anthelmintic to control infection.

3 Implement a good Biosecurity Protocol for bought in Sheep:

Animal movement is a major way in which anthelmintic resistance can spread. In order to prevent bringing resistant worms into the farm incoming sheep should be quarantined treated with either (i) an orange wormer plus a yellow or clear wormer or (ii) a purple wormer plus a yellow wormer. Sheep should then be housed for 48 hours to allow any eggs that might already be in the gastrointestinal tract to pass out. They should then be turned out to pasture recently grazed by sheep. This pasture will contain the worm population found on that farm, which will act as a source of refugia and dilute any resistant worms surviving in the treated sheep.

4 Use Faecal Egg Counts:

Faecal egg counts determine the number of worm eggs in a dung sample and provide a useful indicator of the level of infection in a flock. Faecal egg counts can be used from weaning onwards to determine when treatment is required. A composite faecal sample, from 10-15 lambs in a group, can be submitted to a laboratory for analysis. A faecal egg count of 600 eggs per gram or above may indicate the need to treat. Monitoring faecal egg count will ensure that animals are only treated when necessary and that a susceptible population of worms in refugia is maintained. Faecal egg counts should also be used to determine which anthelmintics are effective on the farm.

Take Home Message:

Knowing which anthelmintics (doses) are effective on the farm is a pre-requisite to ensuring that an appropriate anthelmintic is used. The best anthelmintic is one that works on your farm. Contact your vet or advisor for full details on how to check anthelmintic efficacy.