Control and treatment of stomach worms in sheep

Grazing sheep are continually exposed to gastrointestinal nematodes (stomach worms). In the case of naïve 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.

Even sub-clinical infection can result in production losses in the form of reduced growth rate and light, under finished carcasses. Therefore, good control of these worms is critical in Ireland’s grass-based production system.

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 Éireann 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. 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 of 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.

These classes are 1) benzimidazole (white wormer - 1-BZ), 2) levamisole (yellow wormer - 2-LV) 3) macrocyclic lactones (clear wormer - 3-ML) 4) amino-acetonitrile derivatives (orange wormer - 4-AD) and 5) spiroindoles (purple wormer -5-SI). The last two classes, orange and purple wormers, have been prescription only medicines in Ireland since they launched in 2010 and 2012, respectively, and have not been widely used.

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. The results of this study are presented in Table 1. 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.

Table 1: Prevalence of resistance to white, yellow and clear wormers on Irish sheep farmers

Risks factors for the development of anthelmintic resistance

On each farm, the approach to slow the further spread of anthelmintic resistance in Ireland falls under four main categories:

1. Identify and mitigate any high-risk practices for the development of anthelmintic resistance.

High-risk practices are those that place a selective pressure on worms to develop resistance. Examples include unnecessary or too frequent dosing of sheep, using incorrect dosing technique e.g. under-dosing and dosing and moving sheep to pasture only lowly contaminated with worms e.g. silage after grass.

2. Ensure appropriate anthelmintics are used to control infection.

Using an inappropriate wormer will not give good worm control and may select for resistance. Examples include using combination wormer/flukicides when only fluke control is required or using a wormer that is ineffective due to resistance.

3. Prevent buying in resistant worms by implementing a good biosecurity policy.

Worms can only move short distances on grass. Therefore, a major way that resistant worms spread is by animal movement. A closed flock or a good biosecurity policy will prevent bringing resistant worms onto the farm.

4. Ensuring sufficient worms in refugia.

Refugia refers to worms that are not exposed to a wormer and so are not under selection pressure to develop resistance. These worms provide a source of susceptible genes to dilute the resistant worms. The major sources of refugia are worms in animals that are untreated with anthelmintic and worms on pasture.

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:

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.

Untreated ewes will also act as a source of refugia. In order for untreated ewes to be a useful source of refugia, they must graze the same pasture as susceptible lambs. A leader-follower grazing system post-weaning will enable this. As ewes have good immunity, they will ingest more worms than they shed, thus removing worms from the pasture.

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. Yearlings may not have full immunity against stomach worms and so lactating yearling ewes may be under pressure and may benefit from treatment. If ewes are infected with Haemonchus contortus, commonly known as the barber’s pole worm, they may also require treatment as they will not have immunity. This worm is rare in Ireland but outbreaks have occurred.

Use only white wormers to control Nematodirus

As outlined 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.

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.

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. A composite faecal sample after treatment should show no worm eggs remaining post-treatment. The time after treatment that the faecal sample is collected is crucial, for white and clear wormers the post-treatment sample should be collected two weeks post-treatment, while for yellow wormers the post-treatment sample should be collected one week post-treatment.

Knowing which anthelmintics 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. More details on these four key actions can be found here.

This article by Orla Keane and Michael Gottstein first appeared in the Teagasc Sheep Open Day 2022 book.

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