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News > ‘The Hidden Challenge’ – Are worm burdens affecting the performance of your lambs?


‘The Hidden Challenge’ – Are worm burdens affecting the performance of your lambs?

May 13, 2020

Rachel Megarrell, CAFRE Beef and Sheep Adviser

Worm control is vital if you are aiming to achieve good lamb growth rates and a profitable sheep enterprise. Heavy worm burdens in lambs will result in a check in growth rate leading to a reduction in performance and increasing the amount of time required for lambs to remain on farm to reach slaughter weight. Ultimately parasites cost growth and these worm infections affect performance through reduced food intake, impaired food digestion and gut damage. These damaged tissues need repair which means less nutrients are left for growth. In severe cases permanent damage can occur in the gut and this will reduce nutrient absorption therefore, worm control is a priority to minimise the effect that internal parasites may have on lamb performance.

Sources of infection: Worm larvae: where do they come from?
• Over wintered larvae.
• Eggs from ewes with a worm burden (Spring rise).
• Eggs from lambs with a worm burden (Autumn rise).

Nematodirus (battus) is the big risk early in the season, the infective larval stage is very resilient and can survive low temperatures over the winter on pasture still within the egg. After a period of cold exposure the larva hatches once the maximum environmental temperature exceeds 10 degrees over a period of several days. In order to understand Nematodirus it is important to consider the population dynamics in that it is a lamb to lamb disease with the eggs laid in one season becoming infective and hatching the following season. There will be variation from farm to farm and even field to field and sheep farmers are urged to assess their risk based on the history of the field. For example if you had lambs grazing pasture in the latter end of the season in 2019 then there is a high chance that this pasture is a risk to young grazing lambs now or if a peak hatch is expected and lambs are grazing contaminated pastures they will be exposed to a significant challenge.

How do I know when I need to give my first worm dose to lambs?

It is recommended that the Sustainable Control of Parasites (SCOPS) protocols and guidance are followed with regard to choice of product for worms alongside the timing of administration referring to the SCOPS forecast for Nematodirus risk through the early grazing period. The forecast predicts the hatch date for Nematodirus based on temperature data from 140 weather stations across the UK. The SCOPS forecast is a useful management tool, each dot on the map represents a weather station, choose the dot closest to your farm and zoom in to get information related to your area. This will provide the appropriate risk level and an indication of when hatching is predicted to start. This information should be used in conjunction with grazing history to assess the risk of Nematodirus infection in your lambs. For further information on the SCOPS principles visit

Consider the main risk factors prior to dosing:

1. Are the lambs old enough to be eating a substantial amount of grass?
2. Are different aged groups of lambs being mixed together?
3. How do the lambs look? Are they clean and thriving?
4. What is the previous grazing history of the field that they are in? For example ‘low risk’ pasture would be a field that only grazed cattle last year.

Best practice to follow when using wormers;

It is important to dose animals based on weight, do this by weighing a sample of the group and then set the dose rate based on the heaviest animals in the group. If there is a wide range of weights, consider splitting the group and weighing the heaviest in each group. Under estimation of weight is a big problem and leads to issues arising from underdosing.
Ensure that the dosing technique is correct. To do this make sure that the sheep is adequately restrained and place the nozzle of the dosing gun between the molar and the incisor teeth so the liquid goes over the back of the animals tongue.
Always follow the manufacturer’s advice on the use of wormers and ensure that the drenching gun is working effectively. This can be done by calibrating your gun before dosing commences.

Be aware of Anthelmintic Resistance (AR)

There are a number of factors which influence the rate at which anthelmintic resistance develops within a worm population:
1. Frequency of treatment.
2. Use of one family of anthelmintic treatment year on year.
3. Size of the in refugia populations. You want to avoid all of the population of worms being exposed to a drench to both slow resistance development and prolong drench life.
4. Movement of livestock.
5. Under dosing – this kills a lower percentage of worms in the sheep.
6. Narrow spectrum treatments should be used whenever possible to avoid unnecessary exposure to a wormer, increasing the selection pressure for anthelmintic resistance e.g., the use of a combination fluke and worm product should be avoided when fluke is the primary target.

Other causes of anthelmintic failure:

• Dosing with insufficient anthelmintic due to; poorly maintained dosing equipment.
• Failure to follow the manufacturer’s instructions:
• Not storing the products correctly.
• Using products beyond their ‘use by’ date.
• Mixing anthelmintics with other products.

Key areas to consider for effective parasite control on your farm

1. Adopt an effective quarantine strategy to minimize the introduction of new or resistant parasites.
2. Don’t drench and move onto clean pasture as this is very selective for resistance, if possible return the sheep back onto their old field for a few days.
3. As we move further into the season optimize the use of Faecal Egg Counts (FEC). Using this method allows farmers to make an informed decision with regard to the need to dose. The results are presented as ‘eggs per gram’ (epg) of faeces and the number of eggs is an indication of the number of adult worms in the gut of the sheep. Its use has also proved beneficial on farms with suspected resistance to Group 1-Benzimidazoles (1-BZ) white wormer products. With a steady increase being identified in resistance to white products this has become more important in recent years. The fact that there is emergence of parasites that are resistant to some of the compounds available on the market suggests that the intensive use of anthelmintic products is not a sustainable approach. Regular monitoring combined with the use of FEC can help to ensure that we are preventing the damage that worms can cause to lambs, it is important however to remember that FEC is a tool to help management not to replace it.
4. Finally aim to find a balance between effective worm control and reduction in selection for resistance. Seek to maintain a worm population susceptible to wormers by reducing treatment frequency or by only treating the most susceptible individuals in a flock.
If in doubt or you have concerns about the health of your flock then veterinary advice should be sought immediately.

A video has been produced and is available to view on the CAFREtv channel highlighting practical steps that farmers can take to manage work burdens in sheep. It is available to view below:

In the current COVID-19 situation plan ahead with your supplies for all inputs required on the farm, there is no need to panic buy.  Remember to follow social distancing guidelines to all visitors on the farm and maintain good hand washing practice.