The most common health problem of domestic sheep, especially young lambs, is internal parasites (worms). There are many different types of worms that can cause problems to sheep, but stomach worms are the most common. Stomach worms cause many symptoms in sheep and lambs including scour, weight-loss, poor growth rates and can result in death if the symptoms go undetected.

 parasitised lamb

In the past, worms were effectively controlled with anthelmintics, drugs that kill parasites. But nowadays, the worms have become resistant to most of the drugs, making parasite control more difficult. Scientists are working hard to find new ways to control parasites in sheep.

In the recent years there has been widespread resistance of worms to the‘white drenches’ and an increasing resistance to the ‘yellow’ and ‘clear’ drenches. It is very important that we minimise this and this can be done by following these steps:

1. Avoid introducing resistant worms - Quarantine treatment

Without effective quarantine treatment the risk of resistance can increase greatly. Quarantine strategies should be applied to all sheep whether bought from sales or returning home from grazing on other farms. Treat all incoming sheep and keep sheep off pasture for 24-48 hours following treatment to ensure all worms present in the gut have passed through into their faeces. Ensure they are fed and watered during this period. Turn sheep out into‘dirty’ pasture ie. Pasture previously grazed by resident sheep in the same grazing season. If no ‘dirty’ pasture is available, then hold the sheep off pasture for 72 hours following quarantine treatment before allowing them to graze on a small paddock.

2. Test for resistance on farm

If you believe you have a problem on farm then it would be worthwhile checking to see which wormers are working and which are not.

There are a variety of tests that you can carry out. Whether it is a simple faecal egg count test after worming or more detailed reduction tests, speak to us and we can advise you on the best course of action.


3. Effectively administer your wormers·

Always use products that are in date.

Dose to the recommended rate for the heaviest sheep in your flock. Weigh 2 or 3 of your heaviest sheep and dose them all based on those weights. Under-dosing can increase resistance!!

Check the calibration of your dosing gun to make sure it is giving the correct dose.

Dosing technique is important, make sure it goes over the tongue and is swallowed.

Restricting feed before dosing. Some white and clear drenches are more effective if you with-hold food for 24 hours. Ensure water is available at all times. DO NOT WITH-HOLD FOOD FROM HEAVILY PREGNANT EWES.

Only use wormers when needed - Faecal Worm Egg Count tests can be run to see if your sheep need wormed. Adults can gain immunity to worms if they have had enough exposure throughout their life.

4. Dose At Appropriate Times 

Pre-tupping - to minimise health challenges and maximise fertility

Pre-Lambing - To minimise the peri-parturient rise in worms as the ewe's natural immunity wanes 2-4 weeks before lambing

Dosing Lambs - their immunity takes around 6 months to develop, providing they have enough exposure so they must be wormed regularly as young lambs to prevent health problems associated with heavy burdens

5. Using the right wormer

Using the same wormer repeatedly and unnecessarily can increase resistance so it is important to not over worm! It is also recommended to change your wormers every 1-2 years so that the worms are not exposed to the same drench for too long which can cause resistance.

Click below for a link to NADIS for an up-to-date parasite forecast for your area