Subscribe and save

Farmers Weekly from £127
Saving £36
In print AND tablet



We are in the process of making some changes to our website, which we are excited to be revealing soon. As part of these changes, the learning content provided by the Farmers Weekly Academy will soon be moved to the main site. If you have any queries please email

Worms in sheep

Course: Diseases and pests in sheep | Last Updates: 26th November 2015

Lesley Stubbings
Biography >>

Worms are one of the most significant causes of loss of performance in growing lambs. The clinical signs are scouring, weight loss and anaemia, which are sometimes fatal. But sub-clinically, growth rate can be curtailed by up to 50% without any signs of infection. The increase in resistance to wormers is a major cause of lost performance.
This means an integrated control strategy is becoming essential. You need to know what you’re up against and how it is affecting your flock before you can decide with your adviser the best approach to take this spring.

Which worms cause a problem?

There are four key gastrointestinal nematodes causing concern in UK flocks. Nematodirus battus usually strikes in March to May in the south, and May to mid-June in the north. It causes a particularly nasty disease in lambs, hitting very quickly, without warning, so the mortality rate can be high.
Teladorsagia circumcincta (previously known as Ostertagia) strikes during the summer months. It causes scouring and feed intake flops, leading to mild anorexia. Trichostrongylus species, also known as the black scour worm, normally strike at the end of summer or early autumn.
The barbers pole worm, or Haemonchus contortus, affects sheep at any time. This worm is particularly nasty as it also affects adult sheep. It is not present on all farms, so quarantine restrictions on infected flocks are effective at preventing spread. Previously confined to the South East, it is now across the UK, so all farmers should take precautions against it.

How do they become a problem?

The life cycles of these nematodes are similar – the adult lays eggs in the sheep that pass out in faeces and hatch. The larvae infest the pasture and are taken up by grazing sheep, developing into adults in the intestine.
Pastures are only infective if the third-stage larvae are present and climatic conditions are suitable for them to move up onto the herbage, where they can be ingested. Rainfall and temperature have an influence, so monitoring can help assess risk.
Teladorsagia, Trichostrongylus and Haemonchus have life cycles of just a few weeks, so several can occur in one season. Haemonchus has a short life cycle – as little as 14 days – so numbers can build quickly. Faecal egg counts will effectively monitor levels and you can treat accordingly.
The life cycle of Nematodirus is a full year – eggs shed by lambs during one spring hatch the following season to infect the subsequent lamb crop. This means faecal egg counts will not help gauge risk to the flock, so you have to assess this by looking at grazing history, flock health and seasonal risk.
Nematodirus eggs need a period of cold weather followed by warmer temperatures of 10C or more before they hatch. If conditions are right, this can trigger a mass hatch just when young lambs are starting to graze and the result can be devastating.

What damage do they cause?

Lambs are most at risk because adults acquire immunity, with Haemonchus being an exception. The worms damage the lining of the gut, causing scouring, loss of appetite and weight loss. Haemonchus is a blood-sucker, causing anaemia and preventing the animal absorbing nutrients.
Diseases associated with worms can be confused with coccidiosis, which affects young lambs. If they are infected with both parasites at once, it can cause a lot of trouble. If you have a history of coccidiosis you should be on guard.
Nutritional scouring from lush pasture can be confused for worm damage, and an acute fluke problem can be confused with Haemonchus.
But by the time you have seen the symptoms, the damage to performance has been done, so worms need to be controlled before clinical signs are evident.

What are the issues at lambing?

The immunity of adult ewes wanes at lambing time. This is known as periparturient relaxation in immunity (PPRI) and allows the worms to produce more eggs. Normal practice is to treat ewes, but if you treat the whole flock, this will increase selection for anthelmintic resistance (AR). To compromise, the Sustainable Control of Parasites in Sheep recommends not treating the fittest 10% ewes with twins and those with single lambs as they are most likely to maintain immunity.

How has AR built up?

There are four groups of broad-spectrum anthelmintics. A large proportion of lowland farms now have resistance to Benzimidazole (BZ), or “white” drenches. Resistance to levamisole (LM) “yellow” drenches is lower, but still significant.
Macrocyclic lactones (ML), which include avermectins and moxidectin, are available as “clear” drenches or injectables. ML resistance has been detected on UK farms.
Anthelmintics in the same group share the same mode of action, so resistance to one is likely to be present against others in the same group. You may not notice AR until they are less than 80% effective, but resistance can be detected when they are less than 95% effective. Farmers need to take steps to prevent resistance becoming a problem.

How do you manage resistance?

The first crucial step is to understand the level of resistance you have and to which groups. AR should not be seen as a negative – anthelmintics are still effective against nematodes as long as you manage them to keep the level of resistance low. Your vet can advise on how to do a drench test to check.
The three main selection pressures are:

  • Under dosing – it is vital you use the right dose, that you administer anthelmintics properly, and that they are used effectively. Treat for the heaviest animal in the flock or treatment group. Never mix them with anything else prior to use. Take care to store them correctly.
  • Frequent (unnecessary) dosing – use faecal egg counts to determine the need to treat lambs.
  • Treating a large proportion of the worm population at any one time. This is important where the pasture contamination is low and most of the worms are in the sheep. The classic example is putting newly drenched sheep on an aftermath because you then populate the field with only those worms that survived treatment, so are resistant.

Good pasture management will be one of your main tools. Assess which pastures are likely to have the highest worm burden and prioritise the animals most at risk.
A key opportunity is at weaning – for example, you could move lambs to low-risk pasture and leave ewes with better immunity exposed to high worm burdens.
If you have cattle, you can swap pasture use between sheep and cattle to minimise build-up of worms. Positive use of hay and silage aftermath can help – this is often good, clean pasture and ideal for sheep.

Three golden rules

  • Understand the enemy – know what worms you have and their differences and set out your objectives
  • Put in place a proper strategy with your vet or adviser – use the tools to monitor and only use anthelmintics when necessary
  • Choose the right product for the right reason. Be prepared to use more than one group in a season if required.
Please login or register to take this test.