Managing Internal Parasites in Sheep - Part One – Understanding the Problem

lambs-with-mucky-backsideDiarrhoea, or scouring, is usually blamed on internal parasites, but it can also be a sign of disease, such as Border Disease caused by a virus; or a bacterial infection such as Bacterial Enteritis, both of which can cause profuse scouring. It may be a dietary problem – possibly caused by an autumn flush of grass growth, or access to certain plants such as ‘fat hen’, which is known to cause scour in lambs. Left untreated, scouring can quickly cause dehydration and serious illness, so it’s always wise to treat it seriously and investigate the cause – and then you can deal with it appropriately.

Gone are the days when sheep were routinely drenched at set times of the year, or when moved to fresh grazing, or when lambs were seen with mucky backsides. We are now encouraged to use Sustainable Control of Parasites in Sheep, based on best practice and designed to slow the development of wormer resistance in UK flocks. This combines less reliance on chemical wormers, with a more targeted approach to their use in order to preserve the efficacy of the existing compounds for as long as possible.

Why the change? Very simply, the wormers (also called drenches or anthelmintics) that have been used to rid sheep of their internal parasites are no longer effective on many farms. This is due to the parasites having developed resistance to each of the 3 groups of wormers that have been in use for some years, and the chief reason for this is too frequent, and untargeted use. Although a fourth chemical group (Monepantel) with no known resistance was introduced in 2010, it is vital that it is used in a planned, strategic manner to help slow the further development of resistance to the other 3 groups. A 5th group, Startect, from Pfizer, was introduced in 2012. It is the first-ever dual-active wormer in the UK, employing two active ingredients from different wormer groups. One is derquantel from a new anthelmintic class called spiroindoles. The other is abamectin, an ML wormer never previously available in this country for use in sheep.

How do I start with a sustainable regime?

Unfortunately there isn’t a ‘blueprint’ that will suit every farm and smallholding, but there are some easy principles that everyone can follow. The first one of these is to identify whether your sheep are actually suffering from a parasite burden, and which parasite, before treating them!

All grazing animals are at risk from internal parasites – endoparasites, or worms. Left unchecked, they can permanently damage the gut of young animals, limiting their ability to absorb fluids and nutrients. This leads to poor growth and at worst can kill lambs in significant numbers. Mature sheep can acquire a degree of resistance to this damage, although this is suppressed in pregnant and lactating ewes through a process called “Periparturiant Relaxation In Immunity” (or PRII). They can therefore still suffer parasitic gastroenteritis (PGE), which causes scouring and weight loss, and also poses a threat to their lambs as they excrete worm eggs onto the pasture they graze.

worm-lifecycle-diagramThe eggs can survive on pastures for a considerable length of time, and some are able to withstand heat, desiccation and extreme cold, before they hatch and develop into infective larvae, which are swallowed by grazing sheep. The developing worm then attaches itself to the lining of the sheep’s gut, causing damage to the gut. Some also feed on blood, causing anaemia. Once the larvae mature into worms, which takes between 2-3 weeks, they start to shed eggs that are passed out in the sheep’s dung, and the cycle starts over again.

By identifying the various different parasites and understanding their specific life cycles, including the time of year when they are likely to emerge, and some of the other factors that influence their threat to your flock, you can take measures to mitigate their harmful effect. Importantly, you can manage your flock to protect them from infective worms, and, when necessary, treat them with the most appropriate treatment.

  • Roundworm, is the main group of worms that infect sheep, and includes the brown stomach worm, and the black scour worm, both of which cause scouring and impaired growth in lambs, a heavy infection can cause death. They are active from late-spring to late autumn.
  • Nematodirus battus causes serious disease in young lambs, with profuse watery scouring leading to severe dehydration. It generally emerges in spring and can remain a threat until well into the summer months. In recent years this pattern appears to be changing, and cases are also occurring in the autumn. As affected lambs excrete large numbers of worm eggs, which survive the harshest winter, ‘nursery pastures’ should ideally be alternated every year. Adult sheep develop resistance, and rarely show any sign of disease.
  • inspecting-sheeps-eyeHaemonchus contortus, the ‘barbers pole worm’, is active from early spring through to autumn. It’s a bloodsucker that causes serious gut damage even in relatively small numbers, though rarely causes scouring if the only parasite species present. Signs include anaemia (shown most clearly in the conjunctiva of the eye), and swelling under the jaw. Heart and respiratory rates may also be raised. It can multiply rapidly, causing ill thrift and death in both mature sheep and lambs.
  • Coccidiosis is caused not by a gut worm, but by a protozoan parasite, which destroys the gut lining causing profuse, watery scouring, often tinged with blood. Adult sheep will have an acquired immunity, but young lambs, typically in the 3-8 week age range, are very vulnerable, especially if reared intensively indoors or turned out onto pasture that has carried older lambs in the same season.
  • tapeworm-segmentsMonezia expansa is the sheep-specific tapeworm. It rarely causes significant harm to its host – though its appearance, especially in lambs’ faeces, can be alarming! However, sheep are also the intermediary host for canine tapeworms, which cause a serious condition in sheep called ‘gid’, so always ensure any dogs on your smallholding are regularly wormed.
  • Liver Fluke is caused by a flatworm, Fasciola Hepatica, that is most commonly found in wet, marshy or boggy areas, or following unusually wet weather. The disease involves an intermediate host; in this case a tiny water snail. The high-risk time for Fluke is autumn and winter, and some years can be worse than others – your vet should be able to advise on local conditions. Affected animals suffer from liver damage (fascioliasis) causing ill thrift and eventual death. Treatment depends on whether the fluke is adult or immature as different drugs are required for each stage.
  • Lungworm – causes affected sheep to cough persistently, especially young animals.

Of the 7 types of internal parasites described above, some of which exhibit the same or very similar symptoms, only three would be treated by exactly the same method, each of the others would require a very specific treatment for effective control of the disease and to avoid inadvertently selecting for the survival of resistant worms. To complicate matters further, often more than one parasite is involved. So you see why it is critically important to identify the problem before treating.


sheep cover2 smMore information on managing your flock can be found on the “Managing Your Flock for Peak Health” DVD.






Associated articles:

Managing Internal Parasites in Sheep Part One – Understanding the Problem

Managing Internal Parasites in Sheep - Part Two – A Strategy for Your Flock

Managing Internal Parasites in Sheep - Part Three – Sustainable Control


This article first appeared in Smallholder Magazine, August 2012 - written by Carole Youngs of The Smallholder Series.