• Worms

    The worm problem


Worms present a significant production threat to sheep with infection costing the industry millions of pounds a year. The main worms affecting sheep are stomach and gut worms, which are commonly termed roundworms or nematodes. Other worms that affect sheep are tapeworms (cestodes) and lungworms, which are found in the airways.

 Common sheep worm - haemonchus contortus

Common sheep worm - haemonchus contortus


Economically, worms are extremely important, causing clinical effects in animals.
Common sheep worms and their effects 

  • Stomach and intestinal worms cause diarrhoea, appetite loss and weight loss
  • Lung worms cause coughing and unthriftiness
  • Barbers pole worm causes anaemia and bottle jaw due to blood sucking


Roundworms (nematodes) such as stomach, gut and lung worms are cylindrical and vary in length from a few millimetres to several centimetres. They parasitise the small intestine, large intestine, stomach and lungs. 

Generally they are host specific, i.e. sheep nematodes do not infect cattle.  Exceptions are the Barbers Pole Worm and the gut worm Nematodirus battus which can affect either.

To know how to best control parasites, it is vital to understand their life cycles:

Lifecycle of roundworms


Adult female worms lay eggs, which pass out in sheep faeces onto the pasture.  Depending on temperature and humidity, they can take as little as a few days to two weeks to hatch during the summer.


Eggs hatch to form L1 larvae which feed on bacteria and organic matter in the faecal pat, grow, and moult to become L2 larvae.  These continue the same way to become infective L3 larvae which migrate out of the dung onto pasture. Development from egg to third stage larvae takes approximately three weeks. 

Infective larvae (L3)

Picked from the pasture and eaten, L3 larve continue development inside the rumen. 

Adult worm

Moulting twice through L4 and L5 the larvae develops into an adult worm around three weeks after being ingested. 

General nemotode life cycle

Sheep Tapeworms

(Moniezia spp) are the  only tapeworms found in UK sheep as adult worms, and live in the small intestine with the head attached to the intestine wall. Segments are shed in the faeces thirty days after infection in quite long chains. Immunity develops rapidly and, although diarrhoea and unthriftiness can be associated with large tapeworm infections, generally clinical and economic losses are minimal. 

Life cycle of the tapeworm

Dog Tapeworms

Sheep are the intermediate host of some tapeworms whose adults live in dogs. These can cause cysts to form in the brains and muscle of sheep causing illlnes and condemnation of carcases. Products used to kill the sheep tapeworm have no effect on the intermediate stages of the dog tapeworms and effect control can only be achieved by worming the dogs.


The aim of any worm control strategy is to balance the exposure of susceptible animals to high levels of infestation on pasture against too frequent use of anthelmintics. This can be done using:

  1. Use of low risk pasture/clean grazing
  2. Wormer treatments in the correct manner

Control of roundworms by clean grazing alone is impractical on many farms, due to lack of available clean grazing. Wormer treatment is often necessary.  

Wormer choice

Wormer choice depends on a number of factors from time of year and spectrum of worm activity to preferred handling facilities and the need for added selenium and cobalt. 

Due to the growth of resistance, the most important differentiating factor between wormers is chemical class: 

Sheep wormer chemical classes.


What is resistance?

Resistance is the ability of worms to survive the normal dose of a wormer. Worms can be resistant to one class of wormer, two classes of wormer or all three traditional classes of wormer.  There are no worm populations resistant to the newest class of wormers, the 4-AD Orange drench class.

Resistance develops over years, and the good news is it can be slowed down greatly if SCOPS guidelines, including integration of the orange drench into worming programmes, are used to help manage resistance on farms. 

How widespread is resistance?

At present, resistant and multi-resistant worms are threatening the viability of sheep farming in the UK, Ireland and around the world.

As shown in the graph below benzimidazole (white) drench resistance is common throughout the UK. However, with resistance defined as wormers being less that 95% effective, many farmers with detectable resistance may be unaware of productivity losses or a growing resistance problem which will threaten future stock performance.

Know your farms Resistance Status 

Before choosing a wormer, it is important to check your Anthelmintic Resistance (AR) status to find out whether – and which - drench resistance worms are already present.

A simple drench test (also known as a Faecal Egg Count or FEC test) can be carried out by you or your vet to determine your farm’s resistance status. This will help guide your choice of wormer.

Before using any worming product advice should be sought from your vet or medicine prescriber.

Now 4 classes of drench

There are now 4 classes of drench to choose from. A new 4-AD or orange drench has joined the classification system:

1-BZ (white)

2-LV (yellow) 

3-ML (clear) 

4-AD (orange) 

Containing monepantel, and with a unique mode of action, the 4-AD drench (ZOLVIX) is unaffected by resistance to other anthelmintic groups, so it is effective against worms resistant to (pro) benzimidazole, morantel, levamisole, and macrocyclic lactones including multi-resistant populations.

Comparative efficacy against multi-resistant Haemonchus contortus
Efficacy (%) against various resistant GI nematodes

Class 4-AD drench can be used to manage resistance build up in lambs mid-season, as a clean out dose at Quarantine to treat incoming stock, on farms with resistance to current drenches, and as a broad spectrum wormer as recommended by your prescriber.
Zolvix efficacy against adult and larval stage roundworms


SCOPS provides independent guidelines to help prevent the development of wormer resistance. Starting with determining your farm’s AR status using Faecal Egg Count testing, the main principles are:

Quarantine(clean out) Dosing

  • Avoid buying in resistant worms.  Give all incoming sheep a clean out dose with monepantel and injectable moxidectin (for added sheep scab protection).
  • Keep imported sheep off grass for 24-48 hours after treatment, then turn out to dirty pasture.

Rotate between drench chemical classes

Rotate within the season between drenches from the existing chemical classes. Use Drench Tests to ensure the class selected still has efficacy on farm.

  • 1-BZ (white)
  • 2-LV (yellow) 
  • 3-ML (clear) 
  • 4-AD (orange) 

Effective drenching

  • Avoid ‘selecting for resistance’ due to under-dosing, which allows part resistant and resistant worms to survive.
  • Calibrate the drenching gun before use, and dose to the weight of the heaviest animal in the group.

Preserve susceptible worms on farm

A population of susceptible worms needs to be maintained to dilute the resistant worm population and reduce the speed of AR development on farms.

This is done by making sure all the worms on your farm are not exposed to a wormer or are ‘in refugia’ when you treat. 

To preserve ‘in refugia worms’:

  • Some animals in the flock can be left untreated so they continue to produce susceptible eggs. 
  • Consider if leaving 10% of the flock untreated  is feasible

    Be careful when you dose and move

    If you dose all the flock then move them to clean pasture, all the worm eggs shed will be from any surviving resistant worms.  This means only resistant worms will build up on the new pasture which speeds the development of resistance.

    To avoid this either:

    • Leave around 10% of the stock untreated when you dose and move


    • Allow the treated flock to become ‘lightly’ re-infected by leaving them on dirty pasture for 2-3 days after dosing, then move them to the clean pasture.  Take care when using a persistent wormer.

    These strategies allow a satisfactory compromise between making best use of the ‘clean’ pasture whilst still reducing the selection pressure for AR.



    Ewes are relatively resistant to worm infections and need only be treated once or twice a year: 


    Consider a pre-tupping dose for any ewes that are in poor condition to help ensure they go to the ram in prime condition and that ovulation and conception rates are optimal. 


    To remove worm burdens which would contribute to the ‘peri-parturient’ or spring rise and add to the intake of infective larvae by lambs.


    Frequency of treatment of lambs will depend on pasture infection levels. Use FEC to make sure you only dose when necessary, and use the right class of drench at the right time.

    Sheep on extensive systems may require less frequent dosing as the worm challenge will be lower. 

    On farms with a history of Nematodirus problems, treatment will depend on whether rising spring temperatures and the lambing season coincide. Nematodirus causes damage before eggs are produced so FEC have limited usage when treating this worm. Develop a treatment strategy with your prescriber based on previous on farm experience and NADIS predictions.

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