Internal Parasites in Sheep


  • There are three broad types of internal parasite that can cause significant health issues in sheep - worms, flukes and protozoa.
  • Worms are thought to cost sheep owners more than any other disease.
  • Worm infestation is probably the most common cause of "ill thrift" in sheep.
  • Roundworms are the most common internal parasites of sheep, with the Small Brown Stomach Worm, the Black Scour Worm and Nematodirus being the more common worms in Tasmania. The Barber's Pole worm also occurs in Tasmania but it has to date only been the cause of localised problems, generally in the summer and early autumn, when the local climatic conditions have been warm and moist.
  • Drench resistance is an emerging problem in managing sheep worms.
  • Worm infection is a significant animal welfare issue and, if not treated, can cause death.
  • There is only one species of liver fluke known to infect sheep in Australia. It is, however, a problem in some areas of Tasmania.

Worm management plan

It is important that everyone with sheep has a worm management plan - and this includes hobby farmers. An effective plan is simple and will save a lot of money, effort and heartache. The information on this webpage will help you prepare a worm management plan for your situation. Commercial sheep farmers should involve their vet in preparing the plan.

The essential elements of a worm management plan include
  • Worm testing. It is cheap and easy to do. Without regular testing, you won't know whether you have a problem, whether your worm management plan is working or whether you have an emerging drench resistance problem on your property.
  • Grazing strategy to create safe or low contaminant pastures for weaners and lambing ewes.
  • Maintaining good nutrition during periods of poor pasture growth.
  • Building worm resistance in the flock.
  • Biosecurity measures for new sheep arrivals on the property or any outbreak of worm disease in your flock (a sure sign that your worm management plan has failed).
  • Minimising the risk of drench resistance developing on your property.
It is strongly recommended that you routinely do a worm test before you drench your sheep.

Worms - The signs to look out for

Worm diseased sheep.
  • A typical sign of a worm problem is unthrifty sheep. An unthrifty sheep is one that is not eating properly, is losing condition, tends to lag behind the mob when moved and, in severe cases, is clearly weak.
  • A worm problem often (but not always) results in sheep scouring and becoming daggy. In severe cases, affected sheep may scour profusely. Other conditions can however produce these signs.
  • Young sheep are far more susceptible to worms than older sheep.
  • Sheep under stress (eg during the later stages of pregnancy, during lactation, during drought or winter feed shortages) are more susceptible to worms.
  • A high-risk time in Tasmania for worm infections in sheep is an especially long period of cold and wet weather - especially if this coincides with the later stages of pregnancy or lactation.
  • Other signs of worm infection you may see are anaemia or swelling under the jaw (commonly called "bottlejaw").
  • If you suspect a worm problem, it is worth doing a worm test to confirm it. Remember that "ill thrift" and scours, the major signs of a worm problem, can also occur with other diseases. So, acting purely on the clinical signs may result in a wrong (and costly) treatment.

Worm Control - General Principles

  • Most sheep have some worms in their digestive system. This is normal. Indeed, exposure to worms is essential if sheep are to develop and maintain an immunity to worms. Worms are only a problem if the numbers increase to the point at which production losses occur or sheep become susceptible to other disease conditions.
  • There are four components of an effective worm control program. They are drenching, grazing management, nutrition and breeding worm resistant sheep. Drenching alone will not resolve a worm problem. Too frequent drenching may reduce the sheep's immunity to worms and increase the problem of drench resistant worms. Ad hoc or over-frequent drenching is one of the major causes of drench resistance and is, in most cases, a waste of money, time and effort.
  • The overall purpose of a worm control program should be to minimise production losses caused by internal parasites and to maximise the sheep's immunity to worms. If these are achieved, the sheep will need fewer drenches.
  • It is most important that sheep owners take a long-term and 'integrated' approach to worm control. It is important that the design of worm control programs takes into account the major problem of drench resistance.

Drenching - The Basics

  • Drench is the common name for an anthelmintic, a chemical specifically designed to kill worms.
  • Drenches can be "broad spectrum", which means they treat a wide range of internal parasites, or "narrow spectrum", which means they treat a restricted range of internal parasites.
  • There are three main classes of broad spectrum drench:
      • Benzimidazoles or BZs, commonly known as white drenches
      • Levamisoles or LEVS, commonly known as clear drenches
      • Macrocyclic lactones or MLs, commonly known as mectins
    "Combination drenches" are mixes of two or more of these drench classes.
  • In most cases, broad spectrum drenches are used to control worms. The most common use of narrow spectrum drenches in Tasmania is against liver flukes (broad spectrum drenches have little or no effect on the liver fluke).
  • In most cases, drench is administered orally by a drenching gun. When drenching, it is important to:
      • Shake the drench container first, as many drenches settle out.
      • Check that the right dose is being given. The dose rate will be on the drench container label. The dose should be calculated according to the heaviest sheep in the mob and all other adult sheep in the mob should be given that dosage. If the mob includes lambs, the dose rate for all lambs should be calculated according to the heaviest lamb in the mob.
      • Check that the drenching gun is calibrated to deliver the right dose. Drenching guns have a dial of some kind to adjust the dosage. After you have set the dosage rate for your sheep, squirt a single dose into a measuring container or medicine cup and check that amount against the intended dosage. If air is being sucked into the drenching gun as it reloads, this will result in under-dosing.
      • Insert the drenching gun nozzle into the sheep's mouth from the side, between the incisor and molar teeth, and make sure that the nozzle is above the tongue. After the dose has been delivered, make sure that the sheep has not dribbled or spat the drench out.
      • When all the mob has been drenched, clean the drenching gun by pumping cold water through it. Don't use soapy water as this tends to damage the rubber seals in the drenching gun, which may then cause incorrect doses in the future. After cleaning the drenching gun, you can lubricate the rubber seals inside the drenching gun by pumping some vegetable oil through the gun (but do not use mineral-based oils, such as baby oil, as these will damage the rubber).
      • Check the withholding period for the particular drench being used. Different drenches have different withholding periods.
      • Store unused drench in the original container out of direct sunlight.
      • If using old drench, check the expiry date, which should be on the container label.

Drenching - How Often?

  • There is no "one size fits all" drenching program. How often you drench will vary according to conditions on your property.
  • The basic principle in any drenching program should be to minimise the number of times a sheep is drenched - providing, of course, that effective worm control is maintained.
  • Under normal conditions, if worm control has been effective at reducing worm numbers on your property, a typical drenching program may be:
      • For all sheep, one or possibly two summer drenches, using an effective broad spectrum drench - one as the pasture dies off and the second in February or March. If your worm control program is effective, the second drenching may be unnecessary, so do a worm test first.
      • For lambing ewes, an additional drench pre-lambing may be necessary. If so, it can be done at the same time that they are vaccinated for pulpy kidney, tetanus and cheesy gland - that is, 2 or 3 weeks before lambing.
      • Lambs should be drenched at weaning and then put onto a "safe" pasture. Further drenches may be necessary - for example when the weaners are moved onto the next "safe" pasture and again in autumn and mid-winter. To avoid over-frequent drenching of weaners, which will inhibit the development of their worm immunity later in life, use a worm test to help you decide if these additional drenches are necessary.
  • If you see the signs of a major worm disease outbreak in your sheep, you will need to act quickly. There is no "typical" emergency response to a worm disease outbreak, as it will depend on a range of issues specific to your property. You will need to seek professional advice from your vet, your animal health adviser or from NRE Tas. However, the following general suggestions may be helpful:
      • Firstly, you will need to check that worms are indeed the problem. A worm test will tell you whether there is a worm problem and, if so, which worms and how serious the problem is. Worm testing is the essential first step in dealing with any significant worm problem.
      • If worms are the cause of the problem, you will need to determine how this has come about. If you have been using a drench as part of your worm control program, the drench may have failed because of drench resistance. A drench resistance test could be undertaken to determine which drench will be effective in treating the worm problem.
      • You will need clean pasture to put your sheep on after they have had the emergency drench.
      • Each of the above three steps is most important. The emergency treatment is likely to fail if any of them are not done.

Drench Resistance

  • "Drench resistance" refers to the situation where worms have developed an inherited or genetic resistance to specific drench classes. Once worms have developed resistance to a particular class of drench, an increasing percentage can survive a dose of that class of drench. In other words, that class of drench will no longer be fully effective.
  • It is estimated that about 90% of Australian flocks have resistance (less than a 95% kill of worms) to the white drenches (benzimidazoles, or BZs), about 80% to clear drenches (levamisoles or LEVs) and around 60% to combination drenches (BZ and LEV).
  • Resistance to the 'mectin' drenches (macrocyclic lactones or MLs) has not been a major problem in the past in Tasmania but recent test results from our animal health laboratory are indicating there may be an emerging problem of mectin-resistance in at least some flocks. Mectin-resistance is becoming more of a problem in WA, NSW, Qld and SA and is quite widespread overseas.
  • As it takes a long time to develop and approve new drenches, and even though a new drench class will become available in the short to medium term, continued under-dosing and over-use of drenches will make them all ineffective before too long. With worm resistance in the 'mectin' or ML class of drench on the increase nationally, it is most important that sheep owners develop and use a long term worm control program that reduces the overall reliance on drenches.
  • Drench resistance is not the only reason that a drench may fail. Other reasons include:
      • The drench was not mixed before administering and had settled out.
      • The drench was past its expiry date.
      • The drenching gun was not delivering the correct dose. Check that the gun is not sucking air when reloading and that the piston returns fully down the barrel before dosing another sheep.
      • Some sheep missed being drenched or dribbled the drench out.
      • The wrong dose rate was used. Different drenches have different dose rates, so always read the label or contact the manufacturer if you are unsure.

What Can You Do to Delay the Onset of Drench Resistance on your Property?

  • Make absolutely sure you are administering the correct dose. Underdosing is one of the major causes of drench resistance, as it helps the worms develop a drench resistance. Check the required dose on the label on the drench container.
  • Always set the dose by the weight of the heaviest sheep in the mob. Administer this dose to all adult sheep in the mob. If the mob includes lambs, set the dose for all lambs by the weight of the heaviest lamb in the mob. Unless you are very experienced with sheep, it is not easy to guess the weight of a sheep.
  • Check that the drenching gun is delivering the correct dose. If the drenching gun was not properly cleaned after the last time it was used, the drench residue in the gun may have corroded or otherwise damaged the internal seals. It may also cause the piston to be "sticky", which means that it may not fully return after administering a dose, which in turn means that the next sheep would be underdosed. In any event, drenching guns should be serviced regularly. Repair kits for the more expensive drenching guns are not expensive and are available from rural merchandisers.
  • Use the correct drenching technique. In particular, ensure that you are drenching over the tongue rather than squirting the drench into the front of the mouth.
  • Fasting. When using white drenches (BZs) or mectins (MLs), the treatment is generally more effective if the sheep are fasted for 24 hours beforehand. Keeping the sheep off feed for six hours after drenching also helps make the treatment more effective. But note that ewes in the last six weeks of pregnancy, sheep in poor condition or stressed sheep should not be fasted at all.
  • Overdosing will not help. However, with white drenches (BZs), two drenches twelve hours apart can be effective. (This applies to sheep but not to goats). Note that any departures from label recommendations will extend the withholding period and therefore require an off-label use advice note from a vet.
  • After drenching, put the sheep onto clean pasture wherever possible.
  • Rotate between the drench classes (BZs, LEVs, MLs) either at each treatment or each year.
  • Use a worm test to monitor worm faecal egg counts (FECs). If you know the level of worms in your sheep, you will be best placed to avoid drench overuse, slow the development of drench resistance and save on drench costs.
  • If you suspect drench resistance among your flock, do a drench resistance test. This will show you if any of the classes of drench are ineffective in your situation, in which case they should be dropped from your worm control program. If you wish to undertake resistance testing, discuss the process with your vet, animal health adviser, drench manufacturer or NRE Tas staff.
  • It is most important that drenching is not the only strategy for worm control on your property. Drenching should be just part of a program that includes pasture management, nutrition and selective breeding of worm-resistant sheep.
  • Use a "quarantine drench" to treat any sheep coming onto the property. Unless you know the drench resistance status of these new sheep, the most effective "quarantine drench" is a combination drench (ie one containing both white and clear drenches) plus moxidectin, or otherwise just one of the mectins.
  • Some authorities recommend maintaining a "refugia" of susceptible worms within a flock as a way of delaying any onset of drench resistance. Essentially, this involves leaving some susceptible worms (ie susceptible to your drench providing you know that the class of drench you are using is effective on your property) in the flock. The point is to dilute the number of drench-resistant worms building up in the flock. This strategy involves simply leaving 5-10% of the flock undrenched. Of course, the ones you do not drench must be the healthiest looking sheep in the flock. There is some controversy about this "refugia" strategy and it may not be appropriate in all circumstances, so please consult your vet before using it.


Animal Biosecurity and Welfare Branch

13 St Johns Avenue,
New Town, TAS, 7008.