Internal Parasites in Sheep (Continued)

​​​​Worm Control Through Grazing Management

  • Grazing management should be a key component to any worm control program. Indeed, with the growing problem of drench resistance, grazing management is likely to become an even more important component in worm control.
  • The life cycle of worms includes a period in which the worm larvae live outside the sheep - that is, in the pasture. This creates both a problem and an opportunity for sheep owners. Longer pasture can mean less of a larval pick-up when grazing. Clean pastures are important for lambing.
  • In Southern Tasmania, the number of worm larvae on pasture is usually higher in winter than in summer. Species of worms change in predominance from the cooler months to the warmer ones. Drenches that may be effective against the cool season species may be less effective against the ones which predominate in the warmer seasons.
  • There was once a common belief that worm larvae could only live outside the sheep for a short time and that destocking a pasture for 3 or 4 weeks was effective in breaking the worm cycle. We now know that larvae can survive on pasture for longer than that, especially in Tasmania where conditions are generally cooler and wetter than the mainland. In general terms, the higher the rainfall and the cooler the conditions, the longer the larvae can survive in the pasture. In a wet winter in Tasmania, short term destocking of a paddock is likely to have little effect on reducing larvae numbers.
  • If a paddock is cropped or cut for hay or silage, larvae numbers are dramatically reduced.
  • In the case of cropping, the paddock is destocked for several months, normally for the entire winter and spring. This means there is no new contamination with worm eggs during the high risk seasons. Most of the larvae that survive this prolonged destocking will be removed with the crop at harvest. Cropping a paddock is a very effective way of controlling worms.
  • In the case of hay or silage cutting, the principle is much the same as for cropping. That is, most larvae will be removed with the hay or silage. However, the overall reduction in worms may be less than with cropping for several reasons:
      • the paddock is destocked for a much shorter period.
      • the sheep grazing the paddock during the high risk season of winter may have had high egg counts.
      • the residual pasture from a hay or silage crop is generally much denser than a crop stubble, so the penetration of the summer sun may be much less effective in killing any remaining larvae.
  • The aim in grazing management is to create "safe" or low contamination pastures for the more susceptible sheep in a flock - ie the weaners and lambing ewes.
  • The worms that affect sheep are not the same worms that affect cattle, and vice versa. So, grazing a paddock with cattle only during the spring will help create a "safe" paddock for your young sheep at weaning (assuming a spring lambing). In doing so, the cattle will "vacuum up" the worm larvae at no risk to themselves, thereby reducing the larval contamination for at-risk sheep.
  • If you have only sheep, you can use your older and dry sheep to graze a paddock in spring to prepare it as "safe" for your spring-born weaners. In doing so, these sheep, which are the least susceptible to worms, will "vacuum up" the worm larvae, thereby reducing the larvae in the pasture for your weaners. If using this strategy, ensure you drench these older and dry sheep before putting them into the paddock that you will later use for your weaners.
  • "Smart Grazing" is a strategy that has been developed specifically for the control of worms in weaner sheep during that first critical winter. In short, "Smart Grazing" is the preparation of a "safe" pasture for weaners by using high stocking rates for short periods after each of the summer drenches. The aim is to have a winter pasture for the weaners that has had virtually no contamination with worm eggs since the previous November or December (ie the first summer drench).
  • The grazing management strategies above are specifically for sheep. Goats do not appear to develop useful levels of immunity to worms. Therefore, the use of adult goats to prepare a "safe" paddock for weaners by "vacuuming up" the worm larvae as they graze is not recommended. Instead, the overall aim of grazing management for worms in goats should be to enable the goats to graze at least 100mm above the ground. ie forage or longer pastures.
  • Longer pasture can mean less of a larval pick-up when grazing.
  • Clean pastures are important for lambing
  • Species of worms change in predominance from the cooler months to the warmer ones.
  • Drenches that may be effective against the cool season species may be less effective against the ones which predominate in the warmer seasons.

Worm Control Through Nutrition

  • Sheep under twelve months of age and ewes in late pregnancy or lactation have a higher requirement for protein. Weaners need it for their own rapid growth and ewes need it for the growth of the foetus, particularly in the last six weeks of pregnancy, and for milk production after the lamb is born.
  • If pasture quality is poor, such as during a prolonged period of cold, wet conditions, even healthy weaners or pregnant/lactating ewes have difficulty getting enough protein from grazing. If they have a worm infection, the problem is much worse. In short, they will have a major protein deficiency and this will result in rapid loss of condition, which in turn further increases their susceptibility to worms.
  • Research has indicated that the supplementary feeding of certain minerals, where they are already deficient on your property, may increase the resistance of sheep to worms.
  • Research has shown that the supplementary feeding of oats during a period of poor pasture growth (in Tasmania, typically winter and sometimes early spring), when combined with high quality pasture management, can improve significantly the sheep's resistance to worms.
  • Research has also shown that improved energy nutrition can increase a sheep's "resilience" to worm infection, allowing animals to maintain growth rates and milk production even if they have a worm infection. When a pasture is in a period of poor growth, energy nutrition can be improved by the supplementary feeding of grains such as oats, barley and lupins. Note, however, that a sudden introduction of grain into their diet can kill sheep. Grains need to be introduced gradually.

Types of Sheep that are Less Susceptible to Worms

  • Older sheep (ie sheep that are over 15 months and fully grown) are much less susceptible to worms than weaners and lambs.
  • If sheep are under 12 months old, then Nematodirus are more of a consideration and a problem.
  • Wethers and dry ewes are less susceptible to worms than pregnant or lactating ewes.
  • Sheep under stress, such as during a winter feed shortage or in drought conditions, are more susceptible to worms.
  • Sheep in intensive conditions, such as a feedlot, are more susceptible to worms.
  • Some breeds of sheep are less susceptible to worms than others. Breeds that are more suited to wetter conditions (for example, the Romney and the Perendale) have been specifically developed for resistance to worms. Breeds that are more suited to drier conditions (for example, the Merino) can have major worm problems if they are run in high rainfall conditions.
  • The Nemesis Project is a breeding program designed specifically to increase inherited worm resistance in Merino sheep. As a result, it is now possible to buy Merino rams that genetically have a much higher resistance to worms.

Liver Fluke

  • There are two conditions that have to be met for sheep to be at risk of Liver Fluke Disease (fasciolosis). Firstly, they must graze an area that is constantly damp, such as a marshy area around a spring or seepage, land affected by a slow-running stream or even the wet areas around a farm dam or irrigation channels. Secondly, freshwater snails must be present in that area to act as intermediate hosts for the liver fluke.
  • The signs of Liver Fluke Disease are weight loss and anaemia. In chronic cases, scouring often occurs (but not always) and the sheep suffers obvious abdominal pain and is reluctant to move. Infected sheep may also develop "bottle jaw" - a swelling under the bottom jaw. Unfortunately, in some cases, there are no obvious signs before the sudden death of the infected sheep.
  • Liver Fluke Disease is more likely to occur in summer and early autumn.
  • Drenching can be an effective means of preventing Liver Fluke Disease. Note, however, that most worm drenches do not contain a flukicide (ie a drench specifically for the treatment of liver fluke), so check the label to ensure that the drench you are using is registered as a treatment for liver fluke.
  • Drenching with a flukicide is typically carried out three times a year (late winter or early spring, summer and late autumn). Drenching more often than needed is expensive and may lead to drench resistance. Normally, drenching is only necessary for sheep that graze paddocks with fluke-prone (ie constantly moist) areas.
  • Draining or fencing off fluke-prone areas will help reduce the risk of liver fluke disease. Chemical control of snails in fluke-prone areas is generally a poor option. Snails reproduce very quickly and will repopulate treated areas. And the use of poisons so close to waterways is likely to be dangerous to the aquatic environment.
  • For a small additional cost, you can check for the presence of liver fluke when you do a Worm Test.

Special Information for Hobby Farmers

  • While most commercial sheep farms have fully integrated worm management programs in place, hobby farmers do not generally have such a program. There are several reasons for this, including:
      • not knowing that worm infestation is among the most common of sheep health problems in hobby farm situations
      • not knowing or recognising the signs of a worm infestation
      • not being able to rotate their sheep onto clean pastures
      • not knowing where to buy very small quantities of drench and/or not owning a drenching gun
      • not having good sheep handling facilities.
  • It is most important for the welfare of their animals that hobby farmers understand worm control issues, including drench resistance, and that they actually carry out effective worm control. It is important to know that not all worms cause scouring (diarrhoea) and not all scouring is caused by worms (can be bacterial/green feed scours, etc.).
  • Single doses of drench are usually available from your local veterinary practice. Alternatively, very small quantities of drench can sometimes be sourced from other sheep owners in your local area. If you do this, make sure you store the drench out of direct sunlight and that you label the container with the expiry date of the drench.
  • Often, a local shearer who shears small flocks will drench the sheep at the same time as an additional service. If the sheep are shorn in late spring and crutched in late autumn, a drench at each of these times is usually sufficient, providing the sheep are put onto fresh pasture immediately after each drenching. Don't forget to alternate the drench class used at least annually, as this will help delay the onset of drench resistance. So it helps if you keep a written record of what type of drench is used each time.
  • If you have just two or three sheep, and if you do not have access to a drenching gun, the drench can be administered via a syringe (with the needle removed) in much the same way as if you were using a drenching gun. However, when using this method, it is important that the drench is administered slowly, as you are not delivering the drench down the sheep's throat and are relying on the sheep to, in effect, drink it. After use, the syringe should be cleaned by pumping cold water through it, if you intend to re-use it for future drenching.
  • If you have several sheep, it is worth investing in a drenching gun. A brand new plastic drenching gun is not expensive. Providing you remember to pump some cold water through it after each use, it should last many years. If buying a drenching gun secondhand, check that it is in good working order - in particular that it delivers the correct dosage. A repair kit for one of the more expensive drenching guns costs about the same as a brand new plastic drenching gun.
  • When you have drenched your sheep, clean the drenching gun by pumping cold water through it. Don't use soapy water as this tends to damage the rubber seals inside, 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).
  • Your sheep will gain little value from the drench unless you are able to put them onto low contamination pasture afterwards.
  • When buying sheep for your hobby farm, choose animals that are worm resistant. For example, avoid having merinos in areas with high rainfall. Older sheep are generally less susceptible to worms than sheep less than 12-15 months old. And, unless you really intend to breed from your sheep, run wethers or dry ewes as these are less susceptible to worms.
  • Quarantine drench animals before introducing them onto your property.
  • Having a vet adviser and working with them is vital. If in doubt, ask us or your vet adviser.

Testing for Worms, Liver Fluke and Drench Resistance

  • Inexpensive tests are available that may help you avoid costly treatment programs that do not work.
  •   Animal Health Laboratories Current Fees   (257Kb) 
  • Test kits are available from your vet, rural merchandiser or the NRE Tas Animal Healt​​h Laboratory (03 6777 2111).
  • Using these kits, the sheep owner collects dung samples. Instructions are included in the kits to ensure these samples are reliable.
  • The samples are then sent to the Animal Health Laboratory for examination. The turn around time at the labs for a worm and/or liver fluke test is usually 24 hours. Results are sent to the sheep owner and/or their nominated vet.

Worm and Fluke Tests

  • You can use the same WormTest kit for monitoring both worms and fluke in either sheep or goats (test kits for cattle and horses are also available). It is recommended that our clients keep the kits on hand. Charges are only issued after the testing is completed.
  • The standard WormTest allows producers to monitor worm egg counts (known as a faecal egg count or FEC). Worm egg counts are measured in eggs per gram (epg) of faeces.
  • If you want to know what species of worm is present, a larval culture will be needed. This is relatively inexpensive and, as most worms are treated with broad spectrum drenches, not usually necessary.
  • The liver fluke test does not provide results in eggs per gram, but as detected or not detected. Producers usually treat if eggs are detected. However, it is the immature fluke that causes most of the problems and the faecal egg test may not detect these.
  • WormTest kits can also be used to check drench effectiveness. Collect faecal samples 10-14 days after drenching (5-6 weeks if you have used a capsule). Please note that WormTest does not tell you specifically which classes of drench are and are not effective on your property. But it will tell you if adult egg laying worms are present and what that worm burden is. So, the result can tell you if the class of drench you used some 10-14 days earlier was effective.
  • WormTest is excellent value for money because:
      • it can save you the expense of unnecessary drenching or using the wrong drench;
      • it can help you take action before a worm infection becomes a disaster; and
      • it allows you to monitor the effectiveness of your worm control program.
  • Unless you are experienced in using worm or liver fluke test results, you are likely to need some professional help in using your test results to determine whether to drench. There is not a single epg (egg count) result that can be used as a standard threshold above which drenching is necessary. However, in general, for worms:
      • under 300 epg is a low count and drench is probably not needed
      • over 500 epg is a high count and drench is probably needed
      • between 300 epg and 500 epg is a moderate count and you should seek advice on whether to drench, as that will depend on which species of worms are present and the prevailing weather and paddock conditions.
    The above is a general guide only and livestock owners are encouraged to get professional veterinary advice that is specific to their circumstances. For example, there are circumstances where the threshold for weaner sheep may need to be lower.
  • The faecal egg count test may be unreliable for pregnant or lactating ewes in poor condition. You should use a total worm count instead. This requires a post mortem to be performed. While this may not be a viable option for a hobby farmer with a few pet sheep, it may be worthy of consideration by commercial sheep farmers.
  • If the worms present in your sheep are Nematodirus, you should lower the threshold for drenching. These worms lay fewer eggs, so a low count may be masking a high worm burden.
  • We now have a wormtest kit/mail in service for horses.
  • We recommend that our clients keep the kits on hand. Charges are only issued after the testing is completed.​​​​

Drench Re​​sistance Tests

The old DrenchRite™ test is no longer available. It was proving to be somewhat unreliable in detecting resistance to the ML or mectin class of drenches.

Instead, we offer a "faecal egg reduction test" as a replacement. Essentially, this involves two standard worm tests - one using samples taken just before drenching and the other using samples taken 10 to 14 days after. It works out much cheaper for the producer but does involve having to yard the sheep twice.

If a faecal egg reduction test identifies resistance to the class of drench you are using, you should seek further advice about your options from your vet or animal health adviser.

Information on how to do a faecal egg reduction test is available at the NRE Tas Animal Health Laboratory, we can send out information on how to do this test.

Useful Web Links

The Australian Wool Innovation Limited website has a lot of useful information about worm control in sheep on its Wor​​mboss website.

The ​​​​NSW Department of Primary Industries has a lot of useful information on their Agriculture webpage Drench resistance and sheep worm control.

For Further Information:


Animal Disease Enquiries

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

Animal Health Laboratory - Specimen Reception

(Postal) PO Box 46,
(Delivery) 165 Westbury Road,