Parasites & Pests

Worms

What are worms?

There are several types of internal parasite that can cause significant health issues in sheep – usually worms and liver fluke and occasionally 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.

The Barber's Pole worm ( Haemonchus contortus) is the most dangerous internal parasite of sheep as it sucks blood and when present in large numbers can kill sheep.

Drench resistance is a severe problem in managing sheep worms.

Worm infection is a significant animal welfare issue and, if not treated, can cause death.

Liver fluke are a serious problem in some areas of Australia

Lambs will often show signs of tapeworm infection, but this parasite generally causes little harm

What about the Barbers Pole Worm

This worm sucks blood and as such is the one most likely to cause death in sheep. It is the most important parasite in summer rainfall areas. For this reason it is a major problem in northern NSW and SE Qld, but can also be a problem in coastal areas of other States or anywhere receiving sufficient summer rain, or under irrigation. When an outbreak occurs, it is usually initially seen in the weaners. Typical signs include pale mucous membranes, “bottle jaw”, collapse, weakness and death. Similar signs are also seen in severe liver fluke infection.

Because of its size, approx 35 mm in length, it is the easiest worm to find in the abomasum. A red and white spiral can be seen in female worms – this is the winding of the blood filled intestine around the white uterus, full of eggs.

Under good conditions, numbers can increase rapidly, and catch farmers unaware. This worm needs a lot of moisture and higher temperatures for its development. Unlike Ostertagia and Trichostrongylus, it is not very resistant to dry or cold conditions. The female worm is a prolific egg-layer, producing up to 10,000 eggs per day, perhaps to compensate for its poor survival on pasture.

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. Talk to your veterinarian for relevant advice on parasite control in your area.

The essential elements of a worm management plan include:

Worm testing. - faecal egg count monitoring. It is cheap and easy to do. Without regular testing, you won’t know whether your sheep have a worm 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 (eg Quarantine drench) 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.

To ensure that the mob does need to be drenched, do a worm test beforehand.

Worms - The signs to look out for

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.

Other signs of worm infection you may see are anaemia or swelling under the jaw (commonly called “bottlejaw”). This is usually a sign of severe Barbers Pole Worm or Liver Fluke infection.

If you suspect a worm problem, it is worth doing a worm test to confirm it. Remember that “ill thrift” and scours, often 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, as 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 in the case of Barbers Pole worm, the number of parasites sucking blood becomes life-threatening.

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 will increase the selection of drench resistant worms.

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 four main classes of broad spectrum drench:

Benzimidazoles or BZs, commonly known as white drenches
Levamisole or LEVs, commonly known as clear drenches
Macrocyclic lactones or MLs, commonly known as mectins
Amino-Acetonitrile Derivative - Monepantel (Zolvix) a new drench group

“Combination drenches” are mixtures of two or more of these drench classes.

Narrow spectrum drenches such as Naphthalophos ( Rametin) and Closantel ( Closimax) are used for Barbers Pole Worm control. Rametin is often combined with BZ and LEV (Rametin Combo) for broadspectrum worm control.

Narrow spectrum drenches containing triclabendazole and praziquantel are used for treating Liver Fluke and Tapeworm infections. Broad spectrum drenches have little or no effect on these parasites.

As Drench Resistance is becoming an increasing problem, and triple combinations of anthelmintics from different drench groups are being used more frequently.

To determine which drenches are effective on your farm, conducting a Drench Resistance test every 2-3 years is recommended. Seek veterinary advice on doing this..

In most cases, drench is administered orally by a drench 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 drench gun is calibrated to deliver the right dose. Drench guns have a dial of some kind to adjust the dosage. After you have set the dosage rate for your sheep, squirt a 10 doses into a measuring container to check for accuracy.
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 local climate, age of the sheep and worm burden.

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. Use regular worm testing to monitor the worm burden, and only drench when necessary.

Seek local advice on recommended programs for your area. For many areas of Southern Australia 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 DPI. 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) is an increasing problem in mainland flocks and is quite widespread overseas. Barbers Pole Worm and Ostergia parasites in particular have developed severe ML resistance.

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?

Conduct a drench test to find out which drenches or combinations of drenches remain effective on your farm.

Only drench sheep when worm tests show it is required.

Rotate between effective drenches - Don't rely on the same drench every time you treat your sheep, and where possible use a narrow spectrum instead of a broad spectrum drench.

Lambs and young sheep are most at risk from internal parasites, and may require more frequent drenching than older sheep.They should also be given the best source of nutrition.

Give a quarantine drench to introduced stock, including rams and keep them isolated from the main flock for several months. For best results use a 3 way combination including Zolvix as the inittial quarantine drench and a Rametin drench the following day.

Make sure you are administering the correct dose. Check the required dose on the label of the drench container. Underdosing is one of the major causes of drench resistance, as it helps the worms develop a drench resistance.

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. Drench guns should be serviced regularly.

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. Don't lift the head while drenching - Keep the sheep's head in a horizontal position to avoid drenching into the lungs

After drenching, put the sheep onto clean pasture wherever possible.

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..

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.

Based on information from Tasmanian Department of Primary Industries, Parks, Water and Environment