Levels of drench resistance of sheep worms in WA are some of the highest in the world and without resistance management action at individual farm level, it will continue to worsen and spread. Although effective drench options are generally available, many sheep owners are unaware that resistance has reduced the effectiveness of worm control programs and this can affect profitable sheep production.
Important worms
There are two main groups of gastro-intestinal worms that affect sheep in WA.
Firstly, scour worms — including black scour worm (Trichostrongylus), brown stomach worm (Teladorsagia (Ostertagia)) and to a lesser extent, the thin-necked intestinal worm (Nematodirus) and large mouthed bowel worm (Chabertia ovina). These occur throughout the agricultural areas of WA and larvae are abundant on pastures in winter and spring. Signs of scour worm infection include ill thrift, diarrhoea and in severe cases, death. However, significant production losses (decreased liveweight gains and wool growth rates) usually occur before clinical signs become obvious.
Secondly, barbers pole worm (Haemonchus contortus) occurs mainly in areas that have significant rain during warm weather and pastures with areas that remain green over summer. Larvae are most abundant on pasture in autumn and spring and after significant summer rainfall. This worm sucks blood from the sheep and can cause anaemia (visible as pale mucous membranes of the gums and around the eyes), subcutaneous oedema (bottle jaw) and sheep deaths with little warning, in certain environmental conditions. Barbers pole worm infections do not cause sheep to scour.
Tapeworms (Moniezia) are also common in young sheep but there is little scientific evidence that tapeworm infection significantly affects sheep production and control measures are not considered necessary.
The worm life cycle
Adult worms of each of the species are found in a specific location within the sheep — black scour worm in the small intestine and barbers pole and brown stomach worms in the abomasum, or fourth stomach. Male and female adult worms at this location mate and the females lay eggs, which pass out of the sheep’s gut and onto the paddock in the sheep’s faeces.
To complete the life cycle, eggs in the faeces hatch to release first-stage larvae. These develop through two stages over several days to become infective third-stage larvae (L3s). Infective larvae move from faecal pellets to pasture plants, with most L3s located up to about 25 millimetres (mm) above the ground. If grazing sheep ingest them, L3s develop through further stages inside the sheep to become adult worms within about three weeks.
Development of parasites on the paddock depends on environmental conditions and the type of worm. Barbers pole worm thrives in warm, wet conditions, for example, in areas which experience regular summer rainfall. Black scour worms and brown stomach worms are generally present in largest numbers in the autumn, winter and spring in a typical Mediterranean climate and therefore occur throughout WA.
Larvae survive on pasture for long periods when temperature and moisture conditions are favourable. In winter, some larvae may survive upwards of four months, while in summer, with hot and dry conditions, most larvae are destroyed within a few weeks. In areas with relatively mild summers, small numbers of larvae will survive over summer, protected in dung pellets and can emerge later to infect grazing sheep when conditions improve, such as after autumn rains.
Worm control programs
Effective, sustainable worm control requires more than a drenching program. A total worm control program should be carefully planned, incorporating the latest advice from local veterinarians or sheep advisers.
Key components include:
- planned drenching — for example, time drenches to maintain worm control but also minimise selection pressure for drench resistance
- appropriate drench choice — drench resistance testing is recommended every two years to show which drenches remain effective
- planned monitoring of worm burdens using worm egg count
- breeding worm resistant sheep
- paddock and grazing management — for example, plan grazing rotation to reduce worm larval contamination of pastures and prevent exposure of young, susceptible stock to heavily contaminated paddocks. Good sheep nutrition will also help combat worms.
Drenches and drench resistance
There are several broad spectrum drench groups which treat most important worm species. These are the benzimidazoles (white or BZ drenches), levamisole (clear or LV drenches), the macrocyclic lactones (the MLs) and more recently, monepantel.
A number of combination drenches are also available: BZ/LV combinations, ‘triple combinations’ (an ML, white and clear drench), and combinations of BZs, LV and/or an ML with naphthalophos. Naphthalophos, an organophosphate, is highly effective against barbers pole worm and also against other worms if it is used in combination with other drenches.
Closantel is a narrow spectrum barbers pole worm drench, with prolonged action and efficacy against incoming larvae for up to about five weeks after treatment.
However, due to anthelmintic resistance — drench resistance in some worms due to genetic changes that allow them to withstand particular classes of chemical — some drench groups are far less effective than when first introduced. Resistance to the BZ (white) and LV (clear) drenches and the ML, ivermectin, is present in worms on virtually all WA sheep farms and on 25-50% of farms to abamectin and moxidectin.
A fully effective drench is one shown to be more than 95% effective (preferably 100% effective) in a drench resistance test completed within the last couple of years. Using less effective drenches can be a waste of time and money and can lead to severe problems with worm control and worsening drench resistance.
Drench resistance develops when a parasite population is exposed to any of the drenches. More frequent exposure increases the rate of development of resistance. Drench resistance can also be introduced onto the property in worms inside bought-in sheep.
Environmental effects can also increase the rate of development of drench resistance (the so-called ‘selection pressure for drench resistance’). An example of this is the traditional summer drenching program in WA.
Summer drenching: a double-edged sword
Summer drenching provides extremely effective sheep worm control but also places very high selection pressure on sheep worms for drench resistance. This is because the only worms left in the sheep after a summer drench are those resistant to the drench given. In areas with hot, dry conditions over summer, very few worm eggs and larvae remain on the pasture after summer, so most of the future worm population develops from eggs laid during the following autumn by the resistant worms surviving in the sheep. Consequently, there is increased resistance in the worm population in the next season.
However, if summer drenching of all sheep on a farm was simply abandoned without alternative worm control measures, major worm problems affecting sheep production and profitability are likely to occur during the following winter and spring.
Research by the Department of Agriculture and Food, Western Australia (DAFWA) led to new recommendations to modify summer drenching so that the selection pressure for drench resistance is reduced but good worm control is maintained. For the ‘summer-autumn drenching’ program, weaners and hoggets continue to receive drenches in summer but healthy mature sheep are drenched in autumn, if necessary.
Pre-lambing and marking drenches
Monitoring ewes with a faecal worm egg count (WEC) before lambing will indicate whether drenching is necessary. Ewes can also be monitored before lamb marking if high worm burdens are suspected. If the results are high, a drench can be given when they are in the yards at marking time.
Drenching lambs at marking is rarely justified as lambs seldom acquire significant worm burdens until at least three months of age. However, when ewes and lambs are under nutritional stress or ewes carry significant worm burdens, worm problems in lambs may occur at an earlier stage than usual. Faecal WEC monitoring will indicate whether drenching lambs before weaning may be worthwhile.
Weaning drenches
Drenching lambs weaned at 12-14 weeks after lambing commenced is recommended because lambs have not developed their natural immunity to worms at this stage. Lambs should be moved out of the lambing paddock as this is usually heavily contaminated with worm larvae. Where possible, a ‘clean’ paddock (one with low levels of infective worm larvae) with good pasture nutrition should be provided.
Delaying a first drench to lambs beyond about 16 weeks risks outbreaks of worm disease and if not weaned by this time, either a drench should be given or a WEC conducted.
Quarantine drenching
Background
Resistance by sheep worms to drenches in WA continues to increase. There are sheep worms on virtually all farms that are resistant to white (benzimidazole or BZ) and clear (levamisole or LV) drenches and BZ/LV combination drenches (containing a white and clear drench) only remain fully effective on less than 20% of properties. More than 80% of properties also have brown stomach worms (Teladorsagia (Ostertagia)) resistant to the macrocyclic lactone group of drenches, the MLs (active ingredients include ivermectin, abamectin and moxidectin).
Although the recently released new drench type (monepantel) will be very useful, it is essential that we maintain the effectiveness of all drench groups and prevent the development of resistance to new types for as long as possible. One strategy that farmers can use to help reduce the risk of introducing new strains of resistant worms on their property is effective quarantine drenching of introduced sheep.
To prevent the introduction of new strains of resistant worms onto a farm, purchased sheep or those returning from agistment should be given a quarantine treatment before loading or on arrival. The current minimum recommendation is to treat with three or four different drench types, including monepantel (Zolvix). The sheep should then be released into a wormy paddock to dilute any remaining 'super-resistant' worms.
What should I do?
All introduced sheep should be treated immediately before or after they enter the property.
Treatment with products from four different drench groups is recommended. If the drench resistance status of the source of the sheep is unknown the current minimum recommendation is:
- Monepantel (Zolvix) plus abamectin (an 'ML', or macrocyclic lactone), white drench (benzimidazole) and a 'clear' drench (levamisole).
Giving these three types is most easily done by using a triple combination drench (the ML should be abamectin), such as Hat-Trick, Pyrimide, Q–Drench, Triguard or Napfix. (The drenches can be given individually and abamectin could be substituted by moxidectin — Cydectin, Moximax, Moxitak, SheepGuard).
Zolvix (monepantel) is an important choice as this product, from the latest amino-acetonitrile derivatives (AAD) group of anthelmintics, is the only option guaranteed to have no resistance in WA. Giving this product at the same time as other products, including one of the most potent macrocyclic lactones, will minimise the risk of any resistant worms making it on to the property.
Drenched sheep should be released into a wormy paddock to help dilute any surviving 'super-resistant' worms amongst the resident population of worms already on the property. If a wormy paddock is not available then other options should be discussed with your local vet or sheep adviser.
For more information talk to your local vet or sheep adviser or contact your local WormWise contact at the department.
General drenching tips
Correct drenching technique is essential to ensure drenches are effective:
- Under-dosing is a major cause of drench resistance so it is critical that sheep are given the correct dose. A few of the larger animals should be weighed so that the dose for the heaviest sheep in the flock can be calculated. For drench groups with a greater risk of toxicity (especially naphthalophos), flocks with a wide range in liveweight should be drafted into smaller, more uniform groups and each group given its appropriate dose to prevent overdosing.
- Check drench guns regularly to ensure they deliver the correct dose.
- Place the drench gun over the sheep’s tongue during treatment, rather than in the front of the mouth. This helps to ensure that all of the drench is delivered to the rumen for maximum effect.
- To help improve the effectiveness of the white, ML and closantel drenches, feed can be withheld from sheep for up to 24 hours before treatment. This slows gut content flow through the sheep, keeping the drench in the gut for a greater time and increasing its uptake. Sheep should not be fasted before treatments with levamisole or naphthalophos as this could increase the risk of toxicity with these products. (Note: sheep kept off feed should have access to water.)
Faecal worm egg count monitoring
Faecal WECs are the most effective way of checking the burden of worms in sheep. Between 10 (minimum) and 20 (ideal) individual faecal samples from each mob to be checked should be submitted. Contact the WEC service provider before sample collection to obtain specific sample submission requirements. WEC monitoring can be used to confirm a parasitic problem, to indicate whether drenching is needed or not, whether drenching has been effective and also the level of parasite contamination being deposited in a paddock.
Breeding worm resistant sheep
Resistance to worms in sheep is heritable and breeding worm resistant sheep is one of the best long-term solutions to assist worm control and combat drench resistance.
As with all genetic programs, selecting within a flock can take several years to achieve obvious improvements. However, growing numbers of farmers, including ram breeders, have been selecting sheep for worm resistance for some time and have achieved good results.
Paddock and grazing management
Paddock and grazing management can assist worm control by limiting worm contamination on paddocks and ensuring good sheep nutrition to maximise immunity to worms.
Worm contamination
‘Clean’ pastures, that do not harbour a significant number of infective worm larvae, are a valuable asset in a worm control program. Reduced exposure of sheep to worms on paddocks can reduce the pick-up of worm larvae and therefore worm burdens. This can reduce the impact of worms on the sheep and reduce the frequency of treatments needed.
In general, worm larvae survive on dry pasture in summer for less than one month and four months or more on green pastures after the season’s break. The following list describes contamination levels of paddocks, starting with the cleanest (least contaminated):
- stubbles or crops
- pasture that has been de-stocked (for more than four months in winter or one month in summer)
- pasture grazed only by cattle
- pasture grazed only by adult dry sheep.
Paddocks grazed by pregnant or lactating ewes or by lambs or weaners are often highly contaminated. Problem paddocks should be stocked with animals with better immunity to worms such as older dry sheep or cattle (few worm species will establish in both sheep and cattle) to minimise the impact of the contamination.
Sheep immunity
Sheep develop a natural immunity to worms after they have been exposed to larvae for some months and are usually relatively immune by the time they reach about 12 months of age.
Good immunity to worms results in fewer larvae establishing to develop into adult worms, adult worms being rejected and passed out of the sheep and adult worms producing fewer eggs.
Scour worm immunity may fluctuate but it remains throughout the animal’s life, whereas immunity to barbers pole worm is lost several months after exposure to worms or larvae ceases. Immunity of ewes is reduced for up to two months after lambing, resulting in an increased egg output and more severe effects of worms on the ewes. Lambing paddocks can therefore become heavily contaminated with worm larvae and lambs should always be moved from the lambing paddock after weaning.
Sheep nutrition
Good general sheep nutrition is an important component of an overall worm control program as sheep in poor condition have a reduced ability to deal with the added stresses of a worm burden. To minimise worm impact, weaners should be well grown and reach liveweights of at least 25kg by the beginning of their first summer. Older sheep should be maintained above condition score two. Mineral and trace element deficiencies can also add to the stresses on sheep and local advice on the need for supplements is recommended.