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.
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.