by Garry Levot, New South Wales Department of Primary Industries
Sheep can be treated for lice infestations by immersion in pesticide solution. Dipping equipment may be in-ground or mobile plunge dips or immersion cage dips. Mobile plunge or cage dipping services are provided by contractors in many areas. To find dipping contractors in your district contact the Livestock Contractors’ Association or look for advertisements in local papers or in telephone directories.
Irrespective of the apparatus used to dip sheep, it is essential for lice control that all sheep are wet to skin level at all sites on the head and body. Wool that is more open is easier to wet than tight, fine wool. Merino fleeces resist wetting, so it is important that dipping is done thoroughly and in accordance with label directions. This includes periodic checking of sheep for wetness, for example, by using a water-soluble marking pencil (e.g. Columbia ‘Copperplate’ Red Copying 2100) on the skin under, and on the back of the neck (see page 3 and 4 for details).
The dilution rate indicated on product labels must be used and nothing, other than a bacteriostat in some instances, should be mixed with registered products in a dip. Increasing the dip concentration or use of mixtures containing more than one dip product cannot counteract substandard application and should not be considered.
Immersion dipping is costly due to high labour and chemical costs. Dipping requires people to move sheep into, through and away from the dip and may include the hire of a contractor. Dipping costs include the construction of the dip, the water supply and the chemical used to charge the high volume of dip wash. Nevertheless, immersion dipping is the most thorough and robust means of treating sheep for lice infestations. However, to avoid infection through shearing cuts, sheep must be re-mustered for treatment a few weeks after shearing, with increased labour input.
At the end of dipping a large volume of spent dip solution containing chemical remains. Even if the dip will not be used again for several months, at some time it will need to be pumped out. Mobile operations need to pump out before they can move off-site. In preparation for dip disposal a bunded area with growing pasture should be prepared for this purpose. The bund is used to contain the dip wash within an identifiable area where it will soak in and where sunlight and soil bacteria will degrade the pesticide. Stock should not graze this area for at least 3 months unless a shorter or longer interval is indicated on the product label. It is unacceptable to allow spent dip wash to enter dams, ponds or any watercourse.
The addition of product at the dilution rate indicated on the label at the beginning of dipping.
The selective uptake of pesticide from the dip solution at a faster rate than the removal of water. As a result, the chemical is removed faster than the dip wash, which gradually decreases in concentration as dipping proceeds.
The ‘constant’ addition of fresh pesticide solution from supply tanks into the dip sump during dipping to maintain a constant volume (and concentration) of dip wash. Advantages of constant replenishment are less fluctuation in dip concentration and no interruption of dipping to replenish or reinforce the dip.
The regular addition of pesticide, but not water, to the dip. Reinforcement replaces the pesticide removed from the dip wash by stripping.
The addition of water and pesticide to the dip to replace the dip wash taken out by the sheep. If product label directions say so, topping up should occur after reinforcement, every time the dip volume drops to no less than 75% full.
The addition of product only (reinforcement) towards the end of dipping to minimise the amount of used dip wash for disposal. By reinforcing without topping up, dipping out allows the dip volume to drop to 50% full. To determine when to start dipping out, estimate the rate at which wash is being removed from the dip. Calculate how many sheep will take the dip to half its initial volume. Keep the dip at full volume until that number of sheep remain, then begin dipping out. Reinforce when the dip falls to 75% of its initial volume. Continue to dip out until the dip reaches half its initial volume then stop dipping and clean the dip. A dip must not fall below half of the initial level even when dipping out. The dip level should never be low enough to allow the sheep to walk in the dip.
Plunge dips may be in-ground or mobile, straight, ‘U’- or ‘S’-shaped. A schematic diagram of an in-ground plunge dip appears in Appendix 1, at the end of this note. At least two people and good dogs are needed. One operator will act as forcer to persuade sheep into the dip and the other will dunk sheep during their swim.
To achieve effective wetting of sheep and ensure eradication of lice by plunge dipping, the swim length should be at least 9m. The sheep should be dunked twice, not including the ‘splash’ entry, to completely wet the head and neck, with a preference towards backward dunking. A large spray nozzle can be used to replace one dunk and would be advantageous in maintaining dip wash circulation.
Only sheep that are fit and in good condition should be plunge dipped. All sheep need to be able to complete the swim and walk up the inclined exit ramp. Heavily pregnant ewes, weaners or sheep in poor condition are more susceptible to stress and should be dipped early in the day. Pregnant ewes and sheep in poor condition may be unable to climb out of the dip. Lambs should be drafted from ewes to avoid being trampled or drowned in the dip. If sheep ‘pile up’ at the exit point, dipping becomes inefficient and sheep can drown unless prompt action is taken.
Avoid dipping sheep on very hot days or in wet or very windy conditions. The sheep may be chilled severely by winds and will not dry out quickly in wet conditions. Start early in the day and finish early enough to allow the sheep to dry before nightfall. Hot, tired sheep should be rested prior to dipping, as skin absorption of chemical and drinking of dip wash can result in losses due to poisoning. The sheep should be yarded overnight, prior to dipping, with access to water, but not feed, to allow them to empty out. This reduces contamination of the dip by sheep faeces.
Different sized sheep should be drafted off and dipped separately to prevent smothering and drowning. Young sheep are more susceptible to infections and should be dipped first when the dip wash is cleanest. Sheep heavily infested with grass seeds are more prone to infection. Draft off diseased sheep, such as any with dermo, pink eye or open abscesses. Do not dip any other sheep after these before cleaning out the dip.
Forcing pens and the race leading to the dip entry should have slatted or concrete floors to reduce organic matter (faeces, soil, plant matter etc.) contaminating the dip. Sheep do not enjoy swimming and remember the experience; previously plunge-dipped sheep may become reluctant to enter the dip and may need to be encouraged into the dip. A range of design features can be used to improve sheep flow. The design of the lead up to the dip can incorporate a V-belt conveyor, hock bars, a curved lead up race and decoy sheep to attract the other sheep onto a dip slide entry after which it is too late for the sheep to choose an alternative path. Sheep may be delivered into the dip via a VE conveyer. This labour-saving device has the added advantage of positioning the forcer further from the dip splash zone.
Draining pens should be cleaned regularly to reduce the amount of organic matter carried into the dip. Sheep should not be held in the draining pens, but encouraged to return to their paddock as soon as possible to dry out. This will reduce the risk of mycotic dermatitis and subsequent flystrike.
Management of the dipping process in a calm and organised manner will ensure that sheep are not lost through drowning or by inhalation or ingestion of dip wash. Infection can be managed by ensuring skin is intact with sufficient healing time following shearing, yards are free of sharp projections, dogs are muzzled and sheep are free of grass seed infestations, as any hole in the skin can provide a site for bacterial infection from dip wash.
When using a plunge dip, particular attention should be paid to the following points:
Cage dipping is an extremely efficient means of treating large numbers of sheep quickly. Most cage dips are operated by contractors, and a single operator with good dogs can dip several thousand sheep in a day. There are several sizes of cage dip currently in operation. ‘Richards’ design cage dips operated by ‘Richards-trained’ staff are able to use diazinon to dip sheep for lice. This is because of several unique design features that protect the operator from exposure to dip spray or splash. It is illegal to use diazinon in any other cage dip design or plunge or shower dips. However, there are effective, safer to use alternative products available.
During the dipping process sufficient sheep to fill the cage are persuaded to enter by the use of decoy sheep. The cage lid is lowered and the cage is then lowered into the dip sump below, until sheep in the cage are completely submerged. Operator procedures vary a little. Some will immerse the sheep such that their backs are covered by solution, then raise the cage, before again completely submerging the cage momentarily (i.e. they are dunked twice). Others will simply submerge the cage for about 10 seconds (to achieve the equivalent of two short dunkings) before raising the cage and releasing the sheep. Hydraulic operation of the dip requires a reliable power supply or a dedicated generator. In the event of power failure dips have a ‘fail safe’ release system to either raise the lid, release the dip solution, or both.
Cage dipping is physically less stressful on sheep than plunge dipping. The same sheep management and dip hygiene precautions needed for plunge dipping also apply to immersion cage dipping.
Most dip chemicals are moderately toxic, particularly when concentrated.