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This article was published in 1951
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INSTITUTE OF INSPECTORS OF STOCK OF N.S.W. YEAR BOOK.
Sheep Lice and Ked Control by Fogging
G.J. SHANAHAN, B.Sc.Agr., Entomologist, Department of Agriculture. N.S.W.
It is not always possible to free sheep infested with lice and ked by a single treatment in either a plunge or power spray dip. For years the Department has recommended two dippings, two to three weeks apart, especially in cases of heavy infestations by these ectoparasites, but very few graziers have adopted this practice.
Although partial control only is obtained in some instances with a single dipping, the number of graziers who have difficulty in controlling these parasites is relatively small. Seasonal conditions during the ensuing twelve months largely determine whether dipped sheep are infested or not at the next shearing; at least, favourable conditions limit the degree of infestation. This idea can be expressed in another way. Whilst sheep are dipped yearly on some properties, probably with a particular proprietary dipping material each year, and it can be assumed with a similar thoroughness, infestation of sheep by these parasites is met occasionally. Incomplete mustering, and re-infestation from a neighbour's property, from infested sheep purchased during the year, or from the stray sheep "generously" donated by drovers to graziers who have stock routes traversing their properties cannot be blamed always for the unusually heavy infestations sometimes found on the type of properties, managed by careful pastoralists, to which reference is made above. The explanation is offered that a light infestation can be maintained on a few sheep and in times when the level of nutrition is low or temperature and humidity conditions particularly favour rapid breeding by the parasites, then, in spite of dipping, medium or heavy infestations by ked and lice occasionally occur.
On properties where these parasites are found each year, and this is more usual with ked than lice, incomplete mustering is undoubtedly the main reason for the continuity of infestation. Hence, the treatment of sheep immediately off shears has much to commend it. Since it can be dangerous to treat sheep in a plunge bath or power spray dip before the shear cuts have healed, the Department does not recommend the treatment of sheep immediately off shears. In accordance with this recommendation most sheep are dipped two to eight weeks off shears.
Fogging.
An alternative method of treating sheep for control of lice and ked, namely fogging, has received much publicity during the past two years. This development has the advantage that the sheep can be treated immediately off shears. The removal of the necessity to muster for dipping appeals to graziers and it is possibly this factor which interested many of those who have already purchased fogging machines for use in controlling lice and ked.
It has been mentioned already that a single dipping of sheep in a plunge bath or shower spray dip cannot always be relied upon to eradicate lice and ked. However, it has been stated also that a second dipping increases the chances of achieving eradication of these parasites. Since there is now a new method, in fogging, available for use against these ectoparasites the question is "Will fogging completely free infested sheep of lice and ked?" Partial control is insufficient.
Whilst additional work is required to establish the limitations of fogging as a means of combating lice and ked the tentative conclusion can be drawn from the investigations so far that sheep lice cannot be eradicated by a single treatment with either 10% DDT or 2% gamma isomer BHC, when heavily infested animals are treated immediately off shears. No definite statement can be made concerning its value for eradicating keds.
Further research, in the form of a large scale comparative trial, is required also to show whether fogging with either of these insecticides or a combination of both will give a degree of control similar to that obtained from a single treatment in either a power spray or plunge dip. The value of fogging as a method of controlling lice and ked is discussed in this article. This discussion will be preceded by a brief account of the manner in which sheep are fogged.
Method of Fogging.
The usual procedure is to fog sheep in a tent, the dimensions of which are 30 feet x 20 feet, 5 feet high at the sides and 6 feet high in the centre. Tents about half this size and sheds, suitably adapted for the operation, are used occasionally. The sheep are fogged just off shears for 3 to 5 minutes. Two hundred sheep can be treated at a time in the 30 ft. x 20 ft. tent.
Insecticidal formulations with high concentrations of either DDT or BHC or these insecticides in combination are used. The concentration of either DDT or BHC may be as much as 100 and 400 times, respectively, higher than that used for normal dipping. Fogging is more expensive than dipping and costs about 2d. per head.
Effect of Fogging on Lice and Ked.
(a) Lice. When heavily infested sheep are fogged just off shears for four minutes with either 10% DDT or 2% gamma isomer BHC, either treatment will reduce the population during the next five weeks, but complete control is not obtained. BHC is more effective in reducing the lice population than DDT. Since BHC is a volatile insecticide and its fumes are toxic to insects, it is possible that the explanation for its superior ability in killing lice is due to its effect as a fumigant, in addition to its stomach and contact poison qualities. For example, BHC at this concentration will reduce markedly the lice infestation of fowls when used as a roost paint. Its action in this case is largely as a fumigant.
Whilst a high proportion of lice, both nymphs and adults, are killed or show symptoms of toxicity on fogged sheep, two to three days after treatment with either DDT or BHC, colonies of lice can be found with comparative ease in the long tufts of wool, such as are left when heavily infested sheep are shorn. Most of the lice in the closely shorn wool are dead by then although a few survivors can be found after careful searching in sunlight.
Live lice are difficult to find on fogged sheep two to three weeks after treatment. This applies particularly to BHC fogged sheep. Five weeks from treatment there is an indication that the surviving lice are beginning to multiply although the infestation at this stage is still light; more so from BHC than DDT treatment. It is possible that better control, and perhaps eradication of lice, can be obtained if lightly infested and well shorn sheep are fogged, especially with BHC, than would be the case with heavily infested sheep. Since lice can survive for five weeks following fogging, and furthermore, as it is difficult to find the parasites on sheep which have been fogged two to three weeks previously, work should be conducted to ascertain whether a second fogging then will eradicate the ectoparasite. If it can be shown that two fog treatments will eradicate the pest many graziers probably would be prepared to adopt this measure, since fogging is more rapid and less laborious than dipping in either a plunge bath or power spray dip.
(b) Ked. Insufficient work has been conducted to show whether fogging will control ked completely when infested sheep are treated immediately off shears. Preliminary work indicates that all keds are killed by the second day following fogging with either 10% DDT or 2% gamma isomer BHC. No live keds can be found on treated sheep for approximately three weeks after treatment. However, it is not known whether re-infestation can be established by the few keds which hatch from pupae shortly afterwards. The method looks promising. Here again, consideration might be given to a second fog treatment about three weeks after the initial fogging.
Effect of Fogging an Sheep and Operators.
Since many thousands of sheep have been fogged in this State during the past two years, with very few deaths, it is clear that fogging should not cause many mortalities. In instances where deaths have occurred it has been ascertained that the sheep have been fogged in practically air-tight enclosures or have been treated before being allowed to cool off after yarding. Sheep should be yarded several hours before fogging if they have been grazing on either pastures or crops containing a high proportion of legume plants. Sheep heavily infested with lung worm can be affected adversely by fogging and some deaths might occur within a few minutes of treatment.
There is reason to believe that the solvents, with which the DDT and BHC fogging formulations are prepared, are responsible for causing much of the distress to sheep both during and for about an hour after treatment. During fogging, sheep cough violently and leave the tent with streaming eyes and an increased respiration rate. These symptoms are more pronounced with the BHC than the DDT formulations.
Whilst there is little danger from acute toxicity by either the insecticides or the solvents, the chronic effect to sheep of repeated exposure, as may occur if fogging proves valuable for sheep blowfly prevention, will require investigation.
It is unlikely that operators will expose themselves unduly to insecticidal fogs. Both DDT and BHC fogs, which are generated for controlling ectoparasites, affect the eyes and irritate the respiratory passages, promoting coughing. Nevertheless, the warning is given that operators should avoid inhaling the fog and take special care to ensure that the high concentration fogging preparations do not make contact with the skin. Repeated exposure to either the fog or fogging preparations might eventually impair the health of the operators of fog machines.
Conclusions
The fogging of heavily lice infested sheep with either DDT or BHC or a combination of both insecticides will not eradicate this ectoparasite in a single treatment. The population of lice is reduced by fogging, especially with BHC. The suggestion is made that a second fog treatment with this insecticide two to three weeks after the initial fogging might eradicate the pest from infested sheep. Further work is required to ascertain whether keds can be controlled completely by fogging. The method appears promising for controlling sheep ked.