On the Protection of Sheep from Maggot-fly
Authors: MacLeod J. 
Published: July 2009

Research on the control of maggot-fly on sheep has in the past been seriously handicapped by the uncertainty of natural incidence of strike ; many experiments have been rendered invalid by the failure of the controls to react, the absence of controls, or by reason of the fact that the incidence of strike was so low as to make differences between comparative groups of no significance. For these reasons much of the published evidence does not bear critical examination.

There are now available, however, critical methods for the estimation of the protective value of dips and sprays, and it is essential, if further progress in maggot-fly control is to be made, that use be made of these methods for the accurate assessment of the larvicidal and repellent properties of experimental dressings, and for the duration of effect of these properties. A continuation of the present tendency to general statements that such and such substances were found to protect sheep for so long, without specification of what precisely the sheep were being protected against, and especially when unaccompanied by detailed protocols which will.allow the reader to exercise an independent judgment of the reliability of the results, will only lead to further discordance in a subject already suffering from a lack of uniformity in the standards of different workers, and the absence, even, of any precise definition of much of the technical jargon associated with it.

In the first part of this paper the nature and potentialities of the three possible functions of a maggot-fly dip or spray have been outlined, and the appropriate terms defined. It has been pointed out that field tests, for which it is difficult and sometimes impossible to furnish logically acceptable control data, are unreliable and may be definitely misleading ; at best they still leave the investigator uncertain as to which function of the dressing is responsible for the protection conferred, and consequently handicapped in his efforts to improve still further the protective value of the preparation under test.

In dealing with the maggot-fly problem, two principal lines of attack are possible ; the fly may be prevented from laying its eggs, or alternatively, the fleece may be so impregnated with toxic chemicals that the eggs, or any larvae which succeed in hatching from these, are killed. If a completely reliable repellent were available, no larvicide would be necessary ; conversely, if a thoroughly effective larvicide were available, a repellent would be not only unnecessary but also undesirable, for the more eggs which the flies could be induced to lay on sheep the better.

A third possible line of attack is the use of an antiseptic, which neutralises the bacterial activity in the fleece, and so ends the attraction of the sheep for the gravid female fly. Since, however, such neutralisation is so easily overcome by accidental fouling of the fleece, scouring or other cause, it is obviously unsafe to rely on this negative virtue alone for immunity from attack. The overcoming of the antiseptic property is the chief reason why carbolic dips, which rely on their antiseptic value, tend to have no effect, or even to have a harmful effect, after the dipping solution begins to get fouled by urine and faecal matter. With larvicidal dips, this fouling is of much less consequence, for the active protection conferred by the toxic chemicals prevents the blows which result from any such fortuitous “ attractivation ” from developing to strike.

With regard to the two principal lines of attack mentioned above, it may be stated here that prolonged and thorough tests have failed so far to reveal any substance possessing a reliable repellent action, which remains completely reliable for more than a few days. In all the commonly used repellents tested, the protection was found to be uncertain and seldom of a high degree, It appears, therefore, that we have yet to find the ideal repellent, and until such time it is obviously safer to rely on larvicidal protection.

It is equally obvious that, to ensure thorough penetration of the larvicide, it must be applied by actual immersion of the sheep. Spraying, though the ideal method for applying a repellent, cannot be regarded as suitable in the case of a larvicide, since it coats only the outer part of the wool staple with the spray fluid, and does not thoroughly saturate the fleece down to the skin surface.

With regard to the experimental evidence given in detail in the present paper, the results may be held to suggest the following conclusions:—

1. Carbolic dips neither repel the adult fly nor prevent larvae from establishing active strike on the skin.

2. Cresylic acid, incorporated in a mineral oil base, is not effective as a spray or as a dip in protecting susceptible sheep or preventing development of strike from blow.

3. Arsenic remains in the fleece, and continues to exercise a protective effect against development of strike for some weeks after application. This protection may be only partial (Table III) or almost complete (Table II, IV and VII) ; under winter conditions it lasts for about four weeks, and in the case of one form of soluble arsenic at least, it has been shown to be only slightly affected by heavy rain.