BLOWFLY STRIKE IN SHEEP

New Zealand Journal of Agriculture, Volume 65, Issue 5, 16 November 1942, Page 275

BLOWFLY STRIKE IN SHEEP

Considerable economic loss results from the ravages of the sheep blowfly, particularly in the Merino and half-bred country of Marlborough and in Hawke’s Bay, although a smaller number of cases of strike do occur » in other sheep-raising districts of the 'Dominion. Not only is there loss of wool and loss of condition in struck g

sheep but death of the animal will follow a severe strike which is not detected or treated early enough. The labour necessary for supervision and treatment of flocks, especially at the present time, contributes a great deal to the total cost to the farmer of this pest.

gy ”

L. K. WHITTEN,

, Parasitologist,

. i I, „ Animal Research Station, Wallaceville.

THE fly which is responsible for the vast majority of strikes over the greater part of New Zealand is the common brown blowfly, CalV.phora laemica. The greenbottle, Lucilia sericata is more important than the former only in Canterbury and Otago. The bluebottles, C. quadrimaculata and C. erythrocephala are of quite minor importance in connection with strike in sheep although they will infest meat and carrion. Life History During its lifetime the adult female fly lays about 1000 eggs in batches of 50-100 on carrion or on a susceptible sheep. Some flies lay living maggots, others eggs or maggots, depending on the temperature and humidity. The eggs are small yellowish-white elongated bodies; within one or two days they hatch out and the young larvae or maggots begin to feed actively. They grow very quickly and after two moults reach their maximum size in from two days to three weeks. The maggots have a narrow head and a broad posterior end on which can be seen two dark spots, the breathing

pores or spiracles which enable them to breathe while their heads are deeply buried feeding. After maximum growth is reached the maggots crawl away, seek a sheltered position or burrow into the soil where they become pupae, encased ■in hard shells, within which the tissues of the maggots are changed into those of the adult flies. Under suitable conditions of temperature and moisture the adults emerge from the soil, dry their wings and fly away. The pupae may remain dormant over the winter or the adults may emerge in 1-2 weeks during the summer. The adult females require a protein meal before they begin laying eggs, usually three days after emergence. The life cycle from egg to egg takes about one month in the summer time and the adult .■ may live at least a month during which time it can travel considerable distancesten miles and more. Blowfly Strike The common types of strike encountered in New Zealand are crutch strike, tail strike, body strike, head strike in

rams, and a small proportion of infestations of wounds, shear cuts, accidental wounds, marking wounds, facial eczema lesions, footrot lesions, and prolapsed vaginae. Crutch strike is usually associated with urine soiling and occasionally with soiling with faeces. Repeated soiling by a misdirected stream of urine from an irregular or distorted vulva, or by the presence of medial skin wrinkles on the breech produces, after prolonged wetting of the skin with subsequent decomposition, inflammation and exudation, an area which is highly attractive to the fly. Body strike may occur on any part of the body other than the crutch but is most frequent on the back in the region of the withers. Very often it is associated with “fleece rot” or

“water rot.” Back strike or body strike is relatively, more important in New Zealand than it is in Australia It is usually encountered in Merino and. halfbred hoggets after a period of heavy rain when the fleece remains moist and high humidity prevents thorough drying. Prevention In the Merino where wrinkliness of the breech is an important factor in producing a susceptible state, the logical method of attack is to breed for plain bodied sheep. This would be a long and costly process and as most New Zealand Merinos have little development, as compared with those of Australia, this aspect of the problem is less important than in the latter country. The surgical removal of offending skin folds which would become soiled with urine has been used with considerable success. Folds running down each side of the centre of the crutch are removed with shears starting well up towards the side of the tail and finishing 2-3 inches above the inside of the hock. The sheep is held on a rail as for docking, but the hocks should be bent or the folds will be obliterated as the skin is stretched. The folds are raised with the fingers

so that half an inch of skin is removed on each side of the base of the fold. If part of the bare skin is included in the fold, the latter should be rolled outwards so that none of the bare skin is removed. It is a simple matter, to remove the skin without damaging the deeper tissues and the wounds produced, if reasonable care is taken with cleanliness, heal completely within a fortnight leaving an otherwise susceptible sheep quite as immune as one naturally plain in the breech. Australian workers have shown recently that the length at which the tail is docked has a considerable effect on the incidence of breech strike. Sheep with tails four inches long suffered less than those with two-inch tails, while those with tails docked close to the buttocks suffered most. It appears that the longer tails press the breech wrinkles apart and so reduce the liability to wetting. In New Zealand where vulval deformity is a more important factor in urine soiling than the presence of breech wrinkles, the effect of tail length may be different from that observed in Australia. Macfarlane suggests that a tail which extends beyond the upper third of the vulva may produce distortion and subsequent soiling of the breech.

Shearing and Crutching Shearing renders sheep insusceptible to fly strike for some considerable time to where possible this should be done just before the worst fly period. In Marlborough ewes in particular shoulci be shorn before Christmas if necessary . leaving wethers and dry sheep till later. Crutching, the removal of wool from the breech, above the tail and well down the back of the hind legs, is the best method of controlling crutch strike. Crutching of lambs in Decem- , ber or early January will give considerable protection when fly waves occur in late January and February. Crutching normally gives protection for about six weeks. Jetting Jetting is a practice which has been developed and used for some years in Australia with excellent results. Two to three pints of a one per cent, suspension of calcium arsenite is forced into the wool of the crutch, at a pressure of 40-90 lbs. per square inch, from a pump which may be hand or power driven. Fluid which drains away may be re-used if a properly constructed jetting race.is available. Jetting will give protection for about a month and

has the advantage that it can be applied at short notice, requires less labour and expense than crutching and can be carried out by ordinary station hands. It can be applied at the beginning of a fly wave in which case small strikes, less than three inches in diameter, are jetted but considerable risk attends the jetting of severe strikes. Intermediate strikes may be jetted at reduced pressure. Dipping Dipping as carried out for the destruction of ked and lice has little or no effect in blowfly prevention. T rapping Trapping of blowflies has little effect on the incidence of fly strike in sheep unless the concentration of traps is so great that the procedure becomes uneconomic. Fly traps may be used to advantage close to dwelling, sheds, and slaughter-houses to reduce the fly population in the immediate vicinity. The trap most suitable for this purpose is the Western Australian Trap which can be made easily from two

kerosene tins and wire gauze. It consists of a tin, the top and three sides of which have been largely replaced by gauze. The remaining side has a small trap door for the removal of dead flies. Into this tin is soldered a gauze pyramid with an. opening at the top large enough to admit the blowfly. The trap is then mounted on a second tin which contains the bait, usually raw meat, liver or offal with sufficient water to keep it moist. The addition of 3 per cent, sodium sulphide solution makes the bait more attractive and more lasting. On each side of the bait container a horizontal slit is cut three or four inches long and half an inch high. The flap of tin is not removed but bent inwards to make a slightly inclined platform on which the flies can alight before entering the trap. After feeding on the bait the flies fail to return through the slits but are attracted upwards by the light, pass through