The rate of spread of sheep scab within small groups of Merino and Dorper sheep : short communication

Abstract

a Sheep scab, caused by the astigmatid mite Psoroptes ovis, is a highly contagious disease of sheep. Several factors influence the rate at which scab will spread within a flock. These include flock size, contact frequency between uninfested and infested sheep, age of flock members, sheep breed, season and husbandry methods 2 . In addition the extent of infestation on the initial carrier sheep can affect the rate at which the disease spreads. Autumn/winter groupTen healthy, year-old Merino and 10 year-old Dorper sheep were used in the study. During April 1997 (autumn/winter) 1 sheep of each breed was taken from its group and these 2 sheep were placed in separate quarantine camps approximately 1 ha in size. Mites were collected from a Merino donor sheep and the 2 sheep were each infested with 25 ovigerous female and 5 adult male P. ovis by parting their wool and placing the mites on their skin. The mites were held in position by twisting an elastic band around the wool tufts immediately above the site of infestation. The 9 remaining Merino and 9 Dorper sheep were pastured in separate quarantine camps, each approximately 1 ha in size, in which natural grazing was available. Alfalfa hay and maintenance pellets were supplied twice a week as supplementary feed.Both artificially infested sheep were found to have scab lesions 3 weeks after infestation and were returned to their original groups in the quarantine camps. All the sheep in the 2 camps were individually carefully examined fortnightly for the presence of live mites or active lesion development and the results recorded. The presence of a live mite, or active scab lesion was considered a positive infestation. After examination of each sheep the investigators changed their aprons and washed their hands to prevent mechanical transfer of mites. Spring/summer groupEarly in October 1997 10 Merino and 10Dorper sheep were exposed in the same manner as the previous groups. Two weeks after the artificially infested Merino sheep of the autumn/winter group was returned to its group it had scab lesions covering an area 158 cm 2 . None of the other Merino sheep showed any signs of irritation, nor were any sheep scab lesions visible. Four weeks after the introduction of the infested sheep the first signs of scratching and biting in the control sheep were apparent, although no lesions could be found. In the 6th week the artificially infested sheep's lesions covered almost half its body, and it had a very shaggy appearance, at the same time scab lesions were apparent on 3 of the control sheep. At 8 weeks, 6 of the Merino sheep were infested, and the lesions on the artificially infested sheep had increased to 2100 cm 2 , with large parts of its body denuded of wool. Two weeks later a further 2 sheep were infested, and lesions on those sheep that were already infested had rapidly increased in size. Twelve weeks after introduction of the infested sheep all sheep displayed visible lesions (Fig. 1).Two weeks after placing the artificially infested Dorper sheep with its group, no sig...