Evaluation of the therapeutic and protective efficacy of doramectin against psoroptic scabies in cattle.

Three studies were conducted to evaluate the therapeutic and protective efficacy of doramectin when given by injection at a dose of 200 micrograms/kg against induced Psoroptes otis infestations of cattle. The first study investigated therapeutic efficacy. Mite infestations were established on 15 test animals held in stanchions by transfer of material from infested donor calves. Test animals were then allotted on the basis of mite counts to a treatment group (10 animals) which received doramectin and a control group (5 animals) which received saline. Skin scrapings were collected for mite counts on the day before treatment and on days 7, 14, 21 and 28 after treatment. Efficacy assessed on the basis of the proportion of animals cured by day 28 was 100%. The second study was designed to determine the duration of protective efficacy. Forty-eight scabies-free heifers were allotted to a treated group of 32 which received doramectin, and a control group of 16 which remained untreated. These treatment groups were each divided into eight subgroups. Commencing on treatment day and continuing at weekly intervals for 7 weeks, a subgroup of animals from each treatment was placed in stanchions and challenged by transfer of material from infested donor calves. Skin scrapings for mite counts were collected 7 and 14 days later. Infestations were successfully established on all untreated control calves. Doramectin prevented the establishment of infestation for three weeks and significantly (P < 0.05) reduced infestation levels for an additional two weeks. The third study established the duration of residual protection conferred by doramectin and ivermectin under contact transmission. Ninety-six scabies-free heifers were divided into two equal treatment groups. Animals in one group received doramectin and animals in the other group received ivermectin at its recommended dose of 200 micrograms/kg by subcutaneous injection. Each treatment group was then divided into eight subgroups of six animals. Commencing on treatment day and continuing at weekly intervals for 7 weeks a subgroup of animals from each treatment was exposed to purposely infested seeder animals for one week. Three animals from each treatment subgroup were then placed in individual stanchions in which grooming was prevented and the other three were placed together in a pen where normal grooming behavior was permitted. Skin scrapings for mite counts were collected at weekly intervals for up to 4 weeks. Doramectin provided complete protection against infestation for five weeks compared to four weeks for ivermectin. These periods were not influenced by grooming behavior.