Sheep scab control using trans-cinnamic acid

A series of in vitro and in vivo assays were conducted to examine the effects of trans-cinnamic acid ethyl ester on Psoroptes mange mites. In vitro, 24 h after exposure to the test material at concentrations of 10, 1 or 0.1% (v/v), 100, 74 and 20% of mites had died, respectively, compared to 8% following exposure to the control (0.05% SDS only). The different life-cycle stages were affected similarly by the test compound. The concentration required to produce 95% mortality 24 h after exposure to the test compound was 6.29% (95% confidence interval 4.98-8.88). Tarsal contact of the mites with the test compound was also sufficient to achieve high levels of mortality; 100% death was observed when the mites were placed in contact with either sheep skin circles treated to give 0.01 ml/cm2 or polyester cloth circles treated with 0.03 ml/cm2. However, the residual activity of both skin and cloth treated with 0.03 ml/cm2 was completely lost after 7 days. In vivo, trans-cinnamic acid ethyl ester suspended in 2% (w/v) lecithin was applied as a spray formulation to eight sheep with active artificial infestations of sheep scab. Seven of the 8 treated sheep were cured and remained completely clear of scab mites for 56 days. However, 33 days after treatment 2 adult female mites were observed on one of the eight treated sheep and the mite population on this sheep subsequently recovered. In contrast, in a control group of two infested sheep, treated with a 2% (w/v) lecithin only, mite populations increased as expected in a typical scab infestation, but eventually self-cured in one animal. The data suggest that, with appropriate development of suitable application technology, trans-cinnamic acid ethyl ester could have a role as a potential therapeutic treatment for active sheep scab, however the short residual period of activity is likely to limit its use in commercial sheep flocks.