Scabies is a highly contagious skin disease caused by Sarcoptes scabiei var hominis. It is associated with serious life-threatening secondary bacterial infections caused by Group A Streptococcus and Staphylococcus aureus. There is no vaccine and the most commonly used therapeutics, oral ivermectin and topical permethrin, mainly target the parasite nervous system, killing only the motile stages. Therefore, they have sub-optimal efficacies and require repeat treatments. Prolonged use of current drugs and patient incompliance to repeat treatments have led to emerging parasite resistance. This highlights the importance of a single dose treatment that targets all the stages of the S. scabiei life cycle. In addition, current scabicides do not show anti-bacterial properties to manage the complications caused by scabies associated bacterial pathogens.
Two novel compounds (ADF and FDF) were tested in vitro using S. scabiei in bio-assays and pre-clinical, using a porcine scabies model. Their antibacterial effects at the best scabicidal concentrations were tested on S. aureus, S. pyogenes, Acinetobacter baumannii using MIC and MBC assays. In vitro results indicated excellent miticidal and ovicidal effects of compounds ADF and FDF respectively. When used in combination, the efficacy was significantly raised. The lethal time (LT100) to kill all mites and young eggs was 2h, and for late-stage eggs 8h. Four groups of 8 scabetic pigs each were used in a pre-clinical evaluation of the drug combination. Single topical application of the combined formulation on scabetic pigs for 4h or 8h resulted in complete cure of the infection by day 5 post-treatment. The treatment outcome was significantly better than that of two subsequent doses of ivermectin(p<0.05). Pharmacokinetic studies demonstrated rapid absorption and sufficient bioavailability of ADF and FDF. In addition, the combination treatment showed excellent anti-bacterial effects in vitro against scabies associated pathogens with significantly lower minimum bactericidal concentration (MBC) than the scabicidal concentration. We propose that our novel treatment is a promising next generation single application scabicide that will prevent scabies and associated secondary bacterial infections.