The disclosure relates to a topical formulation for treating a lice infestation, comprising an effective amount of an avermectin insecticide dissolved in an oil phase and an aqueous phase comprising one or more preservatives to inhibit the growth of microorganisms. Olive oil is present at a level of about 20% to about 30% by weight of the formulation, and the aqueous phase is buffered with a buffer comprising citric acid and/or sodium citrate to a pH that is from about 4.5 to about 6.2. The oil phase comprises a water-miscible or water-soluble surface active agent, which may comprise polysorbate 80, cetyl acetate, and acetylated lanolin alcohol, olive oil and shea butter as suspending agents, and a non-ionic surfactant selected from the group consisting of: oleyl alcohol, lanolin alcohol, sorbitan tristearate, bees wax, erucyl alcohol, ricinolyl alcohol, arachidyl alcohol, capryl alcohol, capric alcohol, behenyl alcohol, lauryl alcohol, myristyl alcohol, cetyl alcohol, stearyl alcohol, isostearyl alcohol, oleyl alcohol, palmitoleyl alcohol, linoleyl alcohol, elaidyl alcohol, elaidolinoleyl alcohol, linolenyl alcohol, elaidolinolenyl alcohol and combinations thereof. The formulation may exhibit an enhancedresistance to growth of one or more microorganisms selected from the group consisting of Candida albicans, Aspergillus niger, Escherichia coli, Pseudomonas aeruginosa, and Staphylococcus aureus.
