A topical formulation comprising as active ingredients, at least one effective agent derived from Streptomyces avermitilis, i.e. a macrocylic lactone e.g. an avermectin or chemically modified or synthetic derivative thereof, e.g. ivermectin, together with another anthelmintic of the sulphonamide type, e.g. clorsulon, in a carrier that facilitates topical administration and delivery of the active ingredients transdermal Iy, e.g. a carrier that is useful for this purpose comprises alcoholic solvents, such as ethanol, and isopropanol, with optional excipients and formulation aids, which may comprise a polymeric species such as PVP or a poloxamer.