A composition prepared for treating animals suffering from parasites which parasites are known to be susceptible to at least one of the avermectins, milbemycins or salicylanilides, comprises for example ivermectin in an amount of from 0.1 to 10%(w/v), a solvent selected from the group consisting of glycerol formal, propylene glycol, polyethylene glycol and combinations thereof, and a salicylanilide such as closantel in a required dosage amount for the animal to be treated, typically about 2.5 mg/kg live weight of the animal to be treated, a polymeric species selected from the group consisting of polyvinylpyrrolidone and polyoxypropylene/polyoxyethylene block copolymers, the said polymeric species improving the bioavailability of closantel to the extend that blood plasma levels of the said compound greater than about 20 ppm over period of treatment are achievable.
