PROBLEM TO BE SOLVED: To provide an external parasite controlling agent for animals having excellent controlling effects on an external parasites such as lice, fleas, mosquitoes, gnats and flies； and a method for preventing or treating infections from external parasites.SOLUTION: An external parasite controlling agent for animals contain at least one kind of compounds selected from a heterocyclic compound represented by the formula (IV) or the like and salts thereof as an active ingredient. A method of preventing or treating infection from an external parasite comprises administering the external parasite controlling agent for animals to an animal.【課題】シラミ、ノミ、カ、ブヨ、ハエなどの外部寄生虫に対して優れた防除効果を有する動物用外部寄生虫防除剤並びに外部寄生虫由来感染症の予防若しくは治療方法の提供。【解決手段】式（IV）等で表されるヘテロ環化合物およびそれらの塩から選ばれる少なくとも１種の化合物を有効成分として含有する動物用外部寄生虫防除剤。当該動物用外部寄生虫防除剤を動物に投与することを含む外部寄生虫由来感染症の予防若しくは治療方法。【選択図】なし
