Midrash su Levitico 14:47

10) I might think that a beast and a gentile, (who are not subject to tumah) would "rescue" clothing (upon them) in a plague-spot house; it is, therefore, written (Vayikra 14:46) "he shall be tamei until the evening" (followed by [Vayikra 14:47] "and he who eats in the house shall wash his clothes") — One who himself is subject to tumah "rescues" clothing in a plague-spot house. This excludes a beast and a gentile, who, not being subject to tumah, do not rescue clothing in a plague-spot house.