Lincoln, Bruce. 2015. Toward a more materialistic ethics: Vermin and poison in Zoroastrian thought. Studia Iranica 44(1). 83–98.
Absent from the Older Avesta, vermin and poison first appear in a few verses of the Younger Avesta, whose authors misinterpreted Yasna 34.5c (where they mistook adjectival xrafstra- for a substantive) and Yasna 49.11c (whose ‘evil foods’ [akāiš xvarəθāiš] they took to be poison [viša-]). The Pahlavi texts take the argument further, developing a narrative in which these creatures and substances become prime weapons of Ahriman in his assault on Ohrmazd’s Good Creation. Speculation along these lines introduced novel understandings of evil as a lethal substance, rather than a destructive disposition or spirit, moving questions of morality from metaphysics to physics.