Why might a louse fail to establish itself on a new host animal?

A louse may fail to establish itself on a new host animal due to chemical or physical incompatibility with the host as a source of food or as a habitat.1

Because lice are highly specialized, they often require specific environmental conditions to survive and reproduce. For instance, some mammalian lice can only lay their eggs on hairs of a specific diameter. Furthermore, because lice are adapted to the unique peculiarities of their host's feathers or hairs to orient themselves, a new host may lack the necessary physical characteristics for the louse to successfully navigate or remain attached.1

These factors contribute to host specificity, where a louse species is typically restricted to a single host or a group of closely related species.1