Viper Viper is a next generation sheep lousicide containing thiacloprid, a high potency neonicotinoid insecticide that knocks down lice quickly by affecting their nervous system, resulting in insect paralysis and death. Thiacloprid has been shown in laboratory studies to be 6 times more potent against lice than imidacloprid. Viper controls susceptible body lice (Bovicola ovis) when applied to shorn sheep up to 7 days off shears. Viper has a four week residual activity for lice (when applied within 24 hours off-shears), offering the best protection against re-infestation for complete peace of mind. It will continue to kill any lice that hatch from eggs following treatment and protect sheep from reinfestation from stray sheep or other lousy sheep for up to four weeks.