Are there effective medicinal therapies to prevent a listeria infection?

Hand hygiene: A panacea(if you will) against mostly all infectious diseases is practicing good infection control; good hand washing is the paramount among these practices.

Source control: Keep things clean. Cook meats and eggs thoroughly. Avoid hot dogs, deli meats, processed meats unless thoroughly cooked to 170 deg.Avoid unpasturized soft cheese. Do not leave proceesed food for prolonged periods at room temp. Listeria is a very real concern, with a high mortality rate. If you are pregnant, it can be devastating to your developing baby. See your doc if you suspect any infection.