Unfortunately, it's very difficult to prevent cats from becoming infected with worms, as treatments do not prevent infection in adult animals. Ideally, cats should be wormed at least every 3 months. Monthly worming is recommended for cats that regularly hunt, as they may catch birds and mice which may be infected with roundworms or tapeworms. Regular worming will not only kill worms, but will also help to reduce the number of worm eggs shed into the environment.