Immature fleas pick up infection from the environment and may carry the eggs of an immature form of tapeworms. If the flea is accidentally swallowed by an animal whilst grooming, the tapeworm can develop inside the dog or cat's gut. Once inside your pet, the tapeworm continues to grow and may reach as much as 60 cm long. If you have seen fleas on your pet you should treat your pet with a product to remove tapeworms as well as getting on top of flea control.