It is still wise to use a broad-spectrum parasite preventative for your indoor cat. If you also have a dog, they can bring fleas into the house. Sometimes, indoor cats sneak out when they shouldn't. Lastly, you still need to protect your cat against dangers such as heartworm disease and intestinal parasites. It is still wise to use a broad-spectrum parasite preventative for your indoor cat. If you also have a dog, they can bring fleas into the house. Sometimes, indoor cats sneak out when they shouldn't. Lastly, you still need to protect your cat against dangers such as heartworm disease and intestinal parasites.