While some microscopic creatures are perfectly normal for your cat to carry around in his stomach, others are not. Sometimes parasite eggs get into your cat's digestive system and hatch into worms that snatch your pet's food, preventing him from getting the nutrition he needs. The most common of these undesirable parasites are roundworms, tapeworms and coccidia. Roundworms look like thick white threads; if your cat has a bad case, he might throw them up. Their eggs are passed out in your cat's stool, but you can only see them with a microscope. Meanwhile, tapeworms attach themselves to your cat's intestinal lining and grow segment by segment. Once a segment has ripened, it's shed out in your cat's stool. While the eggs can't always be seen with a microscope, sometimes the segments can be spotted sticking to your cat's rectum. They look like little grains of rice. On the other hand, coccidia aren't real worms -- they're microscopic one-celled organisms.
Cats usually pick up worms by eating something that carries them, like fleas with tapeworm eggs or infected birds or mice that they hunt. You can't always spot symptoms that point to a worm problem, although sometimes kittens get diarrhea, gain weight slowly or have a weird pot belly. Adults can also get diarrhea or vomit when they have worms; sometimes they lose weight and have dark stool, too. If you've never checked your cat for worms, take a stool sample to the vet. If your cat has been cleared as worm-free but then got fleas or killed a couple of mice, it's time to check again.
Don't use over-the-counter treatments to worm your cat; they're just not strong enough. Your vet can give you get right medications and the dosages. Once you've wormed your cat, don't be surprised if he has some stomach problems for a little while.
