Signs of Worms in Cats

How do you know if your cat has worms? Several symptoms present themselves when your cat is experiencing a worm infestation, and knowing the signs will protect your cat's health.

Overview
Worms are intestinal parasites. The most common types in cats are roundworms, hookworms, tapeworms and lung worms. Worms need to be treated by a veterinarian, because there are different treatments for the different types of worms, and it isn't always easy for a non-professional to detect or identify worms.
Symptoms
Common symptoms of worms in cats are diarrhea, constipation, worms in stool or near the anus, weight loss, abdominal bloating, vomiting (sometimes with worms in vomit) and trouble breathing.
Roundworms
Roundworms are several inches long and look somewhat like spaghetti. Sometimes they are seen in stool or vomit, but not usually.
Tapeworms
Tapeworms look like grains of rice or cucumber seeds and may be seen around the anus.
Causes
The main way cats get worms is by eating feces from infected cats. Cats can get lung worms by eating infected slugs or snails (or by eating rodents or birds that have been infected by the slugs or snails). For this reason, outdoor cats are more susceptible to worms. Kittens can get worms from their mother.
Prevention
One way to reduce cases of worms is to keep cats inside. It's important to keep cats flea-free. Change the litter box often; wearing gloves while changing the litter can help prevent humans from getting worms.
Worms You Can't See
Hookworms and lung worms are impossible to see. Because it isn't always possible to see worms, a veterinarian will need to test a cat's stool for eggs. During parts of a worm's life cycle, eggs won't be present, so retesting might be necessary.
Resources
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Pet Center
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ASPCA
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Pet Education