These are probably the most common worms seen in cats. A large percentage of kittens are born with roundworm larvae in their tissues, which is transferred from the mother across the placenta. Roundworm larvae may also be transmitted to nursing kittens from the mother's milk. Adult worms develop and mature in the intestinal tract, and may pass eggs through the stool, which may reinfest other cats that come into contact with the stool. When worm eggs hatch inside your cat, young roundworm larvae are released internally and migrate to the lungs. Kittens with active roundworms in the intestines often have a potbellied appearance and don't grow as well as non-infected kittens. In rare and severe cases, intestinal blockages may occur. In adult cats, the roundworm larvae can encyst in the body tissues, and remain dormant for periods of time, later becoming activated during periods of stress or pregnancy. Most common wormers for roundworms only work on the adult parasites in the digestive tract.
Tapeworms are fairly common in juvenile and adult cats, and are typically transmitted to cats that ingest fleas and cats that like to hunt and eat wildlife and rodents. Tapeworms can reach up to six inches in the digestive tract and often have as many as 90 segments, which often may be seen in the stool or around the rectum of infected cats. This is the most common way that tapeworms are diagnosed. In most cases, tapeworms cause very few clinical signs in affected pets, with mild illness and weight loss the most common symptoms.
Although more common in dogs, hookworm infections do occur in cats. These blood sucking parasites are often found in the small intestine of infected animals. Hookworms may be transmitted in the uterus of infected pregnant dogs, as well as through contact with the larvae in stool contaminated soil, or from ingesting the eggs. Hookworms are also transmitted through the milk to nursing kittens. Hookworms most commonly cause diarrhea, which may be bloody, as well as causing severe anemia in heavily infested kittens.