There are two species of roundworms that can affect cats, Toxocara cati and Toxascaris leonina. Because both are treated with the same medication, determining which species is present is not necessary during diagnosis, which can be done with a fecal flotation exam.

Cats can become infected with roundworms by consuming infective worm eggs that have been passed from a host's feces and infected the soil, consuming an animal (such as a rodent) that is carrying developing worms, and nursing from an infected mother.

Once consumed, the eggs of the Toxocara cati hatch in the cat's intestinal tract and the young worms make their way into the other body tissues where they can remain for years. The worms typically encyst in a cat's liver, then migrate to the lungs where heavy infections can lead to pneumonia. Coughing results when the worms are in the upper airways and the worms are coughed up then swallowed and renter the intestinal tract once again where they mate and start the whole process over again.

In nursing mothers, the larvae migrate to the mammary gland, rather than the lungs, and enter the milk consumed by the kittens. Preventing mother to kitten transmission is usually very difficult, and kittens are often automatically dewormed as a precaution.

Roundworm infection can lead to diarrhea, vomiting, and a pot-bellied appearance. Very heavy infections can also result in pneumonia and obstruction of the intestine. In many cases, you may see no symptoms at all, though some cats will vomit up a worm. The worms are long and white, similar to spaghetti, and can be as long as seven inches in length. Fecal testing can determine if roundworms are present.