There are two species of roundworms affecting cats and kittens: Toxocara cati and Toxascaris leonina. Because both are treated with the same medication, it isn’t necessary to determine which species is present once eggs have been seen on a fecal flotation exam. Roundworms are big worms, typically white in color, about the diameter of a spaghetti-strand and about 4 inches long. These roundworms can be seen in your pet’s feces or sometimes your kitten or puppy may vomit one up.

Kittens or puppies may be born with roundworms. We know that Roundworm larva are tiny enough to migrate through the placental blood supply to the fetal liver or lungs. Soon after birth, the juvenile roundworms begin migrating from the liver and lungs to the intestine where they grow into adults and begin laying eggs. Within three weeks after birth, puppies and kittens can begin shedding roundworm eggs into the environment. A second source of infection is through the mother's milk. Roundworm larva can gain entry into the mammary glands and then be passed to the puppies or kittens as they nurse. A third possibility is if a young animal eats an embryonated egg found in fecal contamination of food or water bowls or other environmental sources. The egg hatches within the intestine and a "Blood-Lung" migration begins.

More about worms in cats.