Roundworms in Cats

Ascarids are the most common worm parasite in cats, occurring in a large percentage of kittens and in 25 to 75 percent of adults. There are two common species that infest the cat. Adult ascarids live in the stomach and intestines and can grow to 5 inches (13 cm) long. The eggs are protected by a hard shell. They are extremely hardy and can live for months or years in the soil. They become infective in three to four weeks after being passed out in stool.

The cat passes eggs in her stool or larvae in her milk (1). The larvae infect her nursing kitten. Eggs from the stool (2) develop into larvae (3) and are eaten by rodents (4). The cat then eats the rodents while hunting. If the larvae pass through the kitten before maturing, the mother cat can also reinfest herself while grooming her kittens.

Cats acquire the disease by ingesting the eggs, perhaps through contact with soil containing the eggs, by them licking off their feet, or by eating a host animal, such as a beetle or rodent, which has acquired encysted larvae in its tissues. The larvae are then released in the cat's digestive tract.

Larvae of the common feline ascarid Toxocara cati are capable of migrating in tissues. Eggs, entering orally, hatch in the intestines. Larvae are carried to the lungs by the bloodstream. There, they become mobile and crawl up the trachea where they are then swallowed. This may cause bouts of coughing and gagging. They return to the intestines and develop into adults. This version of migration is most common in kittens.

In adult cats, only a few larvae return to the intestines. The others encyst in tissues and remain dormant. During lactation, these dormant larvae are released, reenter the circulation, and are transmitted to kittens in the mother's milk. When the queen is shedding larvae in her milk, she may not pass any eggs in her stool. Therefore, it makes sense to deworm both mother and kittens starting about 3 weeks of age, even if a fecal exam is negative.

Deworming the queen before or during pregnancy does not prevent all ascarid infestation of kittens after birth, but it will decrease the frequency and severity. Medications do not eliminate encysted larvae.

The second most common feline ascarid is Toxascaris leonina. This ascarid is not passed via the milk into nursing kittens but can be acquired by ingesting the eggs or by eating infected rodents.

Ascarids usually do not produce a heavy infestation in adult cats, but may do so among cats who do a lot of hunting. In kittens, a heavy infestation can result in severe illness or even death. Such kittens appear thin and have a pot-bellied look. They sometimes cough or vomit, have diarrhea, are anemic, and may develop pneumonia as the worms migrate from the blood vessels to the air sacs of the lungs. Worms may be found in the vomitus or the stool. Typically, they look like white earthworms or strands of spaghetti that are alive and moving.

Treatment: Pyrantel pamoate is a safe, effective choice and can be used in nursing kittens. Kittens should be dewormed by 3 weeks of age to prevent contamination of their quarters by ascarid eggs. A second course should be given two to three weeks later to kill any adult worms that were in the larval stage at the first deworming. Subsequent courses are indicated if eggs or worms are found in the stool. Many veterinarians suggest deworming kittens monthly until 6 months of age.

Pyrantel pamoate dewormers can be obtained from your veterinarian. You do not have to fast your cat before using this medication. Be sure to follow the directions of the manufacturer about dosage. Milbemycin, ivermectin, and selamectin are also very effective dewormers, but they are generally used in older kittens and adult cats.

Public health considerations: Ascarids can cause a disease in humans called visceral larva migrans. This is considered to be a serious public health problem and is one of the top zoonotic diseases. Most cases are caused by the canine ascarid, Toxocara canis, but Toxocara cati also can produce this disease. Some cases are reported each year, usually from areas with a mild climate. Children are most frequently affected, and often have a history of eating dirt. Outdoor sandboxes should be covered when not in use to prevent cats from using them as litter boxes, and gloves should be worn when gardening.

This article is excerpted from “Cat Owner’s Home Veterinary Handbook” with permission from Wiley Publishing, Inc.