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Dogs and cats, both young and old, are at risk for these nasty parasites, and because they are transmitted through fecal-contaminated soil, people, especially children, can become infected, as well. Symptoms in pets include coughing, digestive upset, a dull coat, and failure to gain or keep on weight. Younger pets and children can become seriously ill from a roundworm infection. Fortunately, roundworms can be avoided with a monthly preventive, and they are easily treated with deworming medication.


Roundworms are extremely common parasites that spend their adult lives in the intestines of puppies, kittens, dogs, and cats. There are several species of roundworms. Some can grow to about 7 inches in length and cause severe illness, especially in younger pets.

Mother dogs can pass roundworms to developing puppies in the uterus or through milk when the puppies are nursing. Kittens do not become infected in utero, but they can become infected when nursing. Adult roundworms live in the intestines, where they reproduce and lay eggs. An infected dog or cat sheds roundworm eggs into the environment when it passes feces. Once the eggs are in soil, the worms develop to the infective stage within the egg. Other pets can become infected by eating the eggs from contaminated soil, which often happens when pets groom themselves, sniff or lick the ground, or eat grass and other things outside. Alternatively, pets can be infected when they eat infected prey, such as birds and rodents.

Once roundworm eggs are eaten, they hatch in the digestive tract. In most cases, the worms then migrate through the liver and lungs. Once in the lungs, the pet coughs up the young worms and swallows them. They eventually make their way to the small intestine where they mature into adults and reproduce.

Roundworms are considered zoonotic parasites, meaning that they can be transmitted from animals to humans. Children are most at risk for infection. They usually become infected from eating fecal-contaminated soil, which is often found at playgrounds, backyards, and beaches frequented by pets.

In humans, roundworms are a significant cause of several types of larva migrans, an illness caused by migration of young worms through body organs such as the liver, lungs, and nervous system. Young worms may also travel to the eye, where they can cause blindness. This ophthalmic condition is called ocular larva migrans and occurs in the United States but is far more prevalent in developing countries.

If there are children in the household, pet owners should have their pets tested (at least annually) and treated for roundworm infection if they’re found to harbor them. Keeping pets on a monthly heartworm preventive that also controls roundworms is strongly advised. Children should wash their hands after handling pets or going to a playground, beach, or backyard. Keeping sandboxes covered when not in use is helpful in discouraging neighborhood cats from using them as litter-boxes.

Signs and Identification

Puppies and kittens are usually the most severely affected and often look potbellied. Other signs include:

  • Coughing

  • Dull, thin coat

  • Vomiting

  • Diarrhea

  • Failure to gain weight

Veterinarians can diagnose a roundworm infection by finding microscopic roundworm eggs on a fecal exam. Unfortunately, some owners discover that their pets are infected when live roundworms are expelled in vomit or feces.

Affected Breeds

All breeds of dogs and cats are equally susceptible.


Veterinarians routinely treat young pets with an anti-parasite medication several times, interrupting the worm’s life cycle via parasiticide treatment every few weeks until pets can be placed on a monthly heartworm preventive that also controls roundworms and other intestinal parasites.

Roundworm infections are very common in puppies and kittens, but eggs aren’t always apparent in fecal material from infected pets. Your veterinarian may therefore recommend deworming puppies or kittens even if a fecal test does not confirm a roundworm infection. Many anti-parasite medications kill only adult worms in the intestines, not the migrating younger worms or eggs. Therefore, if a pet is infected with roundworms, a veterinarian may recommend two to three rounds of treatment to clear the infection. Generally, fecal samples are rechecked after treatment to make sure the infection has been resolved.


Because roundworm eggs can remain infective in the environment for months to years, pet feces should be removed and disposed of immediately.

When walking your dog, keep him or her on a leash to help reduce exposure to areas that may have been contaminated by other dogs or cats. This will also minimize the chance of your dog eating infected rodents and birds.

If possible, cats should be kept indoors to prevent them from hunting infected prey. However, even indoor animals can catch infected mice. Sharing litter boxes and outdoor bathroom areas can spread roundworms among pets, so new pets should be tested for roundworms and other internal parasites before being introduced to your household.

Always consult a veterinarian about the best ways to protect a pet — and its human family — against intestinal parasites.

A monthly heartworm preventive that includes medication for roundworms is a good start for year-round protection from these parasites. Since it’s easy to forget a monthly treatment and no parasiticide is 100 percent effective, periodic fecal exams are still recommended to ensure that a pet remains parasite free.

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