An ascarid is a roundworm, and it’s one of the most common intestinal parasites in dogs. Two species infect dogs: Toxocara canis and Toxascaris leonina. Of the two, T. canis is far more significant because it’s more widespread, poses a greater health risk to puppies, and its larvae can also infect humans. Adult ascarids are stout, cream-colored worms that live in the small intestine. T. canis adults typically measure 10 to 15 cm long (roughly 4 to 6 inches), while T. leonina is slightly smaller at 5 to 10 cm.
How Dogs Get Ascarids
Puppies are often born with ascarids already inside them. During late pregnancy, dormant larvae that have been sitting quietly in the mother’s tissues reactivate and cross the placenta into the developing puppies. This transplacental route is the primary way T. canis spreads to newborns. Puppies can also pick up larvae through their mother’s milk, though this is a less common route.
Older dogs typically become infected by swallowing ascarid eggs from contaminated soil, grass, or surfaces. A female roundworm can shed tens of thousands of eggs per day into the environment through a dog’s feces. Once in the soil, those eggs develop into an infective stage and can persist for months or even years under the right conditions. Dogs can also acquire ascarids by eating rodents, birds, or other small animals that have ingested the eggs. The larvae encyst in the tissues of these animals, and when a dog eats them, the parasites complete their development into adults in the dog’s small intestine.
Signs of an Ascarid Infection
Many dogs, especially adults, carry ascarids without showing any obvious symptoms. When signs do appear, they’re most pronounced in puppies and young dogs. The classic presentation is a puppy that isn’t growing well, has a dull coat, and looks “potbellied,” with a round, distended abdomen despite being underweight everywhere else.
Other signs include:
- Diarrhea, sometimes with mucus
- Vomiting, occasionally with visible worms
- Coughing, caused by larvae migrating through the lungs in the early stages of infection
- Worms in the stool, which look like pale, spaghetti-like strands
In heavy infections, a mass of worms can physically block the intestine, which is a medical emergency. This is rare but most likely in very young puppies with a large worm burden inherited from their mother.
How Ascarids Are Diagnosed
Veterinarians diagnose ascarid infections through a fecal flotation test. A small stool sample is mixed with a special solution that causes parasite eggs to float to the surface, where they’re collected on a microscope slide and examined. Roundworm eggs have a distinctive appearance that makes them relatively easy to identify. The slide needs to be read within about 15 minutes of preparation for accurate results.
Sometimes the diagnosis is even simpler: you or your vet see actual worms in the dog’s stool or vomit. Adult ascarids are large enough to spot with the naked eye. In puppies under three weeks old, a fecal test may come back negative even when worms are present because the larvae haven’t matured enough to produce eggs yet. For this reason, vets typically deworm puppies on a set schedule rather than waiting for a positive test.
Treatment and Deworming Schedules
Several deworming medications effectively kill ascarids. The most commonly used active ingredients include pyrantel pamoate, fenbendazole, milbemycin oxime, and moxidectin. These are available in chewable tablets, granules, and topical solutions. A single dose kills the adult worms in the intestine, but because it doesn’t always reach migrating larvae, repeat treatments are necessary to catch new worms as they mature.
The Companion Animal Parasite Council recommends starting deworming at just 2 weeks of age, then repeating every 2 weeks until the puppy is 2 months old. After that, monthly deworming continues until 6 months of age, then quarterly for the rest of the dog’s life. This aggressive early schedule reflects how common transplacental infection is. Most puppies are assumed to have ascarids unless proven otherwise.
Many popular monthly heartworm preventatives also contain ingredients that treat and control roundworms with each dose. Products combining ivermectin with pyrantel, or milbemycin oxime alone, provide ongoing protection against ascarids as part of routine parasite prevention. If your dog is on a monthly heartworm chewable, there’s a good chance it already covers roundworms.
Risk to Humans
Ascarids are zoonotic, meaning they can infect people. Humans are accidental hosts. You can’t catch them from petting your dog; infection happens by accidentally swallowing T. canis eggs from contaminated soil or surfaces, or less commonly by eating undercooked meat from an infected animal. Children who play in dirt and put their hands in their mouths are at highest risk.
In humans, the larvae can’t complete their life cycle, so they don’t develop into adult worms. Instead, they wander through body tissues, causing a condition called visceral larva migrans. Symptoms can include fever, coughing, wheezing, belly pain, and an enlarged liver. If a larva reaches the eye, it causes ocular larva migrans, which typically affects only one eye and can lead to vision problems, eye redness, seeing spots or flashes, or an abnormally colored pupil. Many infected people never develop symptoms at all, but the potential for eye damage makes prevention important.
Keeping Your Yard and Home Safe
Picking up dog feces promptly is the single most effective way to reduce environmental contamination. Freshly passed roundworm eggs aren’t immediately infective. They need time in the environment (usually 2 to 4 weeks, depending on temperature and humidity) to develop to an infective stage. Removing feces daily eliminates most eggs before they become a threat.
Ascarid eggs are remarkably tough once they reach the infective stage. They resist most common disinfectants and can survive in soil for years. Hard surfaces can be cleaned more effectively, but contaminated soil is essentially impossible to sterilize without removing it entirely. This durability is why prevention through regular deworming matters more than trying to eliminate eggs from the environment after the fact. For households with young children, keeping play areas free of dog feces and washing hands after outdoor play significantly reduces the chance of accidental exposure.