Dogs get roundworms in four main ways: from their mother before or shortly after birth, by swallowing infected eggs from contaminated soil, by eating an infected animal, or by ingesting feces containing roundworm eggs. The most common route depends on the dog’s age. For puppies, the overwhelming majority of infections come directly from the mother during pregnancy. For adult dogs, swallowing microscopic eggs from the environment is the primary path.
Transmission From Mother to Puppies
The single most common way puppies get roundworms is from their own mother, before they’re even born. During late pregnancy, dormant roundworm larvae that have been sitting quietly in a female dog’s muscle, liver, kidney, and connective tissue reactivate and migrate into the developing puppies through the placenta. This transplacental transfer is so efficient that roundworms can be found in a puppy’s intestines as early as one week after birth.
Puppies can also pick up larvae through their mother’s milk during nursing. This transmammary route is a secondary pathway, but it adds to the worm burden in an already vulnerable animal. Together, these two routes explain why roundworm infection in young puppies is nearly universal, even when the mother appears perfectly healthy and shows no symptoms herself. The mother may have carried arrested larvae in her tissues for months or years with no outward sign of infection.
Swallowing Eggs From the Environment
Adult dogs most commonly get roundworms by ingesting microscopic eggs from contaminated soil, grass, or surfaces. An infected dog sheds millions of eggs in its feces. Those eggs aren’t immediately dangerous. They need one to four weeks in the environment to develop into an infective stage. Once they do, they’re remarkably tough. The eggs have a thick protective shell that allows them to survive in soil for months or even years under the right conditions.
A dog doesn’t need to eat feces directly. Sniffing contaminated ground, licking dirty paws, or chewing on a stick or toy that touched infected soil is enough. Parks, yards, and any outdoor space where dogs defecate can harbor these eggs long after the feces has been cleaned up or washed away by rain.
Eating Infected Animals
Dogs that hunt or scavenge can pick up roundworms by eating smaller animals that carry dormant larvae in their tissues. Rodents, rabbits, and birds can serve as transport hosts: they swallow roundworm eggs from the environment, and the larvae migrate into their tissues without developing further. When a dog eats one of these animals, the larvae reactivate and mature into adult worms in the dog’s intestine. This route is especially relevant for dogs that live in rural areas or have strong prey drives.
What Happens Inside the Dog’s Body
What happens after a dog swallows roundworm eggs depends on its age. In puppies younger than about three months, the larvae hatch in the gut, burrow through the intestinal wall, and travel through the bloodstream to the liver and then the lungs. From the lungs, they’re coughed up, swallowed back down, and finally settle in the small intestine, where they grow into egg-producing adults. This migration through the lungs is why heavily infected puppies sometimes cough.
In older dogs, this migration pattern changes. The larvae still hatch and penetrate the intestinal wall, but instead of completing the circuit through the lungs, they tend to get stuck in various tissues throughout the body: muscles, kidneys, connective tissue, and the liver. There they enter a dormant state, sometimes for the rest of the dog’s life. These arrested larvae don’t cause obvious illness, but they’re not gone. In a female dog, pregnancy hormones trigger them to wake up and migrate into her puppies, starting the cycle all over again.
Symptoms to Recognize
Dogs of any age can carry roundworms, and many adult dogs show no symptoms at all. Puppies are far more likely to show visible signs because their small bodies can’t tolerate a heavy worm load. Common signs include diarrhea, vomiting, a bloated or pot-bellied appearance, dull coat, weight loss, and stunted growth. In severe cases, the sheer number of worms can physically block a puppy’s intestine, which is a dangerous complication. Young puppies with heavy infections face real nutritional depletion because the worms are competing for every calorie the puppy takes in.
You might also see adult worms in your dog’s vomit or stool. They’re white or light-colored, several inches long, and look like spaghetti. But the absence of visible worms doesn’t mean your dog is clear. The microscopic eggs are invisible to the naked eye, and a fecal test at your vet is the only reliable way to confirm an infection.
Prevention and Deworming Schedules
Because maternal transmission is so reliable, veterinary guidelines recommend starting deworming treatments in puppies at just two weeks of age. Treatments are repeated every two weeks until the puppy begins a regular parasite prevention program. If year-round prevention isn’t maintained, the recommended schedule is deworming every two weeks until two months old, then monthly until six months, and quarterly after that. Adult dogs should receive broad-spectrum parasite treatment at least four times a year.
Beyond medication, reducing environmental exposure makes a real difference. Picking up your dog’s feces promptly, before the eggs have a chance to mature into their infective stage over one to four weeks, breaks the cycle in your yard. Preventing your dog from eating rodents, rabbits, or other small animals eliminates another transmission route.
Risk to Humans
Roundworms from dogs can infect people, making this more than just a pet health issue. Humans pick up the infection by accidentally swallowing eggs from contaminated dirt or unwashed hands, not from direct contact with a dog’s fur or saliva. Children who play in soil and put their hands in their mouths are at highest risk.
In humans, the larvae can’t complete their life cycle, but they don’t just die off harmlessly. They can migrate through the body and cause two forms of illness. Visceral toxocariasis occurs when larvae travel to organs like the liver or central nervous system. Ocular toxocariasis occurs when a larva reaches the eye, potentially causing inflammation, retinal damage, or vision loss, typically in just one eye. Prompt deworming of your dog and basic hygiene, particularly handwashing after handling soil, are the most effective ways to prevent human infection.