Puppies are born with worms because their mother carries dormant larvae deep in her body tissues, and pregnancy hormones reactivate those larvae, sending them through the placenta into the developing puppies. This happens even if the mother shows no signs of infection and even if she was exposed to roundworms years earlier. The vast majority of puppies under six weeks old are infected with intestinal roundworms, making this one of the most common health issues in newborn dogs.
How Dormant Larvae Survive in the Mother
When a dog swallows roundworm eggs at any point in her life, the larvae hatch and burrow through the intestinal wall. Some of those larvae don’t complete their journey to the gut. Instead, they migrate into muscle, liver, kidney, and other tissues, where they form tiny cysts and go quiet. These resting larvae can survive for years without causing any symptoms. The mother’s immune system walls them off with a ring of inflammatory cells, essentially trapping them in place.
This is why deworming an adult dog doesn’t eliminate the problem. Standard deworming treatments kill active worms in the gut but can’t reach larvae locked away in body tissues. A female dog who was infected as a puppy herself will still harbor these dormant passengers when she becomes pregnant, and they’ll reactivate with every subsequent pregnancy, even if she’s never reinfected.
What Pregnancy Hormones Do to the Larvae
The reactivation process is driven by a cascade of hormones, not just one. During the first third of pregnancy, rising progesterone levels do two things simultaneously: they stimulate the larvae directly through hormone receptors on the cysts, and they suppress the inflammatory immune response that had been keeping the larvae contained. The mother’s immune system shifts toward a profile that tolerates the growing fetuses but also, as a side effect, releases the grip on dormant worm larvae.
Later in pregnancy, as progesterone drops, prolactin and estrogen take over. These hormones continue stimulating the larvae and suppressing the immune barriers around them. The result is a steady stream of newly mobile larvae entering the mother’s bloodstream throughout the second half of gestation. By the time the puppies are about halfway through development (around day 42 of a roughly 63-day pregnancy), larvae are crossing the placenta and settling into the puppies’ liver and lungs.
Two Routes: Placenta and Milk
Roundworm larvae primarily reach puppies through the placenta before birth. These larvae are already present in the puppies’ bodies on the day they’re born, and they begin migrating to the intestines immediately. Within about two weeks after birth, those larvae have matured enough to start producing eggs, which means a puppy can be shedding roundworm eggs into the environment before it’s even three weeks old.
The second route is through the mother’s milk. Hookworm larvae in particular travel to the mammary glands and pass into colostrum and milk. The heaviest transmission through milk happens during the first two weeks of nursing, but it can continue for five weeks or longer after delivery. This means that even puppies who somehow avoided infection in the womb pick up worms almost immediately through nursing. Between these two routes, nearly every puppy in a litter ends up infected.
Signs of Worms in Young Puppies
Lightly infected puppies may look perfectly healthy at first. As the worm burden grows, the earliest sign is usually a failure to gain weight at the expected rate. Puppies with moderate infections develop a characteristic pot-bellied appearance, a dull coat, and soft or mucus-covered stool. You might see whole worms in their vomit or feces; roundworms look like pale, rubbery strands several inches long.
Heavy infections are more serious. The worms compete directly with the puppy for nutrients at a stage when rapid growth is critical. Young puppies with severe burdens can develop diarrhea, stunted growth, and in rare cases, an intestinal blockage that can be fatal. Because the larvae also pass through the lungs during their migration, some puppies develop coughing or mild pneumonia in the first few weeks of life.
Why Deworming Starts So Early
Because puppies are infected before or immediately after birth, deworming begins at just two weeks of age. The treatment is repeated every two weeks until the puppy is eight weeks old, then monthly until six months of age. This aggressive schedule exists because no single dose kills all the worms. Larvae that are still migrating through the lungs or liver at the time of treatment won’t be affected, so each round catches a new wave of larvae that have since arrived in the intestines.
Treating the mother during pregnancy can significantly reduce how many worms her puppies are born with. When a pregnant dog receives daily deworming medication starting around day 40 of pregnancy and continuing through two weeks after birth, studies show an 89% reduction in roundworm burden and a 99% reduction in hookworms in her puppies compared to untreated litters. Stopping treatment at the time of birth rather than continuing through nursing is less effective, cutting roundworm numbers by only about 64%, because it misses the larvae transmitted through milk.
Risk to Humans, Especially Children
Roundworm eggs shed by puppies don’t just affect other dogs. The same parasite can infect people, a condition called toxocariasis. Eggs passed in puppy feces need a few weeks in soil to become infectious, so the risk comes from contaminated dirt, sandboxes, or yards rather than from handling a puppy directly. Young children are most vulnerable because they’re more likely to put dirty hands in their mouths or play in areas where puppies have defecated.
Most human infections produce no symptoms at all. When symptoms do occur, the larvae can migrate to internal organs, causing fever, coughing, wheezing, and abdominal pain. In rarer cases, a larva reaches the eye, where it can cause inflammation, retinal damage, or vision loss, typically in just one eye. Keeping puppy living areas clean, picking up feces promptly, and staying on schedule with deworming all reduce the number of eggs that reach the environment in the first place.