Then if that mouse is eaten by a dog the encysted larval forms develop in the dogs intestine. This is referred to as mucosal migration . The encysted second stage larvae in the host tissues are hormonally triggered in the third trimester of pregnancy to migrate to the mammary tissue and uterus where they are passed to the unborn pups. These parasites are mature by the time pups are three weeks of age. Nursing bitches can be infected with immature forms shed in the feces of their pups.
Parasites depend upon their host for survival.
Thus it is in the parasites best interest not to kill its host. However large infestations of ascarid worms can cause significant problems. Large numbers can cause obstruction of the small intestine. They can migrate into and obstruct the bile duct. An owner may notice a general unthriftiness and poor quality hair coat.