Dogs and puppies are vulnerable to two types of roundworms. One, Toxocara leonina, remains in the dog's intestines, while the other, Toxocara canis, travels to his lungs. A puppy can pick up Toxocara canis in his mother's uterus or through her milk. He can pass both forms along in his feces.
Toxocara canis is the most common form of roundworm for a puppy, who can become infected in four ways. In most cases, a puppy picks up the parasite while in his infected mother's womb; or he might pick up Toxocara canis by nursing from his infected mother. If your pup catches and eats an infected animal, such as a squirrel, he's at risk for hosting both forms of roundworm. An infected puppy or dog can spread Toxocara canis and Toxocara leonina when he takes care of his daily business, as roundworm eggs live in feces.
According to Mar Vista Animal Medical Center, even after your pup's poo has melted away in the ground, roundworm eggs live on for months, even through harsh conditions. All your guy has to do is walk through infected soil, pick up a microscopic egg and ingest it during his normal grooming process to become a host. Puppies should be wormed according to your vet's recommendation to ensure they're parasite-free. As well, it's wise to pick up after your pup, avoid areas where poo "minefields" may exist, and have your pup tested for infection regularly.