Certain factors make parasite infestation in puppies more severe, meaning that worming puppies early and regularly is imperative.
Route of infection: Roundworm larvae can be passed via the placenta from the mum to her pups while the pups are still in utero, so pups can be born with existing roundworm infestations. Hookworm larvae can be secreted in the milk, meaning puppies can be infected simply by feeding from their mum.
Smaller size: The smaller size of puppies make them particularly sensitive to parasite infestations. As mentioned above, hookworm infestations can result in a life threatening anaemia which may require hospitalisation and blood transfusion for treatment of severe cases. Severe roundworm burdens in puppies can result in reduced growth rate and failure to thrive.
Immature immune systems: A puppy has an 'novel' immune system, which simply means that it has not been exposed to parasites, bacteria and viruses like adult dogs, and therefore has not had a chance to build up an immunity to fight off these invaders. So, they are often affected by higher numbers of parasites, and these parasites affect them more severely.
Risks of not worming puppies early
Failing to de-worm puppies early poses severe implications to their health, including anaemia (sometimes requiring hospitalisation and blood transfusion) and failure to thrive and grow appropriately.
Puppies that survive severe hookworm infections may continue to be "poor doers" with chronic anaemia. Infection with hookworms can also progress to respiratory disease and pneumonia when puppies are infected with large numbers of worms. Puppies can also often harbour migrating parasite larvae that later mature and commence laying eggs.
Pups with heavy infestations of roundworms may also vomit up a large number of (often live) roundworms at 4-6 months of age (ewww!!)
Since puppies often carry heavy burdens of worms, they act as a source of reinfestation in the environment to other dogs. In the case of whipworm, infective eggs can persist in the environment for several years!
Along with the health implications to the pups themselves, there are also important risks that worms passed from puppies and dogs pose to humans. Both hookworms and roundworms can infect people. Hookworm larvae can penetrate and migrate in the skin (called Cutaneous Larval Migrans), producing intensely itchy lesions on the skin. The larvae can also migrate to the intestines and cause enteritis, resulting in abdominal pain and blood changes. Roundworm larvae, if ingested via contaminated soil, can migrate through the body (called Visceral Larval Migrans) and cause systemic disease including neurological, hepatic, pulmonary and ocular disease, depending on the final destination of the larvae.
So, it's clear that early, and regular, deworming of puppies and dogs is crucial, not only for their own health, but also that of your family.