Internal Parasites in Puppies - Symptoms and Treatment

Knowing how internal parasites spread and how to prevent them is essential to avoid harm to our puppy and their spreading to other dogs or people.

Unlike external parasites like fleas or ticks, internal parasites are usually found in the pet's intestine, so we can't see them with the naked eye, and diagnosis is done through a fecal test (coprology).

Below, we'll explain the steps to follow to prevent internal parasites in puppies from birth through adulthood.

What internal parasites can affect a newborn puppy?

Puppies under 3 months old shouldn't go outside, since they haven't yet completed their vaccination schedule; and although this reduces the risk of infection, it doesn't mean they're 100% protected against internal parasites.

These puppies can become infected through their mother (if she isn't properly dewormed) or if they come into contact with parasites present in the environment (we can carry parasite eggs on our shoes, clothes, or hands if we've been in contact with other dogs, so it's very important to wash our hands before handling puppies and take off the shoes we walked outside in when we get home).

The main parasite affecting puppies before weaning is Toxocara canis, a type of roundworm (similar to spaghetti) that settles in the intestine of infected dogs.

This parasite lays eggs that are released into the environment through the feces of infected animals, and if our female dog swallows these eggs while sniffing other dogs' feces, she can become infected — a high-risk situation for her puppies, even before they're born.

Infection in adult dogs is unlikely to be linked to clinical signs, so it's hard to know if a dog is infected with Toxocara canis unless routine fecal tests are run.

How does a puppy get infected with Toxocara canis?

During pregnancy, if the female isn't properly dewormed and carries Toxocara canis, the larvae in her intestine migrate to the placenta and mammary glands, infecting the puppies before birth or during nursing.

Although the main source of infection for newborn puppies will be their mother, remember that strict hygiene measures should be followed to keep puppies from coming into contact with contaminated environments (shoes, hand washing...).

What symptoms does Toxocara canis cause in puppies?

Puppies under 5 weeks old are especially vulnerable to this parasite, since when they swallow its larvae, these cross the intestinal wall and enter the bloodstream, reaching the lungs. Through coughing, they travel up to the mouth and are swallowed again, reaching the stomach and intestine once more, where they finally become adult worms that lay eggs excreted in the feces.

This migration through the lungs usually doesn't happen in adult dogs, but in puppies just a few weeks old, it's an especially dangerous parasite, since it can cause pneumonia and even death.

The main symptoms seen in an infected puppy are:

• Diarrhea.
• Vomiting.
• A bloated belly.
• Coughing and breathing problems.
• Weight loss and anemia.
• Poor coat condition.

If we notice any of these symptoms, we should take the puppy to the vet urgently so they can start treatment to eliminate the internal parasites and improve the associated symptoms.

How can we prevent internal parasites in puppies?

The best way to prevent infection in puppies is for the mother to be properly dewormed before and during pregnancy/nursing.

There are specific products for deworming pregnant females (e.g., fenbendazole), and a deworming schedule should be followed that's set up and supervised by a vet.

IMPORTANT! Most products commonly used to deworm adult dogs are contraindicated in pregnant/nursing females, so don't give any of these medications without first checking with a vet.

How do we deworm a pregnant female internally?

If we're planning to breed our female dog, we should deworm her internally before mating.

Toward the end of pregnancy, we should deworm her again (giving fenbendazole from day 40 of pregnancy onward prevents Toxocara canis from passing to the puppies).

It's also recommended to deworm the female during nursing (this should be done at the same time as the litter's first deworming treatment).

How do we deworm nursing puppies?

If we don't know the mother's health status and there's a chance she's carrying Toxocara canis and may have infected the puppies, we should do an early deworming of the puppies at 15 days old with specific products (e.g., fenbendazole syrup or paste). This treatment should be repeated every fifteen days until two weeks after weaning. From then on, monthly deworming should be done until 6 months of age.

If we've properly followed a preventive deworming schedule for the mother, we can delay the puppies' first internal deworming until 4-6 weeks of age, before their first round of vaccinations.

How do we deworm puppies who already go outside?

Once the vaccination schedule is complete, around 3 months of age, the puppy can start going outside, and the range of internal parasites that can affect them widens:

• Roundworms (Toxocara canis, Dirofilaria immitis...).
• Flatworms (Taenia spp., Echinococcus granulosus, Dipylidium caninum...).

Because of this, it's a good idea to start using broad-spectrum antiparasitic products that cover both roundworms and flatworms (e.g., milbemycin).

As for how often to deworm, my recommendation as a vet is to keep a monthly schedule until 6 months of age, and then switch to internal deworming every 3 months for life.

In high-risk situations (dogs living in a household with young children, immunocompromised people, or with access to gardens or parks), it's recommended to keep the monthly schedule into adulthood as well.

An alternative to regular deworming would be to run monthly or quarterly fecal tests, and only deworm our pet if they're found to be infected.