Roundworm in Dogs and Puppies

roundworm in dogs and puppies

There are 20,000 different roundworm species and the vast majority of them are parasitical. They can be found in rodents, wild animals, horses, pigs, pets and humans. Both dogs and cats can be infected by roundworms (ascarids) and the two that most commonly infect dogs and puppies are Toxocara canis and Toxascaris leonina. There is a third one, less common but still found in dogs, called Baylisascaris procyonis which is actually a raccoon parasite.

Roundworms in Dogs and Puppies


Dogs can become infested in the womb (specifically T. canis) when the larval form of the parasite travels across the placenta. After birth,  transmission can occur through the mother’s milk. Roundworm infection is common in puppies and a typical symptom of an extreme infestation is a tight swollen belly which may give the appearance of being well fed were it not for the pup’s dull coat, listlessness and failure to thrive.

Roundworm eggs are passed out of an infected dog in its feces and will then stay in the soil where they will eventually become infective  (after about 4 weeks) and will be ingested by a new host. And lastly dogs can also become infected with roundworm by eating a carrier ( a dead mouse or squirrel, for example).

Does My Pup Have Roundworm? 

As mentioned above there is a classic pot-bellied, listless appearance to pups with a roundworm infestation. Symptoms to watch out for:

  • Pot belly
  • Dry, drab coat
  • Failure to thrive
  • Coughing or gagging
  • Bronchitis
  • Diarrhoea
  • Loss of appetite
  • Vomiting

Pups with a particularly heavy burden of ascarids may vomit them up. This is quite horrifying as by this stage the worms will be large and many may be alive and moving.

Toxocara canis LifeCycle(French version)

Toxocara canis LifeCycle(French version) (Photo credit: Wikipedia)

Lifecycle of the Roundworm

Once they have infected the host, roundworm larvae migrate through the liver and lungs, are carried up the mucociliary apparatus, and then are swallowed to develop in the small intestine. When this migration occurs in fetal pups, the migrating larvae wait in the liver and lungs until the pups are born, at which time they resume their migration across the lungs to the airways.

Larvae acquired from ingestion of vertebrate tissues do not migrate in the dog but instead travel to the small intestine to become adult worms. Toxascaris leonina is different from the other dog and cat ascarids in that migration outside the intestinal tract does not occur in their usual definitive hosts. In the intestine the roundworms develop into adults which pass eggs which are then excreted and the cycle begins again.

Encysted Roundworm Larvae

In some cases, the roundworm larvae become encysted in the host tissue. In the case of rodents, the larvae will stay in this state until it current host is eaten by a cat or dog and then it will proceed to infest that new carrier. In female dogs,  encysted larvae spring back to life during pregnancy ready to infest the new generation.

Encysted roundworm larvae are protected from the dogs own immune system and also from most deworming treatments. One which is effective against encysted larvae is Interceptor.

Treatment for Roundworm

An anthelmintic is a drug used to expel parasitic worms and it works by either stunning the worm so that it releases its hold on the intestine or by outright killing them. These are commonly known as vermifuges (stunning) or vermicides (killing). Some, like the one pictured are broad spectrum, meaning they are effective against roundworm, tapeworm and hookworm and are available over the counter. Others, like Interceptor ,which you would use in the case of an encysted larvae, require a veterinary prescription.

Some flea control products also contain anthelmintic chemicals. These are the ones affective against roundworms:

  • Albendazole,– effective against threadworms, roundworms, whipworms, tapeworms, hookworms
  • Mebendazole – effective against pinworms, roundworms and hookworms
  • Thiabendazole – effective against roundworms, hookworms
  • Fenbendazole – effective against gastrointestinal parasites
  • Flubendazole – effective against most intestinal parasites
  • Abamectin – effective against most common intestinal worms, except tapeworms, for which praziquantel is commonly used in conjunction with abamectin
  • Ivermectin – effective against most common intestinal worms (except tapeworms)
  • Pyrantel pamoate – effective against most nematode infections

Many flea control products already contain an anthelmintic so as long as you keep up with the flea treatments your dog should remain free of intestinal parasites.

Note that as mentioned above the roundworm treatments routinely treat the parasite in its adult form, in the intestine. Those that are still in the larvae stage must reach that stage of maturity before they can be dealt with so a repeat treatment after two weeks will be neded to stop the infestation and then a preventative deworming treatment given regularly either as part of your flea control regimen or as a separate treatment.

The Companion Animal Parasite Council
Wikipedia- Anthelmintics
Picture of Toxocara canis taken from a puppy by veterinarian, Joel Mills