What Causes Roundworms in Dogs and How to Get Rid of Them

Cathy Meeks, MS, DVM, DACVIM
By Cathy Meeks, MS, DVM, DACVIM on Nov. 3, 2020

Roundworms are among the most common types of worms found in dogs—along with hookworms and whipworms.

Roundworms can significantly impact a puppy’s health, and in rare cases, they can be fatal. But fortunately, roundworms in dogs can be easily diagnosed and effectively treated.

This article will dive into and explain everything you need to know about roundworms in dogs—from what they are and the symptoms they cause to how to treat and prevent them.

What Are Roundworms?

Ascaridoid nematodes, also known as roundworms, are large intestinal parasites commonly found in puppies. Although they are more commonly seen in puppies, adult dogs can also be infected by roundworms.

There are two important species of roundworms:

  • Toxocara canis (T. Canis)

  • Toxascaris leonina

T. Canis is the species that most commonly infects puppies and can be transmitted to people. 

How Do Dogs Get Roundworms?

There are a few different ways that dogs can get roundworms.

From Their Mother

The most common mode of transmission of roundworms to puppies is through the placenta.

In adults, when eggs are swallowed, the larvae hatch, migrate through the intestines and move into several different tissues of the body. Then the larvae stop developing, and they become dormant in cysts until a dog becomes pregnant.

The larvae then start to develop during estrus and migrate across the placenta and into the puppy fetus. 

Puppies can also be infected while nursing, as larvae can migrate into the mammary glands.

Ingesting Roundworm Eggs

If puppies less than 3 months of age ingest roundworm eggs, the larvae can penetrate the intestines and move into the lungs, where they are coughed up.

The larvae are then swallowed and mature into adults in the intestinal tract. They produce eggs that can be identified under a microscope in a stool sample.

Eating Animals That Carry Roundworm Eggs

Roundworm eggs can also be spread by different animals, including rodents, birds, and earthworms, to name a few. These animals are called paratenic hosts (or dead-end hosts), because their sole purpose is to transport the eggs.

If a dog eats one of these animals, the roundworm eggs can then continue to develop in the dog.

Can You See Roundworms in Dog Poop? How Can You Tell if Your Dog Has Roundworms?

It is possible to easily see roundworms in the stool or vomit without a microscope; however, if you don’t see them, that doesn’t mean they are not there.

A roundworm is white to pale brown in color and can be several inches long (up to 7 inches).

However, it’s more common to diagnose roundworms by identifying the eggs in the feces under a microscope.

After 6 months of age and in adult dogs, the roundworm larvae become encysted in the tissues of the body.

The roundworms stop developing, and very few eggs can be found in the feces. This can make it difficult to diagnose and treat during this stage.

T. canis larvae will then continue to develop during estrus (heat). If a dog is immunocompromised, the larvae can also complete their life cycle.

Roundworm Symptoms in Dogs

The signs of roundworms in puppies can be very subtle in the early stages, and most symptoms are due to the roundworms feeding off of the same nutrients in the small intestines that the puppy requires.

Symptoms can include:

  • Lack of growth

  • Poor body condition

  • Dull hair coat

  • Distended abdomen

  • Vomiting

  • Diarrhea with mucus

  • Seeing a roundworm in dog vomit or feces

  • Coughing

Your vet might also see:

  • Fluid in the abdomen

  • Masses (granulomas) on the kidneys

  • Fatty liver

Can Humans Get Roundworms From Dogs?

If larvae are swallowed by humans, they too can become infected. This is why thorough hygiene when handling puppies and the surrounding surfaces is very important.

Most human infections do not cause symptoms; however, roundworm infection can lead to a condition known as Visceral Larva Migrans, which can cause a fever and enlarged liver.

In rare cases, the larvae can migrate into the eyes and cause visual impairment.

How to Treat Roundworms in Dogs

Roundworms require a veterinarian for treatment. There are no proven home remedies for roundworms for dogs.

Several drugs are approved for the treatment of roundworms in dogs, including milbemycin, pyrantel, and fenbendazole. These medications are safe and effective. It typically only takes a few days after deworming for the adult roundworms to die.

Heartworm preventatives also help to control future infections.

Since most treatments are effective at killing adult worms and not larvae or eggs, multiple treatments two weeks apart will be recommended.

The number of treatments recommended by your veterinarian will depend on the age and current health status of your dog (e.g., pregnancy). 

Reinfection is also possible, so preventing future roundworm infections is an important part of the treatment.  

How to Prevent Roundworms in Dogs

Preventing roundworms in dogs is the key to successful treatment. Follow these tips to keep help prevent your dog from getting roundworms.

  • Pick up your dog’s feces from the yard, dog parks, children’s play areas, etc., as soon as possible. Roundworm eggs live on many surfaces, including soil.

  • Deworm pregnant dogs late in the pregnancy to help prevent newborn puppies from getting roundworms.

  • Deworm your puppy regularly starting at 2 weeks of age.

  • Keep your puppy or dog on heartworm medications to kill adult worms and prevent shedding of eggs in the environment.

  • Practice strict hygiene, including handwashing after handling puppies or feces, and keeping the environment as clean as possible.

Featured Image: iStock.com/Maryviolet

Cathy Meeks, MS, DVM, DACVIM


Cathy Meeks, MS, DVM, DACVIM


Dr. Cathy Meeks started her veterinary career as a veterinary technician while getting her Master's degree in Veterinary Medicine, Forensic...

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