Roundworm Infection in Dogs: Symptoms, Transmission Routes, and Treatment & Prevention Methods

Infection/ParasitesDiseasesMeongsiljang Veterinary Advisory Board

Roundworm infection in dogs is caused by Toxocara canis, an internal parasite that lives in the small intestine and poses a particular risk to puppies. We’ve compiled information on transmission routes, symptoms, and deworming methods based on veterinary advice.

What is Roundworm Infection in Dogs?

A young puppy undergoing a health check-up at the veterinary clinic.

Roundworm infection in dogs is an internal parasitic disease caused by Toxocara canis, a roundworm that lives in the small intestine and causes digestive symptoms and overall health deterioration. Adult worms reside in the small intestine and shed eggs in the feces, and the larvae that hatch from these eggs migrate to the liver and lungs. This condition is particularly severe in puppies, as the larvae can cause clinical symptoms by creating lung lesions as they pass through the lungs. The most important point is to start regular deworming from two weeks of age. Even if there are no visible symptoms, parasites may already be multiplying inside the body.

Three Transmission Routes for Roundworm Infection in Dogs

Placental infection: Larvae that were dormant in the mother dog’s muscle tissue become active during pregnancy and are transmitted to the fetuses.

Milk-borne infection: Larvae are passed to puppies through colostrum and milk during the nursing period.

Environmental infection: Puppies can ingest eggs by licking or consuming contaminated soil, water, or feces from infected animals. Digging in dirt or sniffing feces while on walks are also risky exposure routes.

Even if the mother has completed deworming, it is difficult to completely eliminate dormant larvae. Therefore, if you’ve adopted a new puppy, a fecal test is essential.

Checklist of Major Symptoms of Roundworm Infection

Vomiting: Roundworms can migrate into the stomach, triggering nausea and vomiting. You may even see white, spaghetti-like worms in the vomit.

Diarrhea: Recurrent small-intestinal diarrhea that is sticky or contains mucus.

Distended Abdomen ("Pot Belly"): A characteristic bloated, round appearance of the abdomen.

Growth Retardation: Weight gain may lag behind peers, or growth may stall entirely.

Coughing: Coughing and respiratory abnormalities can occur during the larval migration phase through the lungs.

Lethargy: Accompanied by low energy and decreased appetite.

A puppy receiving an abdominal examination

See a Vet Immediately If You Notice These Symptoms

If vomiting or diarrhea persists for more than a day, or if you see white worms in the vomit or stool, take your pet to a veterinary clinic immediately. In puppies under 8 weeks of age, the simultaneous appearance of abdominal distension, coughing, and lethargy may indicate a heavy parasitic infection. Because larvae migrate through the lungs in young puppies, potentially causing lung lesions and respiratory symptoms, it is safest to seek veterinary care without delay.

Diagnostic Methods at the Veterinary Clinic

Fecal flotation: This is the most basic test, in which a stool sample is placed in a special solution to float and detect roundworm eggs.

Direct smear: A stool sample is smeared onto a slide and examined directly under a microscope.

Even after completing deworming treatment, you cannot be certain that all roundworms have been eliminated. Because fecal tests alone may not detect dormant larvae, follow-up examinations at intervals recommended by your veterinarian are necessary after deworming. In particular, if you have recently adopted a puppy, be sure to request a fecal test during its first veterinary visit.

Treatment Methods — Types of Dewormers and Usage Principles

Pyrantel pamoate: Effective against roundworms and hookworms, but not against whipworms.

Fenbendazole: Broad-spectrum activity against various internal parasites, including roundworms, whipworms, and hookworms.

Milbemycin oxime and Moxidectin: Ingredients found in heartworm preventives that also help prevent roundworms.

Dosage and frequency vary depending on your pet’s weight and age, so a veterinarian should determine the appropriate regimen. Never use human dewormers, as doing so can be dangerous.

Types of Dog Dewormers — Tablets and Liquid Dewormers

Home Care — Preventing Reinfection is Key

The primary source of roundworm infection is contaminated soil. Because the eggs can survive in the environment for extended periods, your dog can become reinfected if exposed to the same area even after deworming. The infection cycle repeats when a dog ingests eggs while digging or eating soil.

Promptly remove feces: Pick up your dog’s stool immediately during walks to reduce environmental contamination.

Wash your hands: Always wash your hands with soap after handling feces. Roundworms are zoonotic parasites that can also be transmitted to humans.

Keep floors and toys clean: Regularly wash and disinfect floors and toys that your dog licks.

Regular deworming: Administer deworming medication consistently according to the schedule recommended by your veterinarian.

A pet owner picking up their dog’s feces during a walk

Essential Checks for Puppies and New Adopters

Even if your newly adopted puppy has been dewormed at the breeder or shelter, dormant larvae may still remain. For safety, have a fecal exam performed at a veterinary clinic shortly after adoption. Deworming a pregnant mother dog does not fully eliminate larvae that have migrated into her muscles, so both the mother and her puppies require separate deworming treatments.

Reviewed by a veterinarian

Dr. Tony — Punnawat Phongkittirak

Veterinarian

A veterinarian who majored in veterinary medicine at Khon Kaen University, Thailand, and completed the IVSA program at North Carolina State University in the United States. Drawing on clinical experience at animal hospitals, he works in the pet healthcare field and is dedicated to building a digital care environment that connects pet parents with veterinarians.

Frequently Asked Questions

Can roundworms in dogs be transmitted to humans?

Yes, it is possible. Roundworm larvae can migrate through the human body, causing a condition known as larva migrans. This is especially risky for children who touch contaminated soil or fail to wash their hands. To prevent infection, always wash your hands thoroughly after handling feces and ensure your dog receives regular deworming treatments.

I found roundworms in my pet’s stool after deworming. Is this normal?

Yes, this is normal. Deworming medications paralyze and kill roundworms, which are then expelled in the stool. Seeing them is actually a sign that the medication is working effectively. However, if symptoms persist, please consult your veterinarian.

When should I start deworming my puppy?

According to veterinary guidelines, puppies should begin deworming as early as two weeks of age, followed by treatments every two weeks for about four rounds. After that, it’s best to maintain regular deworming on the schedule recommended by your veterinarian and confirm whether any parasites are present through fecal testing.

Will Pyrantel dewormer also eliminate pinworms?

No, Pyrantel is effective against roundworms but not against pinworms. To manage pinworms as well, you'll need a broad-spectrum dewormer like fenbendazole or a combination dewormer prescribed by your veterinarian.

Does the absence of symptoms mean there are no roundworms?

Not necessarily. Adult dogs may show few or no symptoms even if they harbor a small number of roundworms. Puppies and immunocompromised dogs can become heavily infected without displaying any clinical signs. Only regular fecal examinations can definitively confirm whether an infection is present.

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References

[1] Schaer M. et al., Clinical Medicine of the Dog and Cat, 4th Edition, CRC Press, 2022

[2] Little S.E. (ed.), The Cat: Clinical Medicine and Management, 2nd Edition, Elsevier, 2022

[3] Bowman D.D. et al., Principles and Practices of Canine and Feline Clinical Parasitic Diseases, Wiley-Blackwell, 2021

[4] Aspinall V. & Cappello M., The Veterinary Workbook of Small Animal Clinical Cases, CRC Press, 2018