Roundworms in Pets: A Common Parasite Every Pet Owner Should Know

What Are Roundworms?

Roundworms are large, spaghetti-like intestinal parasites that represent the most common worm infection in dogs and cats worldwide. The primary species affecting dogs is Toxocara canis, while cats are mainly affected by Toxocara cati. Adult roundworms live in the small intestine, where they feed on partially digested food and can grow 5–15 centimeters long. Females produce enormous numbers of eggs — up to 200,000 per day — which are shed in feces and can remain viable in soil for years.

Roundworms are so prevalent that studies consistently find that nearly all puppies are born already infected. This occurs through transplacental transmission (larvae cross from the mother to the developing puppy in the uterus) and through nursing (larvae are shed in the mother's milk). This means that deworming puppies and kittens is not optional — it is a foundational aspect of neonatal care.

Causes and Risk Factors

Roundworms infect pets through several routes:

In utero transmission — T. canis larvae migrate from mother to puppies during pregnancy
Nursing — larvae present in mammary tissue shed into milk
Ingesting infective eggs from contaminated soil, grass, water, or feces
Eating infected prey — rodents, birds, and earthworms can serve as paratenic (transport) hosts
Ingesting infected cockroaches or other invertebrates

Risk factors include:

Young age — puppies and kittens are almost universally infected
Hunting or outdoor lifestyle
Mothers not dewormed during pregnancy and lactation
Access to contaminated soil (sandboxes, parks, gardens)
Immunosuppression

Warning Signs and Symptoms

Adult dogs and cats with light infections often show no symptoms. Heavy infections — particularly in young animals — can cause significant illness:

Pot-belly or distended abdomen — classic in heavily infected puppies and kittens
Diarrhea — may be soft, mucousy, or contain visible worms
Vomiting — may contain visible white spaghetti-like worms
Poor coat condition and failure to thrive
Coughing — during the larval migration phase, larvae pass through the lungs (Löffler's syndrome)
Decreased appetite alternating with increased appetite
Lethargy and weakness
Visible worms in feces or vomit — round, white/off-white, several centimeters long

In puppies with massive infections, intestinal obstruction, pneumonia, and even death can occur without deworming.

When to Go to the Vet Immediately

Seek prompt veterinary care if:

A puppy or kitten is severely pot-bellied, lethargic, or failing to gain weight
A young animal is coughing combined with digestive signs
A puppy appears to be in severe gastrointestinal distress
You observe a large mass of worms in vomit (can cause intestinal obstruction)

Use the TailRounds AI Triage for assessment and Book a vet appointment at Happy Paws for deworming guidance and fecal testing.

At-Home Care and Monitoring

Roundworm treatment is highly effective and straightforward:

Deworming medications — pyrantel pamoate, fenbendazole, or milbemycin effectively kill adult roundworms; repeat doses are required to kill maturing larvae that were not yet adults at the first treatment
Puppy/kitten deworming schedule — typically at 2, 4, 6, and 8 weeks of age, then monthly until 6 months, then every 1–3 months; discuss exact protocol with your vet
Deworm nursing mothers concurrently
Fecal testing to confirm clearance 2–4 weeks after treatment
Clean up feces immediately from the yard and dispose of carefully

Log deworming dates, fecal consistency, and any visible worm shedding using the TailRounds Daily Log.

Prevention and Long-Term Management

Roundworm prevention requires ongoing vigilance, especially in households with children:

Year-round broad-spectrum parasite prevention — many monthly heartworm preventatives also control roundworms
Annual or biannual fecal testing for all pets
Prevent pets from eating feces, soil, or prey animals
Wash hands thoroughly after handling pets or cleaning up feces
Toxocara is zoonotic — in humans, larval migration causes visceral larva migrans (in internal organs) or ocular larva migrans (in the eye, potentially causing blindness in children)
Keep sandboxes covered when not in use — cats frequently use them as litter boxes, contaminating children's play areas

Protect Your Pet and Family with TailRounds

Regular deworming is one of the most important things you can do for your pet's health — and your family's safety. Track deworming schedules and fecal test results using the TailRounds Daily Log. Book deworming appointments at Book a vet appointment at Happy Paws. Find a Clinic Near You for local fecal testing, and use the TailRounds AI Triage for any concerns about your pet's digestive health.