Can Roundworms Cause Diarrhea in Cats? | Veterinary Insights

Most cat owners picture roundworms as harmless spaghetti-like squiggles, but these intestinal parasites can stir up more than just the occasional upset stomach. Kittens, in particular, are vulnerable, and the symptoms can range from mild tummy trouble to serious illness that weakens the whole body.

The short answer is yes — roundworm infections can trigger diarrhea in cats, especially in kittens whose immune systems are still developing. But the symptom picture varies widely by age, parasite load, and overall health, so knowing what to watch for can help you act quickly.

What Roundworms Look Like and How They Affect the Gut

Roundworms (Toxocara cati and Toxascaris leonina) are long, pale worms that can reach three to six inches — about the length of a piece of cooked spaghetti. They live in the small intestine, where they feed on partially digested food.

As they move and multiply, they irritate the intestinal lining. This inflammation is what often leads to loose stools or full-blown diarrhea. In kittens, the irritation can be severe enough to interfere with nutrient absorption, which contributes to poor growth and a pot-bellied appearance.

While adult cats may carry a few worms without showing obvious signs, a heavy burden can cause vomiting, weight loss, and intermittent diarrhea even in mature animals.

Why Roundworms Are a Common—but Overlooked—Cause of Cat Diarrhea

Many cat owners assume diarrhea is always from food changes or stress, but intestinal parasites like roundworms are surprisingly widespread. Routine fecal exams often miss low-level infections, so the link between worms and digestive upset can go unnoticed for weeks.

• Pot-bellied appearance: A distended belly is a classic sign in kittens with roundworms, often accompanied by a dull coat.
• Vomiting and diarrhea: Both can occur separately or together; diarrhea may be soft, frequent, or mucus-covered.
• Poor growth and weight loss: Worms compete for nutrients, so kittens may fail to gain weight normally.
• Lethargy and depressed appetite: Parasite burdens sap energy, making young cats less lively.
• Visible worms in stool: Adult roundworms sometimes appear in feces or vomit, confirming the diagnosis.

Not every infected cat shows all these signs, and some adult cats remain asymptomatic. But when diarrhea appears alongside other clues, roundworms become a strong suspect.

The Real Danger of Heavy Roundworm Loads

In kittens and debilitated or older cats, large numbers of roundworms can cause life-threatening problems. The combination of diarrhea, vomiting, and reduced appetite leads to dehydration and anemia, which in turn makes the cat more susceptible to viral and bacterial infections. Cornell’s Feline Health Center notes that an infected cat’s weakened state can open the door to secondary infections — this is one reason routine deworming and prompt disposal of feces is so important.

The CDC highlights that very young animals are most vulnerable. Its educational materials point out that puppies of only two to three weeks of age can be infected, and the same holds true for kittens. You can review the roundworm infection age document for more on early-life risks.

These patterns come from clinical observation by major veterinary hospitals; individual cats may show a different mix of signs.

What to Do If You Suspect Roundworms

If your cat has diarrhea and you notice any of the additional signs above, the next steps are straightforward but important. Acting quickly can prevent the infection from worsening and protect other pets.

1. Collect a fresh stool sample. Your veterinarian can perform a fecal flotation test to check for roundworm eggs. A single sample may miss low egg counts, so follow your vet’s advice on repeat testing.
2. Schedule a veterinary visit. Prescription dewormers are safe and effective; over-the-counter options may not cover all species of roundworms and could be less reliable.
3. Administer the full course. Roundworms often require two or three doses spaced two to four weeks apart to kill newly hatched larvae. Skipping doses can leave the infection active.
4. Clean the environment. Roundworm eggs can survive in soil and on surfaces for years. Dispose of feces promptly and disinfect litter boxes with hot water and bleach to reduce reinfection risk.

Remember that roundworms are zoonotic — they can be transmitted from cats to humans, especially children, causing a disease called toxocariasis. Protecting your cat also protects your family.

Preventing Roundworm Infections and Keeping Diarrhea at Bay

Prevention is far easier than treatment. Regular deworming schedules, starting when kittens are a few weeks old, are the gold standard. Most veterinarians recommend a deworming protocol based on the cat’s lifestyle — indoor-only cats may need less frequent dosing than outdoor hunters.

Routine fecal exams at least once or twice a year help catch infections before they cause symptoms. The Cornell University College of Veterinary Medicine emphasizes these steps in its prevent roundworm infections guide, noting that prompt disposal of feces and good hygiene are critical.

Because roundworms are just one of several parasites that cause diarrhea in cats — Giardia, hookworms, and coccidia are other common culprits — a fecal test is the only way to be sure which organism is responsible. Treating the wrong parasite wastes time and leaves the underlying problem unresolved.

The Bottom Line

Yes, roundworms can cause diarrhea in cats, particularly in kittens and young adults. The symptom pattern — diarrhea combined with a pot belly, poor growth, or visible worms in the stool — is highly suggestive. A quick fecal test from your veterinarian can confirm the diagnosis, and prescription dewormers are generally safe and effective when used as directed.

If your kitten has soft stools and seems less energetic than usual, a fresh stool sample at your next vet visit can rule out roundworms and other parasites — no need to guess when a simple test gives a clear answer. Your veterinarian is the best person to recommend the right deworming schedule for your cat’s age, weight, and lifestyle.