What are parasites, and which ones affect dogs and cats?

Parasites are organisms that live on or inside another animal and take what they need to survive from them. Some are easy to spot, like fleas in your pet’s coat or worms in their stool. Others are microscopic and can only be detected through veterinary testing.

Parasites generally fall into two categories:

• Internal parasites: Worms and single-celled organisms that live inside your pet’s body, often in the intestines or other organs.
• External parasites: Fleas, ticks, mites, and lice that live on your pet’s skin or in their coat.

You may also hear about ringworm when researching pet parasites. Despite its name, ringworm is a fungal skin infection, not a worm or parasite. Read more about ringworm

What are internal parasites?

Internal parasites live inside a dog’s or cat’s body. Some affect the digestive system, while others, like heartworms, can damage the heart, lungs, and blood vessels.

Common intestinal worms include:

• Roundworms: intestinal worms that can cause diarrhea, vomiting, poor growth, or a potbellied appearance, especially in puppies and kittens.
• Hookworms: small worms that attach to the intestinal lining and feed on blood, potentially causing anemia.
• Whipworms: intestinal parasites that primarily affect dogs and can cause diarrhea and weight loss.
• Tapeworms: flat, segmented worms that pets may acquire by swallowing an infected flea or eating infected prey.

Other internal parasites include single-celled organisms, such as giardia and coccidia, which can cause digestive problems.

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What are external parasites?

External parasites live on your pet’s skin or in their coat. They can cause irritation, itching, hair loss, and skin infections. Some also transmit diseases.

Common external parasites include:

• Fleas: tiny insects that bite pets, cause itching, and can contribute to flea allergy dermatitis or transmit tapeworms.
• Ticks: parasites that attach to the skin and feed on blood. Certain ticks can transmit serious diseases.
• Mites: microscopic parasites that can cause ear problems, mange, itching, or hair loss.
• Lice: small insects that live in the coat and can cause skin irritation.

Not every itchy pet has parasites, but persistent scratching deserves attention.

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What are signs my pet has parasites?

Signs of parasites in dogs and cats vary depending on the parasite, your pet’s age, and how heavily they’re infected.

Common signs are:

• diarrhea or vomiting
• unexplained weight loss
• a dull or unhealthy-looking coat
• scooting or irritation around the rear end
• frequent scratching, licking, or biting at the skin
• visible worms or worm segments in stool or around the rear end
• fleas or flea dirt (small, dark specks) in the coat

Puppies and kittens may develop a potbellied appearance, poor growth, or ongoing digestive problems.

Some pets with parasites show no obvious symptoms, especially early in an infection. That’s why routine fecal testing and preventive care matter, even when your pet seems perfectly healthy.

How do pets get parasites?

Dogs and cats can encounter parasites in several everyday ways, including:

• Contaminated environments: swallowing parasite eggs or hard shells (cysts) from soil, feces, or contaminated water.
• Fleas and other insects: swallowing an infected flea during grooming or being bitten by an infected mosquito.
• Hunting or scavenging: eating infected rodents or other prey.
• Other animals: encountering parasites in environments shared with infected pets or wildlife.
• From their mother: some parasites can pass to puppies or kittens before birth or while nursing.

And yes, indoor pets can get parasites, too. Fleas can enter homes on other animals, contaminated material can be tracked indoors, and mosquitoes can find their way inside.

Your pet’s exposure depends on where you live, their lifestyle, and other factors. Your veterinarian can help identify the risks that matter most.

How do I prevent parasites in my dog or cat?

The best approach combines year-round, veterinarian-recommended parasite prevention with routine testing and regular checkups. No preventive eliminates every risk, but consistent protection makes a meaningful difference.

Here are a few ways to help protect your pet:

  Use year-round parasite prevention. Depending on the product, preventives can protect against fleas, ticks, heartworms, and certain intestinal parasites.

  Keep up with fecal testing. Routine stool tests can detect intestinal parasites that aren't causing visible symptoms.

  Choose protection for your pet’s needs. Your veterinarian will consider your pet’s species, age, breed, health, lifestyle, and local parasite risks.

  Protect every pet in your household. Consistent, species-appropriate prevention helps reduce opportunities for parasites to spread or return.

  Clean up stool promptly. Regular waste removal helps reduce environmental contamination.

  Check for fleas and ticks. Look over your pet after outdoor activities and pay attention to changes in their skin or coat.

Never use a dog parasite preventive on a cat. Some ingredients used in dog products are toxic to cats.

If one pet is diagnosed with parasites, your veterinarian can determine whether other pets in the household need testing or treatment.

Can parasites spread from pets to people?

Yes. Some parasites that infect dogs and cats can also infect people. These are called zoonotic parasites.

Roundworms and hookworms are two examples. Exposure can occur through contaminated soil or environments, and young children may be at greater risk because they often play close to the ground.

Simple hygiene habits can help lower the risk:

  Wash your hands after handling pet waste or cleaning litter boxes.

  Pick up dog stool promptly and clean litter boxes regularly.

  Keep children's sandboxes covered when not in use.

  Follow your veterinarian’s parasite prevention recommendations.

Your veterinary team can explain which parasites may pose risks to people and how to reduce exposure. Pregnant people should take additional care with litter box hygiene and ask their healthcare provider about appropriate precautions.

Parasites FAQs

When to see the vet

Contact your veterinarian if your pet develops ongoing diarrhea, vomiting, unexplained weight loss, persistent scratching, or visible worms in their stool.

Puppies and kittens should also be checked if they have a potbellied appearance, poor growth, or recurring loose stools.

Even without symptoms, regular veterinary visits are a good time to review parasite prevention and schedule routine fecal testing. Current guidance recommends testing at least four times during the first year of life and at least twice yearly for adult pets, with additional testing for pets at higher risk.