Dogs carry a wide range of parasites, both internally and on their skin. A large U.S. study of over 48,000 dogs found that roughly 12% tested positive for at least one intestinal parasite, and that number only accounts for what shows up on a single fecal test. The actual rate is likely higher, since many parasites shed eggs intermittently and can be missed. Some of these parasites stay confined to your dog, while others can spread to people.
Internal Parasites Dogs Commonly Carry
The most frequently detected intestinal parasites in dogs are roundworms, hookworms, whipworms, tapeworms, and two single-celled organisms: coccidia and Giardia. In the U.S. study, Giardia was the single most common finding at 5.27% of dogs tested, followed by hookworms at 3.14%, roundworms at 2.07%, coccidia at 1.95%, and whipworms at 0.88%.
Roundworms are especially common in puppies, who can be born with them or pick them up through their mother’s milk. Once swallowed, roundworm eggs hatch inside the dog’s body, and the larvae travel through the lungs and liver before settling in the small intestine. Hookworms take a different route. Their larvae can penetrate a dog’s skin directly, then travel through the body to latch onto the intestinal wall, where they feed on blood and tissue. Heavy hookworm infections can cause anemia, particularly in small or young dogs. Whipworms embed themselves in the lining of the large intestine and also feed on blood, though they tend to produce eggs less consistently, making them harder to detect on a single test.
Tapeworms have an unusual life cycle. The most common type in dogs, sometimes called the flea tapeworm, requires a flea as a go-between. Flea larvae swallow tapeworm eggs, and the parasite develops inside the flea as it matures. When a dog swallows an infected adult flea during grooming, the tapeworm finishes its development in the dog’s intestine. You might notice tapeworm segments that look like small grains of rice in your dog’s stool or around their rear end.
Heartworm: A Mosquito-Borne Threat
Heartworm is a parasite spread entirely through mosquito bites, not through contact with infected dogs. When a mosquito feeds on a dog that already has heartworm, it picks up microscopic larvae from the dog’s bloodstream. Over the next 10 to 14 days, those larvae mature inside the mosquito into an infective stage. The next dog the mosquito bites receives those larvae through the bite wound. From there, it takes about six to seven months for the larvae to grow into adult worms that live in the dog’s heart and blood vessels, where they mate and release offspring back into the bloodstream.
Because the transmission cycle depends entirely on mosquitoes, heartworm prevention is recommended year-round in most regions. Dogs should be tested for heartworm annually even if they’re on preventive medication.
External Parasites on Dogs
Fleas, ticks, ear mites, and mange mites are the main external parasites that live on or in a dog’s skin. Each causes different problems.
- Fleas cause itching that ranges from mild to severe. Heavy infestations can lead to open sores, skin infections, and even anemia in small or young dogs. Fleas also serve as the intermediate host for tapeworms, connecting two parasite problems.
- Ticks attach to a dog’s skin to feed on blood and can transmit serious infections to both dogs and people during that feeding.
- Sarcoptic mange mites burrow into the top layer of skin, causing intense itching, hair loss, rashes, and crusting. This type of mange is contagious to other animals and to people.
- Demodectic mange mites typically show up in young dogs as scaly, red patches around the eyes, mouth, legs, or trunk. This form is not contagious in the same way.
- Ear mites cause intense irritation inside the ear canal and are common in puppies.
Which Dog Parasites Can Spread to People
Several dog parasites pose a real, if generally low, risk to humans. The most significant is roundworm. The CDC classifies the infection it causes in people, called toxocariasis, as a notable public health concern. People pick it up by accidentally swallowing roundworm eggs from contaminated soil or unwashed hands after contact with dog feces. Children who play in areas where dogs have defecated are at the highest risk. Once inside a human body, roundworm larvae can migrate to the eyes, liver, lungs, or brain, potentially causing vision problems or organ damage. The infection does not spread person to person.
Hookworm larvae in contaminated soil can burrow through human skin, typically on bare feet, causing an itchy, winding rash. The flea tapeworm can infect people too, though only if someone accidentally swallows an infected flea, which happens most often with young children.
Giardia is worth a separate note. While dogs frequently carry it, the strains that infect dogs are generally not the same ones that make people sick. The CDC states you are unlikely to get a Giardia infection from your dog or cat. Still, basic hygiene (washing hands after handling pet waste, wearing gloves while gardening) reduces even that small risk further.
How Parasites Are Detected
Most intestinal parasites are diagnosed through a fecal sample. The standard method, called centrifugal fecal flotation, uses a sugar solution to separate parasite eggs from the rest of the stool so they can be identified under a microscope. This catches most common worms and coccidia reliably.
Some parasites are trickier. Whipworms produce eggs irregularly and in small numbers, so a single negative test doesn’t rule them out. Giardia sheds cysts on and off, which is why vets may recommend testing stool samples from three different days over a span of six to ten days. An antigen test for Giardia can also be run on a single sample and approaches the sensitivity of those repeated flotation tests. Heartworm is detected through a blood test, not a stool sample, since the worms live in the cardiovascular system rather than the intestines.
Keeping Your Dog (and Family) Protected
Veterinary guidelines from the Companion Animal Parasite Council recommend a layered approach. Puppies should start deworming treatment at just two weeks old, with repeat doses every two weeks until eight weeks of age, then monthly until six months. After that, adult dogs benefit from broad-spectrum deworming four times a year.
Fecal testing should happen at least four times during a puppy’s first year and at least twice a year for adult dogs. Year-round preventive medication that covers heartworm, intestinal parasites, fleas, and ticks is the single most effective step you can take. Even in colder climates where mosquitoes and fleas seem seasonal, gaps in prevention create windows for infection.
On the human side, the basics matter most: pick up dog waste promptly from your yard, wash hands after handling pets or soil, keep children’s sandboxes covered, and avoid walking barefoot in areas where dogs frequently defecate. These simple habits dramatically cut the chances of any parasite making the jump from your dog to your family.