Common Intestinal Parasites in Dogs

Dogs commonly harbor a range of intestinal parasites, from microscopic protozoa like Giardia and coccidia to larger worms such as roundworms, hookworms, whipworms, and tapeworms. Many infections produce no obvious symptoms at all, which is part of what makes them so persistent: a dog can shed parasite eggs or cysts into the environment for weeks before anyone notices a problem. Some of these organisms also pose a real threat to human health, and a worrying trend of drug resistance in hookworms has veterinary parasitologists increasingly concerned.

The Most Common Worms

The intestinal worms that veterinarians encounter most frequently in dogs fall into a few familiar categories. Roundworms (Toxocara canis and Toxascaris leonina) are especially common in puppies, which can acquire them before birth through the placenta or shortly after through their mother’s milk. Hookworms (Ancylostoma caninum) latch onto the intestinal lining and feed on blood, sometimes causing anemia in heavily infected young dogs. Whipworms (Trichuris vulpis) take up residence in the large intestine and cecum, and tapeworms like Dipylidium caninum grow in segments that break off and pass in the stool.

Whipworms are particularly sneaky. In a retrospective study of 45 dogs with confirmed Trichuris vulpis infections, about 93 percent showed clinical signs. The most frequent were diarrhea (roughly half the dogs), weight loss (about 38 percent), and bloody stool (around 29 percent). A subset of infected dogs developed low sodium and high potassium levels, a combination that can mimic Addison’s disease closely enough to send a veterinarian down the wrong diagnostic path before the real culprit is found.¹ In one published case, a Rottweiler with intermittent vomiting, diarrhea, and weakness was suspected of having hypoadrenocorticism until abdominal imaging revealed worms floating in the bowel lumen, and the dog recovered fully after deworming.²

Tapeworms like Dipylidium caninum follow a different route into the dog entirely. Their life cycle requires an intermediate host, typically a flea or a chewing louse. A dog becomes infected by swallowing an infected flea while grooming. Inside the small intestine, the larval stage attaches to the wall and matures into an adult tapeworm within two to three weeks, after which egg-filled segments start appearing in the feces.³ This means that controlling fleas is inseparable from controlling this particular tapeworm.

Protozoan Parasites

Not all intestinal parasites are worms. Single-celled organisms cause their own share of illness, and two groups stand out in dogs: Giardia and coccidia.

Giardia duodenalis infects dogs through contaminated water or fecal-oral contact and causes watery or mucoid diarrhea, especially in young or immunocompromised animals. The question of whether dogs readily transmit Giardia to people is more complicated than it might seem. Giardia comes in different genetic groupings called assemblages, and not all of them infect both species. A study of dogs in rural Cambodia found that only about 2 percent carried assemblages with zoonotic potential, suggesting that dog-specific and human-specific cycles can run in parallel with little crossover.⁴ However, research in southern Brazil painted a different picture: children and household dogs were found to share the same sub-assemblage (BIV), indicating that cross-transmission does happen in some settings.⁵ The honest answer is that it depends on geography, the specific Giardia assemblage involved, and living conditions. In settings where dogs and children share close quarters and sanitation is limited, the risk goes up.

Coccidia in the genus Cystoisospora are common in kennels and breeding facilities, and they hit puppies hardest. In experimental infections, untreated puppies began shedding oocysts by the sixth day and developed diarrhea that ranged from watery to bloody; four of six untreated puppies in one study died by day twelve.⁶ The drug toltrazuril, given as a single oral dose in the third or fourth week of life, proved effective at preventing oocyst shedding and clinical disease in that same study. Coccidiosis in adult dogs with functioning immune systems tends to be mild or subclinical, but in breeding and shelter environments it can spread rapidly and become serious.⁷

Why Standard Fecal Tests Miss So Much

The routine test for intestinal parasites, fecal flotation, works by mixing a stool sample with a dense solution so that parasite eggs float to the surface and can be viewed under a microscope. It is a good screening tool, but it has real blind spots. A head-to-head comparison of fecal flotation against a molecular PCR panel found that PCR detected significantly more infections overall, and about 2.6 times as many co-infections (cases where a dog had more than one parasite at once).⁸

Tapeworms are a particularly glaring gap. Dipylidium caninum sheds eggs in packets inside proglottid segments rather than individually, so those eggs often do not float properly. In a study of shelter and pet dogs across six U.S. states, standard fecal flotation identified D. caninum in essentially zero pet dogs and only 0.3 percent of shelter dogs. When coproantigen testing and perianal swabs were added, detection jumped to about 2 percent in pets and over 12 percent in shelter dogs.⁹ Coproantigen testing also improved detection rates for Giardia (roughly 15 percent versus 3 percent by flotation alone) and hookworms (about 14 percent versus 8 percent). The practical takeaway is that a single negative fecal flotation does not guarantee your dog is parasite-free, especially for tapeworms and Giardia.

Drug-Resistant Hookworms

Perhaps the most alarming development in canine parasitology over the past several years is the emergence of hookworms that no longer respond to available medications. Cases of Ancylostoma caninum resistant to every anthelmintic drug class approved for hookworms in U.S. dogs were first reported in 2019. The problem was initially concentrated in retired racing greyhounds, where dogs had received frequent anthelmintic treatments in confined kennel settings, creating ideal conditions for resistance to develop. Researchers confirmed multiple-drug resistance in several hookworm isolates from greyhound populations, with very high prevalence and infection intensity.¹⁰

The problem has since spread well beyond greyhounds. A 2025 position paper from the American Association of Veterinary Parasitologists reported that cases of multiple anthelmintic drug resistance have now appeared in pet dogs across the United States, and molecular testing has revealed resistance mutations at an alarming frequency, with some reports emerging from Canada as well.¹¹ There is no new drug class on the immediate horizon for treating hookworms in dogs, which makes this a genuine veterinary crisis rather than a theoretical concern. If your dog has hookworms that keep coming back despite treatment, drug resistance is a real possibility worth discussing with your vet.

Zoonotic Risks Worth Knowing About

Several dog parasites can infect people, though the risk and consequences vary widely depending on the organism. Toxocara canis is among the best-known. When a person accidentally ingests roundworm eggs from contaminated soil (children playing in parks or sandboxes are the classic scenario), the larvae cannot complete their life cycle in a human body. Instead they migrate through tissues, causing a condition called visceral larva migrans. A published case of a human patient infected with Toxocara showed massively elevated white blood cells with extreme eosinophilia, and abdominal imaging revealed multiple lesions scattered throughout the liver.¹² Larvae can also end up in the eye, where they cause a condition called ocular larva migrans that can impair vision.

Echinococcus granulosus, a small tapeworm that lives in the dog’s intestine without causing it much trouble, is another serious zoonotic concern. Dogs are the definitive host, typically becoming infected by consuming raw offal from sheep or other livestock that contain hydatid cysts. The eggs shed in dog feces are directly infectious to humans, and when a person ingests them, slow-growing cysts can develop in the liver, lungs, or other organs. The disease is most prevalent in rural areas of Africa, the Mediterranean, South America, Central Asia, and Eastern Europe where dogs, humans, and livestock live in close contact.¹³ In an Australian survey of rural dogs, half the owners fed raw meat and some fed raw lamb or mutton offal, and few dewormed often enough to keep their dogs cestode-free.¹⁴

A related species, Echinococcus multilocularis, causes an even more aggressive form of the disease in humans called alveolar echinococcosis. This tapeworm typically cycles between wild canids (foxes, coyotes) and rodents, but domestic dogs can pick it up and bring it into closer contact with people. Surveillance has confirmed E. multilocularis in coyote feces in the Winnipeg metropolitan area¹⁵ and in coyotes in Washington State, where researchers concluded the parasite’s range in western North America may be much larger than previously recognized.¹⁶

Environmental Contamination

Dog parasites do not stay on dogs. Parks, playgrounds, beaches, and any shared green space where dogs defecate become reservoirs of infective eggs and cysts. Studies from around the world have documented high rates of soil contamination with dog-origin helminths in public and recreational areas.¹⁷ A survey of public parks and playgrounds in Hue City, Vietnam, found Ancylostoma caninum eggs in 80 percent of the sites tested, with contamination rates higher during the rainy season than the dry season.¹⁸ A similar survey of parks in Niš, Serbia, identified seven different dog-origin parasites in soil and sand samples, including Toxocara, hookworms, whipworms, and coccidia.¹⁹

Hookworm and roundworm eggs can survive in soil for months or even years under the right conditions. Warm, moist environments favor persistence and embryonation (the development of the egg into an infective stage). This is why simple hygiene measures, like picking up dog feces promptly and washing hands after soil contact, are not trivial public-health advice. The eggs are microscopic and invisible in soil, and no amount of visual inspection will tell you a patch of grass is safe.

How Raw Diets Factor In

The popularity of raw-meat-based diets for dogs has introduced an additional parasite transmission pathway that conventional kibble does not carry. A parasitological investigation of 89 pets fed raw (sometimes labeled BARF) diets found several shedding eggs of Dicrocoelium dendriticum, a liver fluke not typically associated with pet dogs. DNA from Fasciola hepatica, another liver fluke, was also confirmed in one case. Coccidia oocysts, including Cystoisospora canis, were detected as well.²⁰ These are parasites that dogs eating commercial cooked diets almost never encounter, because the cooking process kills the infective stages. If you choose to feed raw, regular fecal testing becomes even more important.

What Parasites Do to the Gut Beyond the Obvious

Beyond the expected diarrhea and weight loss, intestinal parasites reshape the microbial community living inside the dog’s gut. This is an area of growing research interest. Dogs with natural Giardia infections showed shifts in their intestinal microbiome, with increases in Clostridium and Lactobacillus and decreases in Bacteroides, a group of bacteria considered important for normal gut function.²¹ Toxocara canis infection similarly decreased the overall diversity of the intestinal flora and shifted the ratio of major bacterial groups, with increases in potentially pathogenic bacteria like Clostridium sensu stricto and Staphylococcus.²² Environmental sampling at dog parks in Appalachia found the same pattern: fecal samples that tested positive for parasites also showed greater bacterial diversity associated with gut-disease-linked taxa like Clostridia.²³

These microbiome shifts may explain why some dogs continue to have digestive problems for weeks after the parasites themselves have been cleared by treatment. The worms or protozoa are gone, but the bacterial community they disrupted has not fully recovered. This is still an emerging area of research, but it adds a layer of understanding to the chronic loose stools and intermittent gastrointestinal upset that veterinarians sometimes see in post-treatment dogs.

How Worms Manipulate the Immune System

Intestinal helminths are not passive passengers. They actively modulate the host’s immune response to improve their own survival. Laboratory work using products secreted by hookworm (A. caninum) and roundworm (T. canis) larvae showed that these substances increase the frequency of regulatory T cells, which are immune cells that dampen inflammation. The parasite-derived products also boosted secretion of the anti-inflammatory molecule IL-10 and prevented dendritic cells from maturing properly, essentially blunting the immune system’s ability to mount a full attack.²⁴ This immune suppression helps the worms persist but also creates a vulnerability: dogs with heavy worm burdens may be less able to fight off other infections simultaneously. Shelter dogs arriving with intestinal parasites, for instance, frequently carry multiple species at once. A survey of new arrivals at shelters in northwestern Spain found that nearly 87 percent of dogs positive for lung-dwelling nematodes were co-infected with gastrointestinal parasites as well.²⁵

A Shifting Landscape

The world of canine parasites is not static. Climate change, increased global pet travel, and the movement of rescue dogs across borders are reshaping which parasites show up where. Warmer winters in temperate zones allow parasites with soil-dwelling stages to survive year-round in areas where hard freezes once kept them in check. Dogs imported from tropical or subtropical regions may arrive carrying species that local veterinarians have never encountered. A recent review described this as a challenge to current understanding across multiple scientific fields, noting that some parasites appear to have emerged in new regions, others have switched hosts or developed drug resistance, and still others may have been present but overlooked due to limited diagnostic tools.²⁶ The expanding range of E. multilocularis in North American coyotes is one concrete example of this phenomenon. The spread of multi-drug-resistant hookworms from greyhound kennels into the general pet population is another. Routine deworming protocols that worked well twenty years ago may not be sufficient for today’s parasite landscape, making regular veterinary screening and updated prevention plans more valuable than ever.