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Roundworms in Small Animals


Andrew S. Peregrine

, BVMS, PhD, DVM, DEVPC, DACVM, Department of Pathobiology, Ontario Veterinary College, University of Guelph, Guelph, Ontario, Canada

Last full review/revision Sep 2014 | Content last modified Jun 2016
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The large roundworms (ascaridoid nematodes) of dogs and cats are common, especially in puppies and kittens. Of the three species Toxocara canis, Toxascaris leonina, and Toxocara cati, the most important is T canis, not only because its larvae may migrate in people (as do larvae of T cati), but also because infections are generally common and may impact puppy health. Also, fatal infections may occasionally be seen in young pups. T leonina is seen in adolescent/adult dogs and cats. These species also infect wild carnivores, especially those in zoos or other captive settings.

In puppies, the usual mode of infection with T canis is transplacental transfer. If pups <3 mo old ingest embryonated infective eggs, the hatched larvae penetrate the intestinal mucosa, reach the lungs via the liver and bloodstream, are coughed up, swallowed, and mature to egg-producing adults in the small intestine. However, when infective eggs of T canis are swallowed by older dogs, the larvae hatch, penetrate the intestinal mucosa, and migrate to the liver, lungs, muscles, connective tissue, kidneys, and many other tissues, where development is arrested. In pregnant bitches, these dormant larvae mobilize and migrate into the developing fetus; they can be found in the intestine of puppies as early as 1 wk after birth. Some larvae migrate to the mammary gland, so pups may also be infected via the milk. During this perinatal period, the immunity of the bitch to ascarid infection is partially suppressed, and substantial numbers of eggs may be passed in the feces of the bitch. Development of these patent infections appears to be associated with maturation of arrested larvae in the bitch, which migrate to the intestine via the lungs. Patency may also occur as a result of ingestion and maturation of larvae that are passed in the feces of puppies.

After ingestion of infective eggs, larvae of ascaridoid nematodes may migrate into the tissues of many animals and thus provide an alternative source of infection, particularly for cats and wild carnivores. Such migration also occurs if larvated eggs of Toxocara spp are swallowed by people. Most human infections are asymptomatic, but fever, persistent eosinophilia, and hepatomegaly (sometimes with pulmonary involvement) may occur, resulting in a condition known as visceral larva migrans. Rarely, a larva may settle in the retina and impair vision, resulting in a condition known as ocular larva migrans.

The life cycle of T cati is similar to that of T canis, except that no prenatal infection occurs. Furthermore, transmammary transmission appears to occur only when queens acquire infection during late gestation. Thus, overall, this route of infection appears to play a minor role in transmission. With T leonina, migration is restricted to the intestinal wall so that neither prenatal nor transmammary transmission occurs.

Clinical Findings and Lesions:

The first indication of infection in young animals is lack of growth and loss of condition. Infected animals have a dull coat and often are “potbellied.” Worms may be vomited and are often voided in the feces. In the early stages, migrating larvae may cause an eosinophilic pneumonia, which can be associated with coughing. Diarrhea with mucus may be evident.

In puppies with severe infections, verminous pneumonia, ascites, fatty liver, and mucoid enteritis are common. Cortical kidney granulomas containing larvae may be seen.


Infection in dogs and cats is diagnosed by detection of eggs in feces. Distinguishing the spherical, pitted-shelled eggs of Toxocara spp (T canis 80–90 × 75 μm; T cati 65 × 75 μm) from the oval, smooth-shelled eggs of Toxascaris leonina (75–85 × 60–75 μm) is important because of the public health significance of the former.

Treatment and Control:

In dogs, compounds approved for treatment of roundworm infections include fenbendazole, milbemycin, moxidectin, piperazine, and pyrantel ( see Table: Drugs for Intestinal Helminths of Dogs Approved in the USA and UK Drugs for Intestinal Helminths of Dogs Approved in the USA and UK Drugs for Intestinal Helminths of Dogs Approved in the USA and UK ). In Europe, selamectin is approved to treat T canis infections with a single dose, whereas in Canada approved treatment requires two doses 1 mo apart. Preventive programs for heartworm infection using milbemycin, milbemycin/lufenuron, milbemycin/praziquantel, milbemycin/spinosad, moxidectin/imidacloprid, ivermectin/pyrantel, or ivermectin/pyrantel/praziquantel also control intestinal ascarid infections. In addition, selamectin is approved for this indication in some countries but not in the USA ( see Table: Drugs for Intestinal Helminths of Dogs Approved in the USA and UK Drugs for Intestinal Helminths of Dogs Approved in the USA and UK Drugs for Intestinal Helminths of Dogs Approved in the USA and UK ).


Drugs approved for treatment of ascarid infections in cats include emodepside, fenbendazole, milbemycin, moxidectin, piperazine, pyrantel, and selamectin ( see Table: Drugs for Intestinal Helminths of Cats Approved in the USA and UK Drugs for Intestinal Helminths of Cats Approved in the USA and UK Drugs for Intestinal Helminths of Cats Approved in the USA and UK ). Heartworm-preventive programs that use milbemycin, milbemycin/praziquantel, moxidectin/imidacloprid, or selamectin also control ascarid infections in cats ( see Table: Drugs for Intestinal Helminths of Cats Approved in the USA and UK Drugs for Intestinal Helminths of Cats Approved in the USA and UK Drugs for Intestinal Helminths of Cats Approved in the USA and UK ).


Environmentally resistant larvated eggs on the ground and somatic larvae in the bitch are the main reservoirs of infection. Perinatal transmission of infection can be greatly reduced by treating bitches with 1) daily doses of fenbendazole (25 mg/kg, PO) from day 40 of gestation to day 2 after whelping (approved in the UK), 2) ivermectin (0.3 mg/kg, SC) on days 0, 30, and 60 of gestation, and 10 days after whelping, 3) ivermectin (0.5 mg/kg) on days 38, 41, 44, and 47 of gestation, or 4) ivermectin (1 mg/kg) on days 20 and 42 of gestation; these uses of ivermectin are extra-label. Otherwise, to minimize egg output, pups should be treated as early as possible; ideally, treatment should be given 2 wk after birth and repeated at 2-wk intervals to 2 mo of age, and then monthly to 6 mo of age. Nursing bitches should be treated at the same times as puppies. In cats, perinatal transmission can be greatly reduced by treating queens with a single dose of emodepside/praziquantel spot-on in the last week of pregnancy. Kittens should be treated with an appropriate anthelmintic at 3, 5, 7, and 9 wk of age, and then monthly to 6 mo of age. Nursing queens should be treated at the same time as kittens. In other animals, the appropriate frequency of preventive treatment for roundworms should be based on a risk assessment of the animal's environment.

Because the eggs adhere to many surfaces and become mixed in soil and dust, strict hygiene should be observed by people, particularly children, exposed to potentially contaminated animals or areas.

Baylisascaris in Dogs

The raccoon roundworm, Baylisascaris procyonis, is a common infection of raccoons in parts of North America and Europe. Adult parasites reside within the small intestine of raccoons and occasionally in the small intestine of dogs. It is thought that dogs become infected via ingestion of infective eggs or paratenic hosts (eg, rodents, rabbits, birds).

Ingestion of infective eggs results in visceral and neural larva migrans in many species, including people. As a result, the parasite is a significant zoonotic concern.

Clinical disease has not been reported in dogs with intestinal infections. However, signs are likely to be similar to those of T canis infections. In contrast, severe or fatal neural larva migrans has occasionally been reported in dogs.

Patent intestinal infections appear to occur uncommonly in dogs. However, it is likely that such infections are underdiagnosed; the eggs of B procyonis are similar in appearance to those of T canis, except they are slightly smaller (63–75 × 53–60 μm) and darker in color. Eggs of B procyonis may be found in dog feces as a result of ingestion of raccoon feces; shedding of such eggs should cease within 1–2 days.

Adult B procyonis and T canis passed in feces can be difficult to differentiate; female parasites should be dissected, and the eggs examined for identification.

Because of the zoonotic potential of B procyonis eggs, it is important that patent infections in dogs are diagnosed promptly and that appropriate treatment administered immediately. Ivermectin/pyrantel and milbemycin oxime/lufenuron at approved dosages have been shown to exhibit significant activity against intestinal infections in dogs with cure rates of 100% and 75%, respectively, after treatment with a single dose. However, these are not approved treatment protocols. Dogs should be confined and feces/parasites collected and disposed of for at least 3 days after treatment.

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