The large roundworms (ascaridoid nematodes) of dogs and cats are common, especially in puppies and kittens. Two species (T canis and T leonina) infect dogs and are found globally. Three species infect cats: T cati and T leonina occur globally, and Toxocara malaysiensis is found in cats in parts of eastern Asia. For all species, adult parasites are stout cream-colored nematodes that are found in the small intestine and typically range from 3 to 15 cm in length.
In dogs, the most important roundworm is T canis, not only because its larvae may infect humans but also because infections are generally common and may impact puppy health. Also, fatal infections may occasionally occur in young pups. In puppies, the most common mode of infection is transplacental transfer. If pups < 3 months old ingest embryonated infective eggs, the hatched larvae penetrate the intestinal mucosa and reach the lungs via the liver and bloodstream. Then they are coughed up and swallowed, maturing 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 intestines of puppies as early as 1 week after birth. Some larvae migrate to the mammary gland, so pups may also be infected via the milk. Lastly, dogs may also become infected by ingestion of paratenic hosts (eg, rodents), which, after ingestion of infective eggs, have dormant parasites in their tissues. The prepatent period ranges from 2 to 3 weeks after prenatal infection to 4 to 5 weeks after ingestion of eggs or paratenic hosts.
Dogs typically develop immunity to T canis with age. However, during the perinatal period, the immunity of the bitch to 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.
Eggs of T canis become infective (larvated) after ~2–4 weeks in the environment. If infective eggs of T canis are swallowed by humans, systemic migration of larvae may occur. Most human infections are asymptomatic; however, 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.
T leonina is present in adolescent/adult dogs and is a less common infection than T canis. Dogs become infected via ingestion of infective eggs and infected paratenic hosts (eg, rodents). Infections are not zoonotic.
The most important of the three roundworms affecting cats is T cati because it is the most common, infections may impact the health of kittens, and it is zoonotic. 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.
As in dogs, T leonina infections are acquired via ingestion of infective eggs and larvae in paratenic hosts, eg, rodents; migration is restricted to the intestinal wall so that neither prenatal nor transmammary transmission occurs. Infections are not zoonotic.
Toxocara malaysiensis is a common infection in cats in countries such as China, Malaysia, and Vietnam. However, both the life cycle and zoonotic potential are currently unclear.
Clinical Findings and Lesions With Roundworms in Small Animals
Infections with all roundworms are often subclinical. Disease is most common in young puppies and kittens. 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 “pot-bellied.” Worms may be vomited and are often voided in the feces. Diarrhea with mucus may be evident.
In the early stages of infections with Toxocara spp, migrating larvae may cause an eosinophilic pneumonia, which can be associated with coughing. In puppies with severe T canis infections, verminous pneumonia, ascites, fatty liver, and mucoid enteritis are common. Cortical kidney granulomas containing larvae may occur.
Diagnosis of Roundworms in Small Animals
Identification of adult parasites in vomitus or feces
Examination of feces for parasite eggs or roundworm antigen
Adult roundworms are cream colored and typically 10–15 cm in length (T canis), 5–10 cm in length (T leonina), 3–10 cm in length (T cati) and 1–8 cm in length (T malaysiensis). T canis, T leonina and T malaysiensis are similar in morphology with tapered ventral and dorsal ends. By contrast, T cati has a distinctive arrow shape at the ventral end.
Infection in dogs and cats is diagnosed by detection of eggs in feces. Sensitivity is maximized by using a centrifugal fecal flotation method. Distinguishing the spherical, pitted-shelled eggs of Toxocara spp (T canis 80–90 × 75 mcm; T cati 65 × 75 mcm) from the oval, smooth-shelled eggs of Toxascaris leonina (75–85 × 60–75 mcm) is important because of the public health significance of the former.
At least one commercial diagnostic company offers a roundworm fecal antigen test for dogs and cats that detects antigen produced by Toxocara spp, T leonina, and Baylisascaris procyonis. However, fecal examination for parasite eggs is required to determine which parasite(s) are responsible for the presence of roundworm antigen.
Treatment and Control of Roundworms in Small Animals
Multiple anthelmintics are available for treatment and prevention programs
Routine fecal monitoring
Prompt disposal of feces
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 US and UK Drugs for Intestinal Helminths of Dogs Approved in the US and UK ). For drugs with activity against only adult parasites, two treatments 10–14 days apart should be administered to eliminate an infection. Fecal examination 7–14 days after the second treatment should be performed to confirm the efficacy of treatment. In Europe, selamectin is approved to treat T canis infections with a single dose, whereas in Canada approved treatment requires two doses 1 month apart.
Preventive programs for heartworm infection using milbemycin, milbemycin/afoxolaner, milbemycin/lufenuron, milbemycin/lufenuron/praziquantel, milbemycin/praziquantel, milbemycin/spinosad, moxidectin, 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 US ( see Table: Drugs for Intestinal Helminths of Dogs Approved in the US and UK Drugs for Intestinal Helminths of Dogs Approved in the US and UK ).
Drugs approved for treatment of ascarid infections in cats include emodepside, eprinomectin, fenbendazole, milbemycin, moxidectin, piperazine, pyrantel, and selamectin ( see Table: Drugs for Intestinal Helminths of Cats Approved in the US and UK Drugs for Intestinal Helminths of Cats Approved in the US and UK ). As in dogs, many drugs only kill adult parasites. Thus, when used to treat infected cats, such drugs should be administered twice, 10–14 days apart. Fecal examination 10–14 days after the second treatment should be used to confirm the efficacy of treatment.
Heartworm-preventive programs that use eprinomectin/praziquantel, eprinomectin/praziquantel/fipronil/methoprene, milbemycin, milbemycin/praziquantel, moxidectin/fluralaner, moxidectin/imidacloprid, selamectin, or selamectin/sarolaner also control ascarid infections in cats ( see Table: Drugs for Intestinal Helminths of Cats Approved in the US and UK Drugs for Intestinal Helminths of Cats Approved in the US and UK ).
Environmentally resistant larvated eggs on the ground and somatic larvae in the bitch are the main reservoirs of infection. For control of T canis, perinatal transmission of infection can be greatly decreased by treating bitches with one of the following (note that these uses of ivermectin are extra-label):
daily doses of fenbendazole (25 mg/kg, PO) from day 40 of gestation to day 2 after whelping (approved in the UK)
ivermectin (0.3 mg/kg, SC) on days 0, 30, and 60 of gestation, and 10 days after whelping
ivermectin (0.5 mg/kg, SC) on days 38, 41, 44, and 47 of gestation, or
ivermectin (1 mg/kg, SC) on days 20 and 42 of gestation 1 References Roundworms (Toxocara spp, Toxascaris leonina) are common infections of dogs and cats, particularly young animals. Toxocara canis and Toxocara cati are zoonotic. Adult... read more (Note: Not recommended for Collies or related breeds due to increased risk for ivermectin toxicity)
Otherwise, to minimize egg output, pups should be treated as early as possible; ideally, treatment should be administered 2 weeks after birth and repeated at 2-week intervals to 3 months, and then monthly to 6 months. Nursing bitches should be treated at the same times as puppies. In other animals, the appropriate frequency of preventive treatment for roundworms should be based on a risk assessment of the animal's environment.
For control of T cati in cats, perinatal transmission can be greatly decreased 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 2 weeks, then at 2-week intervals to 12 weeks, then monthly to 6 months. Nursing queens should be treated at the same time as kittens. In older animals, the appropriate frequency of preventive treatment should be based on a risk assessment of the animal's environment. An appropriate control program for T cati will control other roundworms.
In both dogs and cats, routine monitoring of feces should be carried out at least annually to ensure that the roundworm preventive program is efficacious. Feces should be picked up promptly and disposed to minimize environmental contamination. Because Toxocara spp eggs adhere to many surfaces and become mixed in soil and dust, strict hygiene should be observed, particularly in children, when exposed to potentially contaminated animals or areas.
Epe C, Pankow WR, Hackbarth H, Schnieder T, Stoye M. A study on the prevention of prenatal and galactogenic Toxocara canis infections in pups by treatment of infected bitches with ivermectin or doramectin. Appl Parasitol. 1995 May;36(2):115–123. PMID: 7550440.
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 humans. As a result, the parasite is a noteworthy zoonotic concern.
Clinical signs of disease have 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, particularly in young 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 mcm) 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. Thus, to confirm that eggs in feces represent a genuine infection, a second fecal sample should be collected 7 days after the first sample and analyzed; animals that test positive for only the first sample most likely have a spurious infection.
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 appropriate treatment administered immediately. Ivermectin/pyrantel and milbemycin oxime/lufenuron at approved dosages have been shown to exhibit substantial 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.
Roundworms (Toxocara spp and Toxascaris leonina) are common infections of dogs and cats, particularly puppies and kittens. Infections are acquired via multiple routes.
Adult parasites reside in the small intestine. While infections are often subclinical, diarrhea and/or loss of condition may be observed.
Diagnosis is typically based on identification of parasite eggs and/or roundworm antigen in feces.
Larvated eggs of Toxocara canis and Toxocara cati are potentially zoonotic. Feces should therefore be disposed of promptly to minimize this risk.
Multiple anthelmintics are available for treatment and prevention.
All dogs and cats should be treated preventively for roundworms.
For More Information
Ascarid for Dog. Companion Animal Parasite Council. Accessed October 3, 2022. https://capcvet.org/guidelines/ascarid/
Roundworms. Tropical Council for Companion Animal Parasites Ltd. Accessed Oct 3, 2022. https://www.troccap.com/canine-guidelines/gastrointestinal-parasites/roundworms/
Ascarids for Feline. Tropical Council for Companion Animal Parasites Ltd. Accessed Oct 3, 2022. https://www.troccap.com/feline-guidelines/gastrointestinal-parasites/ascarids-feline/
Endoparasites. European Scientific Counsel Companion Animal Parasites. Accessed Oct 3, 2022. https://www.esccap.org/parasites/Endoparasites/1/#c7