The large roundworms (ascaridoid nematodes) of dogs and cats are common, especially in puppies and kittens. Of the 3 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 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 the puppies as early as 1 wk after birth. Some larvae migrate to the mammary gland, so that 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 feces. 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.

Toxocara canis ova

Toxocara canis

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 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. 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.

Diagnosis

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.

Roundworm eggs

Treatment and Control

In dogs, compounds licensed for treatment of roundworm infections include fenbendazole, milbemycin, moxidectin, nitroscanate, piperazine, and pyrantel (see Gastrointestinal Parasites of Small Animals: 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, while in Canada approved treatment requires 2 doses 1 mo apart. Preventive programs for heartworm infection using milbemycin, milbemycin/lufenuron, milbemycin/praziquantel, 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 Gastrointestinal Parasites of Small Animals: Drugs for Intestinal Helminths of Dogs Approved in the USA and UK).

Table 1
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Drugs for Intestinal Helminths of Dogs Approved in the USA and UK Drug/Drug Combination Dosage (mg/kg) Route of Administration Helminths Active Againsta Approved only in USA Dichlorophene 220 PO Dipylidium caninum, Taenia pisiformis Fenbendazole 50, sid for 3 days PO Toxocara canis, Toxascaris leonina, Ancylostoma caninum, Uncinaria stenocephala, Trichuris vulpis, T pisiformis Epsiprantel 5.5 PO D caninum, T pisiformis Milbemycin oxime 0.5 PO T canis, T leonina, A caninum, T vulpis Moxidectin 0.17 SC A caninum, U stenocephala Piperazine (various salts) 47–133b PO T canis, T leonina Praziquantel 5–12.5 PO D caninum, T pisiformis, Echinococcus granulosusc, E multilocularisc 5–11.4 SC, IM D caninum, T pisiformis, E granulosusc, E multi-locularisc Pyrantel pamoate 5 (10 for dogs <2.3 kg) PO T canis, T leonina, A caninum, U stenocephala Ivermectin + pyrantel pamoate 0.006/5 PO T canis, T leonina, A caninum, A braziliense, U stenocephala Praziquantel + pyrantel pamoate 5/5 PO T canis, T leonina, A caninum, A braziliense, U stenocephala, D caninum, T pisiformis Praziquantel + pyrantel pamoate + febantel 5/5/25 PO T canis, T leonina, A caninum, U stenocephala, T vulpis, D caninum, T pisiformis, E granulosus, E multilocularis Praziquantel + pyrantel pamoate + ivermectin 5/5/0.006 PO T canis, T leonina, A caninum, A braziliense, U stenocephala, D caninum, T pisiformis Approved only in UK Fenbendazole 100 PO T canis, T leonina, Ancylostoma spp, Uncinaria spp, Trichuris spp, Taenia spp 50, sid for 3 days PO T canis, T leonina, Ancylostoma spp, Uncinaria spp, Trichuris spp, Taenia spp Nitroscanate 50 PO T canis, T leonina, A caninum, U stenocephala, D caninum, T pisiformis, Taenia hydatigena, E granulosusd Praziquantel 5 PO D caninum, T pisiformis, Taenia multiceps, T hydatigena, T ovis, E granulosus, E multilocularis 3.5–7.5 SC, IM D caninum, T pisiformis, E granulosus, E multilocularis Selamectin 6 Topical T canis Febantel + pyrantel embonate 15/14.4 PO T canis, A caninum, T vulpis Milbemycin oxime + praziquantel 0.5/5 PO T canis, T leonina, A caninum, T vulpis, D caninum, Taenia spp, E granulosus, E multilocularis, Mesocestoides spp Praziquantel + pyrantel embonate + febantel 5/14.4/15 PO T canis, T leonina, A caninum, U stenocephala, T vulpis, D caninum, Taenia spp, E granulosus, E multilocularis Praziquantel + pyrantel embonate + oxantel embonate 5/5/20 PO T canis, T leonina, A caninum, U stenocephala, T vulpis, D caninum, Taenia spp Approved both in USA and UK Milbemycin oxime + lufenuron 0.5/10 PO T canis, T leonina, A caninum, T vulpis Moxidectin + imidacloprid 2.5/10 Topical T canis, T leonina, A caninum, U stenocephala, T vulpis a Many of these drugs are also approved for use against other types of parasites (eg, fleas, heartworms) not listed here. b Repeat in 1–20 days c Only some products d Aid in control Drugs for Intestinal Helminths of Dogs Approved in the USA and UK Drug/Drug Combination Dosage (mg/kg) Route of Administration Helminths Active Againsta Approved only in USA Dichlorophene 220 PO Dipylidium caninum, Taenia pisiformis Fenbendazole 50, sid for 3 days PO Toxocara canis, Toxascaris leonina, Ancylostoma caninum, Uncinaria stenocephala, Trichuris vulpis, T pisiformis Epsiprantel 5.5 PO D caninum, T pisiformis Milbemycin oxime 0.5 PO T canis, T leonina, A caninum, T vulpis Moxidectin 0.17 SC A caninum, U stenocephala Piperazine (various salts) 47–133b PO T canis, T leonina Praziquantel 5–12.5 PO D caninum, T pisiformis, Echinococcus granulosusc, E multilocularisc 5–11.4 SC, IM D caninum, T pisiformis, E granulosusc, E multi-locularisc Pyrantel pamoate 5 (10 for dogs <2.3 kg) PO T canis, T leonina, A caninum, U stenocephala Ivermectin + pyrantel pamoate 0.006/5 PO T canis, T leonina, A caninum, A braziliense, U stenocephala Praziquantel + pyrantel pamoate 5/5 PO T canis, T leonina, A caninum, A braziliense, U stenocephala, D caninum, T pisiformis Praziquantel + pyrantel pamoate + febantel 5/5/25 PO T canis, T leonina, A caninum, U stenocephala, T vulpis, D caninum, T pisiformis, E granulosus, E multilocularis Praziquantel + pyrantel pamoate + ivermectin 5/5/0.006 PO T canis, T leonina, A caninum, A braziliense, U stenocephala, D caninum, T pisiformis Approved only in UK Fenbendazole 100 PO T canis, T leonina, Ancylostoma spp, Uncinaria spp, Trichuris spp, Taenia spp 50, sid for 3 days PO T canis, T leonina, Ancylostoma spp, Uncinaria spp, Trichuris spp, Taenia spp Nitroscanate 50 PO T canis, T leonina, A caninum, U stenocephala, D caninum, T pisiformis, Taenia hydatigena, E granulosusd Praziquantel 5 PO D caninum, T pisiformis, Taenia multiceps, T hydatigena, T ovis, E granulosus, E multilocularis 3.5–7.5 SC, IM D caninum, T pisiformis, E granulosus, E multilocularis Selamectin 6 Topical T canis Febantel + pyrantel embonate 15/14.4 PO T canis, A caninum, T vulpis Milbemycin oxime + praziquantel 0.5/5 PO T canis, T leonina, A caninum, T vulpis, D caninum, Taenia spp, E granulosus, E multilocularis, Mesocestoides spp Praziquantel + pyrantel embonate + febantel 5/14.4/15 PO T canis, T leonina, A caninum, U stenocephala, T vulpis, D caninum, Taenia spp, E granulosus, E multilocularis Praziquantel + pyrantel embonate + oxantel embonate 5/5/20 PO T canis, T leonina, A caninum, U stenocephala, T vulpis, D caninum, Taenia spp Approved both in USA and UK Milbemycin oxime + lufenuron 0.5/10 PO T canis, T leonina, A caninum, T vulpis Moxidectin + imidacloprid 2.5/10 Topical T canis, T leonina, A caninum, U stenocephala, T vulpis a Many of these drugs are also approved for use against other types of parasites (eg, fleas, heartworms) not listed here. b Repeat in 1–20 days c Only some products d Aid in control

Drugs licensed for treatment of ascarid infections in cats include emodepside, fenbendazole, milbemycin, moxidectin, piperazine, and selamectin, (see Gastrointestinal Parasites of Small Animals: Drugs for Intestinal Helminths of Cats Approved in the USA and UK). Pyrantel is also licensed in Canada. Heartworm-preventive programs that use milbemycin, milbemycin/praziquantel, moxidectin/imidacloprid, or selamectin also control ascarid infections in cats (see Gastrointestinal Parasites of Small Animals: Drugs for Intestinal Helminths of Cats Approved in the USA and UK).

Table 2
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Drugs for Intestinal Helminths of Cats Approved in the USA and UK Drug/Drug Combination Dosage (mg/kg) Route of Administration Helminths Active Againsta Approved only in USA Epsiprantel 2.75 PO Dipylidium caninum, Taenia taeniaeformis Ivermectin 0.024 PO Ancylostoma tubaeforme, A braziliense Milbemycin oxime 2 PO Toxocara cati, A tubaeforme Piperazine (various salts) 47–103b PO T cati c, Toxascaris leonina Praziquantel 4.6–10 PO D caninum, T taeniaeformis 5–10 SC, IM D caninum, T taeniaeformis Praziquantel + pyrantel pamoate 5/20 PO T cati, A tubaeforme, D caninum, T taeniaeformis Approved only in UK Fenbendazole 50, sid for 3 days; 100 PO T cati, T leonina, A tubaeforme, A braziliense, Uncinaria stenocephala, Taenia spp Praziquantel 5 PO D caninum, T taeniaeformis, Echinococcus multilocularisc 8 Topical D caninum, Taenia spp, E multilocularis 3.5–7.5 SC, IM D caninum, T taeniaeformis, E multilocularisc Milbemycin oxime + praziquantel 2/5 PO T cati, A tubaeforme, D caninum, Taenia spp, E multilocularis Praziquantel + pyrantel embonate 5/57.5 PO T cati, T leonina, D caninum, T taeniaeformis Approved in both USA and UK Selamectin 6 Topical T cati, A tubaeforme Emodepside + praziquantel 3/12 Topical T cati, A tubaeforme, D caninum, T taeniaeformis, T leoninad, E multilocularisd Moxidectin + imidacloprid 1/10 Topical T cati, A tubaeforme a Many of these drugs are also approved for use against other types of parasites (eg, fleas, heartworms) not listed here. b Repeat in 1–30 days c Only some products d Only in the UK Drugs for Intestinal Helminths of Cats Approved in the USA and UK Drug/Drug Combination Dosage (mg/kg) Route of Administration Helminths Active Againsta Approved only in USA Epsiprantel 2.75 PO Dipylidium caninum, Taenia taeniaeformis Ivermectin 0.024 PO Ancylostoma tubaeforme, A braziliense Milbemycin oxime 2 PO Toxocara cati, A tubaeforme Piperazine (various salts) 47–103b PO T cati c, Toxascaris leonina Praziquantel 4.6–10 PO D caninum, T taeniaeformis 5–10 SC, IM D caninum, T taeniaeformis Praziquantel + pyrantel pamoate 5/20 PO T cati, A tubaeforme, D caninum, T taeniaeformis Approved only in UK Fenbendazole 50, sid for 3 days; 100 PO T cati, T leonina, A tubaeforme, A braziliense, Uncinaria stenocephala, Taenia spp Praziquantel 5 PO D caninum, T taeniaeformis, Echinococcus multilocularisc 8 Topical D caninum, Taenia spp, E multilocularis 3.5–7.5 SC, IM D caninum, T taeniaeformis, E multilocularisc Milbemycin oxime + praziquantel 2/5 PO T cati, A tubaeforme, D caninum, Taenia spp, E multilocularis Praziquantel + pyrantel embonate 5/57.5 PO T cati, T leonina, D caninum, T taeniaeformis Approved in both USA and UK Selamectin 6 Topical T cati, A tubaeforme Emodepside + praziquantel 3/12 Topical T cati, A tubaeforme, D caninum, T taeniaeformis, T leoninad, E multilocularisd Moxidectin + imidacloprid 1/10 Topical T cati, A tubaeforme a Many of these drugs are also approved for use against other types of parasites (eg, fleas, heartworms) not listed here. b Repeat in 1–30 days c Only some products d Only in the 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 (licensed 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, and 4) ivermectin (1.0 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. Kittens should be treated 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.

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.

Last full review/revision March 2012 by Andrew S. Peregrine, BVMS, PhD, DVM, DEVPC