Roundworms are relatively large nematodes (3 to 18 cm long) found in the small intestine. Toxocara canis affects virtually all puppies because of prenatal infection. Many kittens are infected with Toxocara cati soon after birth by transmammary transmission. Toxascaris leonina affects both dogs and cats but is less common.
Pregnant bitches have arrested (dormant) T. canis larvae in somatic tissues that become activated after day 42 of gestation. These larvae migrate to the uterus and infect developing fetuses. Postnatal infection may occur as a small amount of larvae are excreted in the milk. Older puppies acquire T. canis after ingestion of embryonated eggs, and larvae may undergo hepatotracheal migration before being coughed up and swallowed, leading to maturation in the small intestine. Pregnant queens also have arrested T. cati that migrate during gestation to mammary glands. Neonates commonly acquire larvae by this route, whereas older kittens are infected by ingestion of larvae or paratenic hosts such as rodents. T. leonina larvae are acquired by ingestion of embryonated ova or paratenic hosts.
Mild infections may be asymptomatic, whereas moderate numbers of worms cause vomiting, diarrhea, lethargy, and an unthrifty appearance. A rounded, fluid-filled abdomen (“pot belly”) is common with heavier worm burdens. Severe complications including intestinal obstruction, intussusception, and death are possible. Larvae migrating through the liver, lungs, and other organs occasionally result in tissue damage.
Roundworm ova are easily identified on routine fecal flotation. Adult worms may be passed in the feces or found in vomitus.
A number of effective anthelmintics are available (Table 19-1). Deworming is ideally started at 2 weeks of age and repeated every 2 weeks for four treatments. Following this, certain heartworm preventatives will control roundworms if continued year-round (see Table 19-1). Prenatal infection with T. canis can be prevented in puppies by daily administration of fenbendazole to pregnant bitches starting at day 40 of gestation and continuing through day 14 postpartum. Selamectin also prevents prenatal transmission in puppies if given at days −40, −10, +10, and +40 before and after whelping. Similar studies have not been reported in cats. Roundworm ova persist in the environment for months and possibly years. Removal and proper disposal of feces at least twice a week will decrease the risk of soil contamination.
T. canis and, to a lesser extent, T. cati cause visceral larval migrans and ocular larval migrans in humans. Accidental ingestion of soil contaminated with embryonated ova is considered to be the main source of infection, but roundworm ova have also been found in hair samples clipped from the perianal areas of dogs. Households with children or immunosuppressed individuals should be informed of the risk of toxocariasis.