1. Roundworms

Three species of roundworms occur in dogs: Toxocara canis, Toxascaris leonina, and Toxocara cati. Roundworms are extremely common. It has been estimated that up to 80% of pups born in the United States are infected at birth.

Adult roundworms live in the intestinal tract of dogs. Eggs are passed in the feces. Ingested eggs hatch within the intestinal tract of the new host and may migrate throughout the body, including through the liver and lungs. Some larvae may migrate into muscle or other tissue and become dormant. In pregnant bitches carrying these dormant larvae, the stress of pregnancy may reactivate the larvae, allowing their migration across the placenta and into the pups; this usually happens in the last third of gestation. Some larvae may migrate into the mammary glands and be ingested by the pups as they nurse. These reactivated larvae also may migrate into the bitch’s intestinal tract, mature, and begin to reproduce.

Adult worms are spaghetti-like in appearance, ranging in length from 3 to 12 inches (4 to 18 cm). Roundworm eggs are slightly ovoid with a thick, slightly pitted shell containing a round embryo (Figure 4-1).

Figure 4-1 Roundworm (Toxocara canis) egg.

Roundworm infection usually is diagnosed by fecal flotation. Clinical signs in infected pups include general unthriftiness, abdominal distension, vomiting and diarrhea, coughing, seizures, and intestinal obstruction. Adult dogs may carry roundworms with no clinical signs of disease and have negative fecal flotation results. Puppies also may have negative fecal flotation results when carrying roundworms; one study of 4-week-old pups demonstrated positive fecal flotation in one third of the pups that were carrying roundworms.

Anthelmintics used to treat roundworms include piperazine, pyrantel pamoate, and fenbendazole (see Chapter 2). Environmental contamination is possible; the eggs can overwinter and are resistant to chemicals. Control of roundworms in a facility includes (1) treatment of all bitches late in gestation, (2) treatment of all pups beginning at 2 weeks of age, (3) regular fecal flotation testing of all dogs in the facility, and (4) excellent sanitation.

Roundworm eggs may persist in the environment, especially in soil, for years. Measures that may be used to attempt decontamination of the environment include the following:

2. Hookworms

One species of hookworm occurs commonly in dogs: Ancylostoma caninum. Two less common species, Ancylostoma braziliense and Urcinaria stenocephala, are found occasionally. Hookworm infection is common in the United States, especially in areas with moderate climate.

Adult hookworms live in the small intestine, where they attach to the wall and ingest large volumes of blood. Female hookworms lay thousands of eggs daily. The eggs are passed in the feces and hatch outdoors with development of the infective larval stage within 4 to 8 days. The larvae penetrate the skin and enter the circulatory system and lungs from where they are coughed up and swallowed, allowing them to colonize the intestinal tract. Passage from the pregnant bitch to pups through the placenta is uncommon; however, larvae form cysts in female dogs that remain latent until the bitch is stressed with pregnancy. Reactivated larvae will migrate through the bitch’s body and may migrate into the mammary tissue; passage of hookworm larvae to pups from the nursing bitch through the milk is not uncommon.

Adult hookworms are tubular, up to 1 inch (2.8 cm) long, and are red. The anterior (“mouth”) end is bent, creating the hook for which they are named. Hookworm eggs are thin-shelled and oval and contain two to eight round cells (Figure 4-2).

Figure 4-2 Hookworm (Ancylostoma caninum) egg.