Oncicola canis and other species are rarely found in the small intestine of dogs and cats. They are white and ~8–15 mm long, and their thorny heads are embedded in the mucosa. The females lay brown, thick-shelled, embryonated, wide oval eggs (43–50 × 67–72 μm). The life cycle is not completely known, but it is thought to include an arthropod intermediate host and paratenic hosts such as lizards, birds, or armadillos. Most infections cause no clinical signs.
Macracanthorhynchus ingens, naturally a parasite of raccoons, is occasionally found in dogs. The usual observation is of a large (8–12 cm), white, wrinkled worm passed in the feces. No clinical signs have been definitively associated with the infection. The life cycle requires a millipede as an intermediate host, but other animals may serve as paratenic hosts. The eggs look similar to those of Oncicola canis but are larger (~50 × 100 μm). Diagnosis of patent infections is unlikely, because experimentally induced infections did not persist after 1–12 days of patency. No treatment is necessary.