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 mcm). The life cycle is not completely known; however, 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; however, other animals may serve as paratenic hosts. The eggs look similar to those of O canis but are larger (~50 × 100 mcm).
Diagnosis of patent infections is unlikely, because experimentally induced infections did not persist after 1–12 days of patency. No treatment is necessary.