These nematodes are parasites of warm-blooded, fish eating animals, i.e. mammals and birds, as sexually mature adults.

The eggs and the successive stages of their larvae use invertebrates and increasing size classes of fishes as intermediate hosts.

In this fish host, the larvae penetrate the wall of the intestinal tract into the organs and body cavity.

Contracaecum nematodes are common in wild salmonids in the marine environment where they may occur abundantly.

Dependent on the species of Contracaecum the definitive host can be mammals, including humans, birds, and in some cases, fish.

The symptoms of anisakiasis include abdominal pain and distention, diarrhea and nausea, faeces with high proportions of blood and mucus and a mild fever.