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Acanthocephalans (thorny-headed worms, phylum Acanthocephala) are obligate endoparasites (i.e., internal parasites), ranging in size from 1 mm to over 40 cm, with a two-host life cycle involving arthropods and vertebrates. Cases of acanthocephaliasis in humans (which are not very common as far as is known) generally occur in areas where insects are eaten for dietary or medicinal purposes. Nearly all known cases in humans have involved infection of the gastrointestinal tract, although Haustein et al. (2009) reported removing an immature unidentified acanthocephalan from a patient's eye. Bolbosoma acanthocephalans occasionally infect humans (Tada et al. 1983), although the principle agents of acanthocephaliasis in humans are Macracanthorhynchus hirudinaceous and Moniliformis moniliformis. The normal definitive hosts (i.e. the hosts in which the parasites reach maturity) for Bolbosoma species are marine mammals, especially cetaceans. The intermediate hosts for Bolbosoma species are microcrustacea (e.g., copepods and krill), with fish serving as paratenic hosts (paratenic hosts are intermediate hosts not required for normal development in the parasite life cycle). Humans can become infected with Bolbosoma from eating undercooked fish.
(additional source: Centers for Disease Control Parasites and Health website)