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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.
Macracanthorhynchus hirudinaceus is one of the two main acanthocephalans known to infect humans and cause acanthocephaliasis (the other being Moniliformis moniliformis). The definitive host for M. hirudinaceus (i.e., the host in which the parasite reaches maturity) is typically a pig, although carnivores and primates, including humans, may serve as accidental hosts. The parasite's eggs are ingested by an intermediate host (typically a scarabaeoid or hydrophilid beetle), which is subsequently eaten by the definitive host, resulting in infection of the definitive host. In infected human hosts, the worms seldom mature and, if they do, they generally do not produce eggs.