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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.
Moniliformis moniliformis is one of the two main acanthocephalans known to infect humans and cause acanthocephaliasis (the other being Macracanthorhynchus hirudinaceus). The definitive host for M. moniliformis (i.e., the host in which the parasite reaches maturity) is typically a rat, although carnivores and primates, including humans, may serve as accidental hosts. The parasite's eggs are ingested by an intermediate host (typically a beetle or cockroach), which is subsequently eaten by the definitive host, resulting in infection of the definitive host. In infected human hosts, the worms seldom mature or mature but do not produce eggs.
Libersat and Moore (2000) found that cockroaches (Periplaneta americana) infected by M. moniliformis showed reduced predator avoidance behavior, a phenomenon that presumably benefits the parasite by increasing the likelihood that its cockroach host will be ingested by a rodent, a necessary event for the parasite to complete its life cycle.