Macracanthorhynchus hirudinaceus are an acanthocephalan parasite (thorny-headed worms) of the small intestine of pigs. Adults are 10 cm (males) to 65 cm (females) long, 3 to 9 mm thick, and slightly pink with a transversely wrinkled outer covering; superficially, they resemble ascarids (but can also be mistaken for tapeworms). However, unlike ascarids, the caudal end bears a spiny, retractable proboscis used for firm attachment to the jejunal wall. A granulomatous inflammation at the site of attachment results in nodule formation, which will regress approximately 1 month after the parasite is no longer present. The eggs (dark brown, embryonated, with three embryonic envelopes, 90–110 × 50–65 mcm) passed in the feces are ingested by the grubs of various beetles that serve as intermediate hosts.
Pigs become infected by ingesting either grubs or adult beetles; thus, the infection is restricted to pigs with pasture access. The prepatent period is 2–3 months, and longevity is approximately 1 year. The female can lay approximately 260,000 eggs/day for several months.
Clinical signs are generally absent; when present, they are nonspecific. Antemortem diagnosis is difficult, because the ova do not float reliably in many conventional salt solutions and thus should be looked for in the sediment if using solutions of low specific gravity. At postmortem examination, nodules usually can be observed through the serosa. Because the proboscis is longer than the jejunal wall is thick, perforations may occur; however, the inflammatory response generally seals off any perforation. When perforations do occur a fatal peritonitis can result.
Ivermectin is effective for treatment. Control depends on avoiding use of contaminated hog lots or pastures or by regular removal of feces when pigs are kept in sties or small runs.