Adult Macracanthorhynchus hirudinaceus (thorny-headed worms) are usually seen in the small intestine. They are 10 cm (males) to 65 cm (females) long, 3–9 mm thick, and slightly pink with a transversely wrinkled outer covering; superficially, they resemble ascarids. However,unlike ascarids, the anterior end bears a spiny, retractable proboscis or rostellum used for firm attachment to the intestinal wall. There is a granulomatous inflammation at the site of attachment resulting in nodule formation, which will regress ~1 mo after the parasite is no longer present. The eggs (dark brown, embryonated, with three embryonic envelopes, 90–110 × 50–65 μm) 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, and the infection is thus restricted to pigs with pasture access. The prepatent period is 2–3 mo, and longevity is ~1 yr. The female can lay ~260,000 eggs/day for several months.
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 necropsy, nodules usually can be seen 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. Clinical signs are not generally present; however, when perforations do occur a fatal peritonitis can result.
Levamisole and ivermectin are 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.