Macracanthorhynchushirudinaceus (Thorny-Headed Worm) in Pigs

Macracanthorhynchus hirudinaceusare acanthocephalan parasites (thorny-headed worms) of the small intestine of pigs. Adults are large, with females up to 65 cm (25.6 inches) long, 3–9 mm (0.1–0.35 inches) thick, and white to slightly pink, with a transversely wrinkled cuticle. They superficially resemble ascarids; however, they can also be mistaken for tapeworms. The cuticle can look pseudoannulated or pseudosegmented. However, unlike ascarids, the anterior end has a spiny, retractable proboscis used for firm attachment to the jejunal wall.

Macracanthorhynchus egg

Image

Macracanthorhynchus sp egg recovered by fecal sedimentation, 99 × 55 mcm. Eggs are dense, large, and dark in color. The multiple embryonic envelopes give the eggs a 'layered' appearance. 400X total magnification.

Courtesy of Dr. Heather Walden.

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 are large, dark brown, larvated, with three embryonic envelopes (90–110 × 50–65 mcm) (see Macracanthorhynchus egg image). Eggs 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 per day for several months.

Clinical signs are generally absent; when present, they are nonspecific. Antemortem diagnosis of thorny-headed worm infection is difficult, because the ova do not float reliably in many conventional salt or sugar solutions. They 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 can occur and can cause diarrhea and abdominal pain (1, 2).

Ivermectin and doramectin are reported as effective treatments for thorny-headed worms (1). 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.