Thorny-headed Worm Infection

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Infection by parasitic worm Macracanthorhynchus hirudinaceus.


This parasite occurs only in swine. Infection is usually seen in feeder or finishing pigs. These pigs are typically old enough to have had opportunity to root and forage, thus having access to the larvael stages of beetles. Infection is relatively common in pigs raised outdoors in southern states.

Historical information

The parasite was once believed to be a parasite only in southern states, but it is now found farther north into the lower Midwestern states.


Macracanthorhynchus hirudinaceus has an elongated, sack-like body with no internal alimentary canal. There is a protrusible proboscis armed with about six rows of spiny hooks for attachment to the intestinal wall. The worms are 10-40 cm long, and up to 9 mm wide. They are transversely wrinkled, slightly curved, light pink and attenuated posteriorly. The brown ova are up to 110 by 65 µm and have a three-layered shell. The ova are resistant to environmental degradation.


The eggs are passed and persist in swine feces. The eggs are ingested by beetles, their grubs or other arthropods that act as intermediate hosts. Swine then ingest these intermediate hosts. The eggs hatch in the intestinal tract of the pig and the worms attach there, usually in the ileum. The parasites develop by absorbing nutrients from the ingesta.


The worm’s proboscis is inserted deeply into the wall of the intestine. It sometimes penetrates into the peritoneal cavity and introduces infection that leads to peritonitis. Intestinal bacteria usually infect the attachment site and an inflammatory nodule develops around it. The parasite occasionally moves to a new feeding site. The lesions cause hemorrhage, loss of blood proteins and localized intestinal infections. Animals with only a few parasites appear not to be seriously affected. Animals with many parasites tend to become unthrifty from loss of blood, the toxic effect of many localized lesions, and impaired -nutrition.

Clinical signs

There are few or no signs with light infections. Pigs with heavy infections become unthrifty and have rough hair coats. Occasionally a parasitized pig dies as a consequence of intestinal perforation followed by peritonitis.


Scattered inflamed nodules one to two cm in diameter are apparent through the serosa of the small intestine, usually the ileum. Attached worms often are large enough to be palpated through the unopened intestine. In the opened intestine, the sites that worms recently vacated appear as deep, infected ulcers. Old, healed sites appear as firm, fibrous nodules. The parasites are easily found and to the uninitiated, are spectacular in size, up to 40 cm long.


Diagnosis usually is made at necropsy by finding and identifying the parasites. Alternatively, diagnosis can be made through laboratory fecal examinations


Control is through avoidance of exposure to contaminated lots or pastures and to beetles, the usual intermediate hosts. Beetles should be eradicated around feed storage areas and other sites accessible to the swine. There is little data on treatment but levamisole has been recommended. See the table, Anthelmintics and Parasiticides.