A Systematic Overview of Zoonotic Helminth Infections in North America

Blaine A. Mathison, BS, M(ASCP); Bobbi S. Pritt, MD, MS, DTM&H


Lab Med. 2018;49(4):e61-e93. 

In This Article


Acanthocephalans are multicellular parasitic animals commonly referred to as thorny-headed worms. Although members of this group superficially resemble nematodes, they are more closely related to microscopic rotifers, or are possibly even nested within the phylum Rotifera.[84] Acanthocephalans have a complex life cycle involving a vertebrate definitive host and an arthropod intermediate host, with fish, reptiles, and amphibians serving as paratenic hosts. Human infection is rarely documented, and most cases of acanthocephaliasis have been attributed to members of the genera Macracanthorhynchus and Moniliformis. In addition to the species covered herein, there is 1 report of human infection from North America of Corynosoma strumosa, which is normally a parasite of seals, sea lions, and fish-eating birds, in an Eskimo in Alaska.[85]

Macracanthorhynchus hirudinaceous, M. ingens, Moniliformis moniliformis (Acanthocephaliasis).General considerations: Most human cases of acanthocephaliasis are attributed to Macracanthorhynchus hirudinaceous, M. ingens, and Moniliformis moniliformis. Macracanthorhynchus hirudinaceous and M. moniliformis probably occur nearly worldwide in places where their primary definitive hosts (pigs and rats, respectively) live, whereas M. ingens is endemic to the eastern two-thirds of North America, where its primary definitive host is the raccoon. Although most human cases involving Macracanthorhynchus spp. have been attributed to M. hirudinaceous, it has been suggested[86,87] that most cases from the United States are probably caused by M. ingens, especially given that it is a native species, along with the anthropophilic nature of the raccoon host.

Adults reside in the small intestine of the definitive host, where they attach to the intestinal mucosa by means of an armed rostellum. This rostellum is not used for feeding; rather, nutrients taken up by the host are absorbed directly through the cuticle. Mated females release eggs that are shed in the feces of the definitive hosts. Eggs are shed containing a fully developed infective first-stage larva (acanthor). After eggs are ingested by an appropriate arthropod intermediate host, the acanthor molts into the second larval stage, known as an acanthella. Primary intermediate hosts for M. hirudinaceous, M. ingens, and M. moniliformis are scarabaeoid beetles, millipedes, and cockroaches, respectively.

After several weeks to several months, the acanthella becomes an infective cystacanth that remains dormant in the tissue of the arthropod host. The definitive host becomes infected after eating arthropods harboring infective cystacanths.[87] Humans also become infected after the ingestion of infected arthropods harboring cystacanth larvae. In the United States, most cases are seen in young children, who are more likely to put insects in their mouths.

Clinical manifestations and pathologic features: Patients with acanthocephalan infection may experience severe symptoms such as high fever, severe abdominal pain, lack or loss of appetite, nausea, diarrhea, frankly bloody stools, abdominal distension, intestinal perforation, intussusception, ascites, and peritonitis; they may even die.[6,87] In particular, patients with M. hirudinaceus are likely to present with an acute syndrome.[6] Leukocytosis and eosinophilia are common. Treatment is with albendazole or other antihelminth drugs.[2]

Laboratory diagnosis: Typically, acanthocephaliasis is diagnosed by finding adult worms in stool specimens or, less commonly, by the finding of eggs in O&P examination of stool. Moniliformis moniliformis seems more likely to release eggs into the feces of the human host than Macracanthorhynchus spp.

Adults are large pseudocoelomate worms. They may have constrictions or wrinkles but lack true segmentation. Adult female Macracanthorhynchus species (Figure 2E) are 12–32 cm long; males are smaller, at 7–8 cm long. They are often heavily wrinkled or distorted and may be mistaken for mucus strands. Moniliformis moniliformis (Figure 2F) is slightly smaller and more slender, with females measuring 10–27 cm long and males 4–10 cm long; adults have prominent annulations in the anterior two-thirds or so of the body. Adults of both genera have a retractable proboscis that contains an armed rostellum with recurved hooks.[87] In clinical specimens, the proboscis may be retracted, and the worm may require dissection for this feature to be examined.

Adult Macracanthorhynchus species can be separated by the length-to-width ratio of the proboscis or by performing a morphometric analysis of the hooks.[88] Eggs of Macracanthorhynchus species (Figure 3K) are 80–100 μm long × 50 μm wide and have a tick shell that is heavily textured with a lattice-like wrinkling; the eggs of M. hirudinaceous and M. ingens cannot be separated morphologically. Eggs of M. moniliformis (Figure 3L) are slightly larger, measuring 90–125 μm long × 65 μm wide and have a thinner shell. Eggs of both genera contain a fully developed acanthor when shed in feces.[1,87]