The giant thorny headed worm, Macracanthorhynchus hirudinaceus, is primarily a parasite of pigs but can infest other species including dogs and humans. It belongs to the Acanthocephala, a small phylum of highly specialised parasites with complex life cycles usually involving arthropod then vertebrate hosts. They lack an intestinal tract, absorbing nutrients through their integument and use a spiny proboscis to attach to the intestinal wall of their final host (Bowman et al 2002).
Two mature feral pigs killed in August 2014, as part of a pest control program near Palmer’s Oakey, north east of Bathurst, were observed to be heavily infested with ‘tape worms.’ A worm, collected and submitted by the property owner, was subsequently identified as the euphoniously named acanthocephalan parasite, Macracanthorhynchus hirudinaceus.
The females of M hirudinaceus are remarkably fecund, producing about 260,000 highly resistant eggs per day. The eggs, which can live for several years, hatch when ingested by the larvae of scarabs and other species of beetles. The worm larvae eventually become encysted both in the grubs and mature beetles and infect pigs consuming them. Most pigs are only lightly infested. However, because thorny headed worms anchor their proboscis into the intestinal wall, they can cause weight loss and in some cases death through intestinal perforation (Soulsby 1971). In one instance the parasite caused such a high mortality in nuisance feral pigs (in Iranian sugar cane fields) that it was welcomed as a biological control agent (Mowlavi et al 2006).
M hirudinaceus can also occasionally develop in dogs that ingest beetle larvae. As an aside, some American dogs also suffer from infestations of the even larger M ingens which is normally a parasite of racoons, with millipedes the intermediate host. However as Bowman et al (2002) observed,
‘to eat a millipede requires extraordinary cunning, frightful taste, great excitement or utter boredom on the part of the dog because the millipede gives off a potent defensive secretion. The racoon gets around the problem by rolling the millipede about in the dust to exhaust its supply of defensive secretion, but few dogs have learned that trick.’
While a parasite that may cause emaciation and sometimes death in feral pigs is not of major concern, M hirudinaceus could also infect free range pigs and is a potential zoonosis. While M. hirudinaceus seldom matures in people there are numerous reports of its recovery in humans. In some countries people consume beetle larvae either for medicinal or dietary purposes exposing them to the risk of infection (Shapiro, Schmidt 1971).