What are Acanthocephalans

Acanthocephalans, also known as thorny-headed worms, are parasitic pseudocoelomate worms whose larval stages parasitise arthropods while the adult worms live in vertebrate hosts. The common name comes from the curved hooks that the adult sports around the pharynx and which are used to hold the worm in the digestive tract of the host. The hooks are sharp and can damage the wall of the host’s intestine. There are about twelve hundred described species of these parasitic worms.

Acanthocephalan anatomy is relatively simple because of the parasitic lifestyle. The most distinguishing feature is the muscular and spiny proboscis at the head end. there are two basic types: nonperforating acanthocephalans have a short proboscis and do less damage to the host’s intestine. Worms with long probosci are called perforating worms and can cause severe damage to the host’s intestinal wall.

The body of an average acanthocephalan is long, pale and worm-like. The worm has no digestive or respiratory organs, being able to absorb the host’s nutrients through its body wall. The adult worms can be from a few millimetres up to about sixty cm long. They are found in all types of vertebrates: mammals, birds, reptiles, fish and frogs. If the adult inhabits a terrestrial vertebrate, the larva comes from a spider or insect or other terrestrial arthropod. If the adult inhabits a marine or aquatic vertebrate then the larval stage infects a crustacean or other aquatic invertebrate host.

Because these worms don’t need to feed, they can concentrate on reproduction. Sexes are separate in thorny headed worms and fertilisation is internal. The worms find each other in the host’s intestine and mate. The eggs are voided in the feces and then ingested by the arthropod intermediate host, where the eggs hatch and grow into shelled larvae. The egg stage is the only free-living stage of this animal’s life and the eggs therefore are hardy, designed to survive until picked up by the intermediate host. The larva feeds on the host in the same way as the adult, by absorbing the host’s nutrients, Eventually it grows large enough to mature sexually and at this stage, it needs the arthropod to be eaten by the adult vertebrate host. Somehow this simple worm larva seems to be able to influence the arthropod’s behaviour to help this occur. Crustaceans that are eaten by ducks, for instance, are normally photonegative. When infected by acanthocephalans, the crustaceans become photo-positive which makes them more likely to be found and consumed by ducks. How the larval worms do this is not known, but one author called it ‘brain-jacking’ *. Sometimes the larval stage goes through several invertebrate hosts before finally getting a final vertebrate host where it can finish its life cycle, mature sexually and reproduce.

Humans are rarely infected by acanthocephalans and it usually happens accidently when the human ingests raw meat or accidently consumes an infected insect or crustacean. Symptoms of acanthocephalan infection include giddiness, feverishness, abdominal pains caused by the damage to the intestinal wall, oedema, constipation or diarrhea and sometimes can be fatal but not usually. Human cases are rare but evidence of infection in ancient humans was found in 9000 year old human coprolites (fossilised feces) that showed signs of acanthocephalan eggs.

As well as occasionally being medically important, acanthocephalans can also be economically important because they can infest some agriculturally important animals such as pigs, chickens and turkeys. They also infect some species of aquarium fish. Lobster fisheries in Canad have been affected by deaths due to acanthocephalans.

Acanthocephalan relationships are difficult because the animals are so simple due to their parasitic lifestyles. However during development, they show similarities to rotifers and nematodes. They undergo spiral determinate cleavage and develop a pseudocoelom. Genetics indicates that their closest free-living relatives may be the innocuous rotifers. No other group of organisms is so completely parasitic or has such degenerate internal organs as the acanthocephala. This makes them interesting to parasitologists at least.




Meglitsch, P. 1972. Invertebrate Zoology. Oxford University Press