What is Acanthocephalan: Ancient Parasite Worm Species Straight Out of Science Fiction

What is an Acanthocephalan?

An acanthocephalan is a parasitic worm with thorn-like or spiny heads. They are commonly called thorny-headed worms and are named after their shape. Acantha (or Akantha) is Greek for thorn, and Cephalan (or Kephale) means head. There are only about 1,150 species of acanthocephalans [1]. An Acanthocephalan has a characteristic, retractable proboscis with spine-like structures to fasten itself in a host’s intestines. Common types of Acanthocephalan worms are Moniliformis moniliformis, Macracanthorynchus hirudinaceus, and Macracanthorynchus ingens [2].

Their hollow trunks house their excretory, nervous, and reproductive systems. They live in various vertebrate hosts, but infections in humans are rare. Hosts include rats, pigs, raccoons, foxes, bony fishes, and intermediate insect hosts like cockroaches and beetles [3].  

Epidemiology: How is Acanthocephala Transmitted?

Infection by an acanthocephalan is known as acanthocephaliasis. Acanthocephaliasis is not contagious and rarely affects humans. The worms spread when their eggs are excreted along with a host’s feces and ingested by an intermediate host, like an insect or a crustacean. Infected rats have been found in the Middle East, Asia, and Africa (Egypt, Malaysia, Nigeria, and Taiwan).

Acanthocephaliasis cases in humans are often found in places where insects are eaten as food and medicine or by mistake. Human infections have been reported in Italy, Nigeria, Sudan, Madagascar, Belize, Thailand, China, Japan, Iran, Iraq, and the United States [4] [5] [6] [7] [8].

Pathogenicity and Life Cycle of the Parasites

1. Eggs: Acanthocephala lives in the intestines of vertebrates. Their life cycle begins when their eggs pass together with their hosts’ feces. The eggs are oval and already contain a mature embryo or ‘acanthor’ when expelled.  
2. Intermediate host: The eggs are first ingested by an intermediate host before moving on to their main or definitive host. Intermediate hosts are arthropod insects like cockroaches, beetles, and small crustaceans like Gammarus lacustris (called scuds).
   • Once ingested, the eggs hatch and the acanthor becomes an acanthella, a transitional larval stage, a miniature version of the adult worm.
   • The acanthella penetrates the arthropod intermediate host insect’s gut and enters the haemocoel – the body of the insect with circulatory fluid. Here, they develop a body wall and preliminary forms of internal organs.
   • The final stage is the larval stage called the cystacanth. The proboscis develops and is retracted. A protective cyst wall develops around the parasite. Now, the larvae wait until the intermediate host is eaten by a final (definitive) host animal like a pig, raccoon, rat, or even a human.
3. Definitive Host: Once the larvae are ingested by the permanent host, the cyst ruptures, and the proboscis extends and attaches itself to the host’s intestines with its spiny protrusions, dorsal intrafurcal spines. The juvenile worms mature on the walls of the intestine and begin to mate after 8 – 12 weeks [2]. 

‘Brain-Jacking’ Intermediate Hosts

The Acanthocephala do not passively infect their intermediate hosts. Instead, they modify their intermediate host’s behavior to increase their likelihood of being eaten by a definitive host. It’s like something out of a science fiction novel and is referred to as ‘brain-jacking.’

For example, the Gammarus lacustris is an invertebrate crustacean that hides away from light and water surfaces to avoid being eaten by ducks. However, other animals like ducks are attractive hosts for Acanthocephala. When infected with Acanthocephalan worms, the Gammarus lacustris of a specimen begins to adopt self-destructive behaviors. The scud becomes attracted to light and begins to swim on the surface of water bodies. Sometimes, it attaches itself to a rock, making it easy prey for ducks [9].

Cockroaches infected with M. moniliformis – an Acanthocephalan – become more vulnerable to predators. Studies show that infected cockroaches cannot detect potential predators and escape as quickly as their uninfected counterparts [10].

Other intermediate hosts have been shown to exhibit similar ‘self-sabotaging’ behavior to increase the Acanthocephalan’s chances of survival.

Diagnosis of the Species

Acanthocephaliasis is diagnosed through a stool test. If infected, the stool or specimens will contain eggs or adult worms. It is important to note that most humans do not exhibit symptoms because ingested eggs often do not mature.

Symptoms of Acanthocephala

According to the CDC, early or mild infections are often asymptomatic. However, with more severe infections, common symptoms are abdominal pain and digestive problems. Pain may be severe because the acanthocephalan’s proboscis inserts itself into a host’s intestine wall [2].

Treatment of these Worms

Acanthocephaliasis is treated with common anti-parasitic or anthelmintics like pyrantel pamoate.

Prevention of Acanthocephala

Prevention of Acanthocephaliasis infections often involves raising public awareness about the risks of eating beetles and cockroaches as food. Children must be taught to avoid putting insects in their mouths.