Moniliformis moniliformis

Infested rats have been found world-wide. Cases of human infestation by Moniliformis moniliformis have been reported in the United States, Iran, Iraq, and Nigeria.[1]

Acanthocephalans do not have digestive tracts and absorb nutrients through the tegument, the external layer. The scolex of this worm has a cylindrical proboscis and a multitude of curved hooks. The main parts of the worm body are the proboscis, neck, and trunk. Because of horizontal markings on the worm, there is the appearance of segmentation. Acanthocephalans are sexually dimorphic (dioecious) – adult males are generally 4 to 5 cm long while females are longer, ranging from lengths of 10 to 30 cm. Males also have copulatory bursas, used to hold on to the female during copulation and cement glands. Females have floating ovaries within a ligament sac where fertilization of the eggs occurs.[2] The eggs of this parasite are 90–125 μm long and 65 μm wide. They are oval in shape with a thick, clear outer coat.[3]

Usually, the definitive hosts for M. moniliformis are rodents, cats, dogs and red foxes (in Poland); human infestations are rare. The intermediate hosts are usually beetles and cockroaches.[1]

In the life cycle of M. moniliformis, the intermediate hosts ingest the eggs of the parasite. In the intermediate host, the acanthor, or the parasite in its first larval stage, morphs into the acanthella, the second larval stage. After 6–12 weeks in this stage, the acanthella becomes a cystacanth. The cystacanth, or infective acanthella, of M. moniliformis are cyst-shaped and encyst in the tissues of the intermediate hosts. However, most other acanthocephalans have infective larvae that more closely resemble underdeveloped adult worms. The definitive hosts consume the cystacanths upon feeding on infested intermediate hosts. These cystacanths mature and mate in the small intestine in 8–12 weeks. After this time, the eggs are excreted with the feces, to be ingested yet again by another intermediate host and renew this cycle.[3]

The reproduction of the parasite only occurs in the definitive host. In acanthocephalans, adult males have cement glands in their posterior ends. The widely held theory is that the mucilaginous and proteinaceous substance that these glands secrete is used by males to seal up the females after copulation in order to prevent leakage of the inseminated sperm and further insemination by other males. It has also been found that these males may create this seal on other males in order to prevent them from copulating.[4] These seals, or copulatory caps, last for a week.

In what is commonly known as "brain-jacking", the parasite induces a behavioral change in its intermediate host that increases the risk of predation for the host. It is thought that this behavioral change holds an evolutionary advantage for the parasite by increasing its chances of getting to its definitive host. When M. moniliformis infests its intermediate host, the cockroach species Periplaneta americana, it changes the cockroach's escape response. In one study, it was concluded that cockroaches infected by M. moniliformis took longer to respond to wind stimuli simulating the approach of a potential predator and displayed fewer escape responses implying that the parasite infection renders its intermediate host more vulnerable to predation by hindering its ability to detect and escape from its predator. It is thought that serotonin plays a role in upending the communication between giant interneurons and the thoracic interneurons and in turn hampers the escape response of the cockroach.[5] In a similar study, the effects of parasitism on three Periplaneta species are studied. The results show that Periplaneta australasiae uses substrates differently and moves around less when infected with Moniliformis moniliformis.[6] Another study concludes an increased vulnerability of infected Periplaneta americana due to increased phototaxis, more time spent moving (due to slower movement) and movement in response to light (uninfected cockroaches hesitated before moving).[7]

As it requires consumption of raw infested beetles or cockroaches, human acanthocephaliasis is rare.