Parasites & Pestilence- Winter 2010
Acanthocephalans, or thorny-headed worms, represent a phylum (Acanthocephala) of metazoans. They are aptly named for the proboscis and curved hooks present on the scolex of the worm. Within this phylum there are myriad Moniliformis species of the family Moniliformidae but along with Macrocanthorhynchus hirudinaceous, M. moniliformis is different in its ability infect human hosts. The earliest known human infection was found in Utah in the coprolite of a prehistoric man. Specimen analysis showed a significant amount of M. moniliformis eggs in the coprolite. (Moore 1969) Other excavations of both human and animal coprolites contain these eggs as well. In 1888 in Italy, Calandruccio infected himself by ingesting larvae, reported gastrointestinal disturbances, and then shed the eggs in two weeks. This was the first report of the clinical manifestations of an M. moniliformis infection in humans. Since then, there have been very few case reports of the infection in humans, although they do occur. Most recently in 2007, there was a case of a 2-year-old girl in a rural part of Iran, only the third ever reported in the country. (Berenji 2007) The first case in Iran was reported in 1970 in an 18-month-old child. (Salehabadi 2008)
The sparse data on cases of human infection of M. moniliformis make mapping the exact distribution of this parasite difficult. Even so, infected rats have been found generally worldwide and there are case reports of humans infected with this parasite from Iran, the United States (most recently an infant in Florida), Nigeria, Italy, Sudan and Belize (Gideon, Salehabadi).
Acanthocephalans do not have digestive tracts and absorb nutrients through the tegument, the external layer. As mentioned before, 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 as seen in Figure 1, which makes it easy to mistake for a tapeworm. Acanthocephalans are sexually dimoprhic- adult males are generally 4 to 5 cm long while females are longer, ranging from lengths of 10 to 30 cm. In the case of the 2-year-old Iranian girl, one of the worms extracted from the stool sample was 125 mm long, 3.6 mm wide, and the proboscis had 14 rows of hooks, with each row containing 6 to 8 hooks (Salehabadi 2008). 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. The eggs of this parasite are 90-125 mm long and 65 mm wide. They are oval in shape with a thick, clear outer coat as can be seen in Figure 2.
Reservoirs and Hosts
While acanthocephalans rarely infect humans, there have been several cases reported of M. moniliformis causing acanthocephaliasis in humans as their definitive hosts. Usually, the definitive hosts for M. moniliformis are rodents, cats, dogs and red foxes (in Poland). The intermediate hosts are usually beetles and cockroaches.
Life Cycle and Reproduction
Figure 3- M. moniliformis Life Cycle (CDC 2009)
In the life cycle of M. moniliformis, as shown in Figure 3, 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 infected 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. (CDC 2009)
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 are used by males use 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. These seals, or copulatory caps, last for a week. (Animal Parasitology 2005)
The Dynamic Between Larva and Intermediate Hosts- Induced Behavioral Changes
Several studies have been conducted to understand the relationship the parasite has with its intermediate hosts. Libersat et al looked at the dynamic between Moniliformis moniliformis and its intermediate host, the cockroach species, Periplaneta americana by analyzing the escape response in infected cockroaches. 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. (Libersat 2000) It should be noted however, that Libersat does not claim that these changes in the escape mechanisms due to infection are directly tied to higher rates of parasite transmission. This has yet to be determined. In a similar study, Moore looks at the effects of parasitism on 3 Periplaneta species. Specifically, Moore concludes that Periplaneta australasiae uses substrates differently and moves around less when infected with Moniliformis moniliformis. (Moore 1994) In another study, Moore proposes the 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). (Moore 1983) Both studies indicate the cockroachÕs increased risk of predation by the definitive host due to the effects of the parasite.
Acanthocephaliasis in Humans- Clinical Manifestations
While human infection is rare, when by chance, humans become the definitive hosts in the Moniliformis moniliformis life cycle acanthocephaliasis develops. Calandruccio provided the first description of the clinical manifestations of acanthocephaliasis and similar accounts are found in the case studies since. There are not many case studies on acanthocephaliasis but of the ones that exist, many of the patients described were asymptomatic. When the patients exhibited symptoms, they normally experienced abdominal pain, diarrhea, dizziness and anorexia. In the aforementioned case of the young girl, facial edema was another clinical manifestation. Perforation of the intestinal mucosa could also occur, leading to ulcers, inflammation, and hemorrhaging. (Richardson 2003) In some patients, giddiness has also been reported (Gideon) In rodents, acanthocephaliasis is fatal and manifests itself through similar hemorrhaging and gastrointestinal disturbance.
Diagnosis and Treatment of Acanthocephaliasis
Because the abdominal pain of patients with acanthocephaliasis resembles that of appendicitis, there is a risk of improper or delayed diagnosis. The proper diagnosis of acanthocephaliasis in humans is made through fecal analysis, which if the host is infected should contain adult worms or eggs. To obtain the worms from the host, piperazine citrate, tetramisole and bithionol can be administered to the patient. In the case of the 2-year-old Iranian girl, piperazine citrate was administered. However, in this case report it was difficult to obtain eggs or worms in the stool samples possibly indicating the inability of traditional laboratory procedures to detect the helminths and eggs. Once correctly diagnosed with a stool sample, acanthocephaliasis is treated with anti-helminthics. There is considerable debate over the efficacy of anti-helminth drugs on this parasite, but thus far, the drugs seem to be working. Pyrantel pamoate has been shown to be particularly effective in treating patients, as has ivermectin. Mebendazole and thiabendazole have also both been cited to work. (Richardson 2003)
Strategies to Control and Prevent the Spread of Moniliformis Moniliformis
While diagnosis and treatment are relatively simple, because of the route the parasite must take to infect a human host, control strategies are very feasible. In the case of the 2 year-old girl in Iran, it was shown that the pica proclivity left the child at risk for consuming the intermediate host. Because of this, an effective prevention control strategy entails informing and warning the public about the risks of consuming beetles and cockroaches. Caretakers should be warned to be more vigilant of the activities of children exhibiting pica, especially in areas where beetles and cockroaches are widespread. Moreover, the prevention of intermediate host entry into the home and more importantly, into the kitchen where it may be consumed, would also be highly effective. (Neafie 1993)
Moniliformis moniliformis infection in humans is rare as evidenced by the low number of case reports since the first reported clinical manifestation in 1881. However, there is also the possibility that it is not easily detected by routine laboratory procedures and that many cases actually go unreported. Even though prevalence is low, understanding the pathway of this parasite and especially the neurological effects it seems to have on its intermediate hosts can further elucidate the relationship parasites have with their hosts and the ways parasites persist in the environment.
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