Taenia asiatica

Taenia asiatica is commonly known as Asian taenia or Asian tapeworm and is a parasitic tapeworm of humans and pigs. It is one of the three species of Taenia that infect humans and causes taeniasis. Discovered only in 1980s from Taiwan and other East Asian countries, it is so notoriously similar to Taenia saginata, the beef tapeworm, that it was for a time regarded as a slightly different strain. But the taxonomic consensus turns out to be that it is a unique species. Like other taenids, humans are the definitive hosts, but in contrast, both cattle and pigs can serve as intermediate hosts. Moreover, SCID mice and Mongolian gerbil can be experimentally infected.^[1]

Discovery

T. asiatica was first recognized in Taiwan, and subsequently in Korea and other Asian countries; therefore it was originally known as Asian T. saginata, as it appeared to be exclusive to Asia. From 1952 W.H. Huang and his team had recorded that taeniasis was highly prevalent in Taiwan under the assumption that T. saginata was the principal cause. In 1966 S.W. Huang began to suspect that the tapeworm could not be the conventional T. saginata for the obvious reason that the Taiwan aborigines hardly eat beef, and T. saginata is strictly a bovine tapeworm.^[1] From 1970s studies on the biology began to throw light to its difference from the classical T. saginata. Firstly the tapeworm infects visceral organs such as liver, serosa and lungs of pigs, and liver of cattle; while T. saginata is known to infect only the muscle of cattle. Secondly there are significant morphological variations though their resemblance is overwhelming.^[2] By the early 1990s the morphological and genetic differences were firmly established, but the Amrican and Australian parasitologists remained sceptical as to its position as a separate species, which prompted P.C. Fan and coworkers to resolve it to be a sub-species, and was named T. saginata asiatica.^[3] But further analyses imposed its taxonomic revision into a valid species T. asiatica.^[4] Comparison of the mitochondrial genome also provide further supports to its taxonomic status.^[5]

Description

The body is yellowish white in colour, about 350 cm long and 1 cm broad, divided into the anterior scolex, followed by a short neck and a highly extended body proper called strobila. The strobila is composed a series of ribbon-like segments called proglottids. There are more than 700 proglottids in the strobila. The scolex bears of 4 simple suckers. The segments are made up of mature and gravid proglottids. It is an acoelomate animal with no digestive system. It is unique in having posterior protuberances in the gravid proglottid, which are absent in other taenids including T. saginata. The total number of proglottids is less than 1000 (~900), while T. saginata have more than 1000 proglottids. The protoscolex of cysticercus of the Taiwan Taenia has a sunken rostellum while that of T. saginata has only an apical pit. In addition, the rostellum is usually surrounded by two rows of rudimentary hooklets.^[3] The distinct rostellum on the scolex, the large number of uterine twigs and the existence of posterior protuberance in adult were the defining characters. Moreover, the metacestode was different morphologically from that of T. saginata in having wart-like formations on the external surface of the bladder wall.

Life cycle

The life cycle is indirect and complicated, and is completed in humans as the definitive host, and the intermediate host is mostly pigs, and cattle on rare occasion.^[6] The adult worm inhabits the small intestine of humans. Fertilized eggs are released through the faeces along with the gravid proglottid which gets detached from the strobila. Pigs and cattle ingest the infective embryo while grazing. The digestive enzymes will break the thick shell of the egg and allow formation of the zygotes called "oncospheres". These zygotes then penetrate the mucous layer of the digestive tract and enter the circulation of the host. This is where the young larval stages form a pea-sized, fluid filled cyst, also known as “cysticercus”, which migrate to visceral organs like liver, serosa and lungs in pigs, and liver in cattle.^[2]

Epidemiology

The parasite is known in Asian countries including Taiwan, Korea, Indonesia, Thailand and China. In addition, molecular genotyping techniques have revealed that the disease also occurs in Japan, the Philippines, and Vietnam.^[7]

Diagnosis

The basic diagnosis is examination of a stool sample to find the parasite eggs. However there is a serious limitation as to the identification of the species because the eggs of all human taenids look the same. It is extremely difficult to identify T. asiatica from other taenids because of their striking resemblances. The species and T. saginata are frequently confused due to their morphological similarities and sympatric distribution. Identification often requires histological observation of the uterine branches and PCR detection of ribosomal 5.8S gene.^[8][9] The presence of rostellum on the scolex, a large number of uretine branches (more than 57) and prominent posterior protuberances in gravid proglottids, and wart-like formation on the surface of the larvae are the distinguishing structures.

To date the most relevant diagnosis of taeniasis due to T. asiatica is by enzyme-linked immunoelectrotransfer blot (EITB). EITB can effectively identify it from other taenid infections since serological test indicates that immunoblot band of 21.5 kDa exhibited specificity only to T. asiatica.^[10] Even though it gives 100% sensitivity, it has not been tested with human sera for cross-reactivity, and it may show a high false positive result. Loop-mediated isothermal amplification (LAMP) is highly sensitive (~2.5 times that of multiplex PCR), without false positive, for differentiating the taenid species from faecal samples.^[11]

Treatment

Niclosamide (2 mg) was very effective against experimental infection in human.^[4] In general cestode infections are treated with praziquantel and albendazole. Atrabine is quite effective but indicates adverse effects in humans.^[1] The commonly used drugs for tapeworms, benzimidazoles are relatively ineffective. Praziquantel at a single dose of 150 mg is the most effective against T. asiatica without causing sideeffects.^[12]

See also

• List of parasites (human)
• Taenia
• Taenia saginata

References

External links

• Uniprot Taxonomy
• NCBI Taxonomy Browser
• Human Tapeworms at Metapathogen.com
• Images at National Yang-Ming University
• Encyclopedia of Life