Taenia asiatica, commonly known as Asian taenia or Asian tapeworm, is a parasitic tapeworm of humans and pigs. It is one of the three species of Taenia infecting humans and causes taeniasis. Discovered only in 1980s from Taiwan and other East Asian countries as an unusual species, it is so notoriously similar to Taenia saginata, the beef tapeworm, that it was for a time regarded as a slightly different strain. But anomaly arose as the tapeworm is not of cattle origin, but of pigs. Morphological details also showed significant variations, such as presence of rostellar hooks, shorter body, and fewer body segments. The scientific name designated was then Asian T. saginata. But the taxonomic consensus turns out to be that it is a unique species. It was in 1993 that two Korean parasitologists, Keeseon S. Eom and Han Jong Rim, provided the biological bases for classifying it into a separate species. The use of mitochondrial genome sequence and molecular phylogeny in the late 2000s established the taxonomic status. T. asiatica causes intestinal taeniasis in humans and cysticercosis in pigs. There is a suspicion that it may also cause cysticercosis in human. Like other taeniids, humans are the definitive hosts, but in contrast, pigs, wild boars, as well as cattle can serve as intermediate hosts. Moreover, SCID mice and Mongolian gerbil can be experimentally infected. The life cycle is basically similar to those of other taenids. Humans contract the infection by eating raw or undercooked meat – a practice common in East and Southeast Asia – which is contaminated with the infective larva called cysticercus. Cysticercus develops into adult tapeworm in human intestine, from where it releases embryonated eggs along faeces into the external environment. Pigs acquire the eggs from vegetation. The eggs enter the digestive tract, which they penetrate to migrate to other body organs. Unlike other Taenia they preferentially settle in the liver, where they form cysticerci. Asian taeniasis is documented in nine countries in Asia, including Taiwan, South Korea, Indonesia, the Philippines, Thailand, south-central China, Vietnam, Japan and Nepal. The rate of a prevalence is estimated to be up to 21% and resulting in annual economic losses of about US$40,000,000 in these regions. Praziquantel is the drug of choice for treating the infection. As the latest addition to human taeniasis, misidentified for over two centuries, still complete lack of systematic diagnosis, and no control programmes, it is regarded as the most neglected human taenid.
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. 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. By the early 1990s the morphological and genetic differences were firmly established, but American and Australian parasitologists remained adamant as to its position as a separate biological species. In 1992 two Korean parasitologists Keeseon S. Eom, from Chungbuk National University, and Han-Jong Rim, from Korea University, reported the transmission and larval stages (metacestodes) in naturally infected pigs. They also succeeded in experimentally infecting pigs in which cysticerci were formed in the liver. Further the metacestodes recovered from infected liver were used to infect a human volunteer, whom they had given two years earlier. They therefore proposed the scientific name Taenia saginata taiwanensis. They recovered the intact strobilae from the stool after giving the volunteer niclosamide. It was using these metacestodes, strobilae, and adult worms that they gave detailed morphological and anatomical comparisons and concluded it to be a novel species, Taenia asiatica, in 1993. Even then scepticism still persisted. Considering the degree of variations between Taiwan strain and typical T. saginata, Taiwanese parasitologists such as P.C. Fan, C.Y Lin, C.C. Chen and W.C Chung from National Yang-Ming University designated it to a subspecies, and named it T. saginata asiatica. Based on critical assessment on the field reports, experimental infections, morphological and immunological studies available since 1981, they advocated this position. Independent research in Australia also supported the subspecies concept regardless of the genetic variations. But subsequent analyses including epidemiological studies, and phylogenetic analysis using random amplified polymorphic DNA imposed its validity as a distinct species. In 2005 the first complete sequence of its mitochondrial genome was published, and genetic comparison (Cox1gene) with those of Taenia solium and T. saginata provided further support to its taxonomic status. The complete sequence of mitochondrial genome of T. saginata in 2007, and the development of high-resolution multiplex PCR assay in 2009 finally established beyond doubt that it is indeed a new species. The two species separated 1.14 million years ago.
… excerpt ends here. Continue reading the full article.
