Taenia is a genus of tapeworm that includes some important parasites of livestock. Members of the genus are responsible for taeniasis and cysticercosis in humans. There are more than 100 species recorded. They are morphologically characterized by a ribbon-like body composed of a series of segments called proglottids; hence the name Taenia (Greek tainia meaning ribbon, bandage or stripe). The anterior end of the body is the scolex. Not all members of the genus Taenia have an armed scolex (hooks and/or spines located in the "head" region), for example, Taenia saginata has an unarmed scolex, while Taenia solium has an armed scolex.
Proglottids have central ovary, with a vitellarium (yolk gland) posterior to it. As in all cyclophyllid cestodes, there is genital pore on the side of the proglottid. Eggs are released when proglottid deteriorates, and so a uterine pore is unnecessary.
- Taenia asiatica – Asian Taenia. Humans as definitive hosts, pigs and rarely cattle, as intermediate hosts.
- Taenia crassiceps
- Taenia gonyamai, parasite of antelope (larval-) and lions (adult forms).
- Taenia mustelae, which infects small carnivorans.
- Taenia pisiformis, which is common in wild dogs and in rabbits, who serve as intermediate hosts.
- Taenia rileyi, which infects bobcats.
- Taenia saginata – beef tapeworm. Infects cattle and humans, and can only reproduce while in the human gut.
- Taenia solium – pork tapeworm. Like T. saginata humans serve as its primary host, and it can only reproduce by the dispersal of proglottids while in the gut. These reinfect pigs when human faeces is improperly disposed of. This infection is most common in parts of Africa.
- Taenia taeniaeformis, which uses rodents as intermediate hosts and then inhabits cats as the definitive host.
- Taenia serialis, parasite of dogs and foxes which has rabbits as the intermediate host.
The life cycle begins with either the gravid proglottids or free eggs (embryophores) with oncospheres (also known as hexacanth embryos) being passed in the feces, which can last for days to months in the environment. Sometimes these segments will still be motile upon excretion— they will either empty themselves of their eggs within a matter of minutes or, in some species, will retain them as a cluster and await the arrival of a suitable intermediate vertebrate host. The intermediate host (cattle, pigs, rodents, etc., depending on the tapeworm species) must then ingest the egg or proglottids. If the host is a correct one for the particular species of tapeworm, the embriophore then hatch, and the hexacanth embryos invade the wall of the small intestine of the intermediate host to travel to the striated muscles to develop into cysticerci larvae. Here they will grow, cavitate, and differentiate into this second larval form shaped like a bladder (and erroneously believed until the middle of the 19th century to be a separate parasite, the bladderworm) which is infectious to the definitive host when an invaginated scolex (protoscolex) is completely developed. To continue the process, the definitive host must eat the uncooked meat of the intermediate host. Once in the small intestine of the definitive host, the bladder is digested away, the scolex embeds itself into the intestinal wall, and the neck begins to bud off segments to form the strobila. New eggs will usually appear in the feces of the definitive host within six to nine weeks, and the cycle repeats itself.
Taenia saginata are about 1,000-2,000 proglottids long with each gravid proglottid containing 100,000 eggs, while Taenia solium contain about 1,000 proglottids with each gravid proglottid containing 60,000 eggs.
Divergent of Taenia in Humans
It was previously thought that humans acquired Taenia spp. (T. solium, T. asiatica and T. saginata) after the domestication of large mammals. Although the omnivorous diet and forging of early hominid suggest that contact between the ancestral Taenia was established prior to the rise of modern humans and advance agriculture. Evidence, in fact, suggests that domestication of animals by humans consequently introduce Taenia spp. to new intermediate host; cattle and swine.
Morphological data and molecular data suggest that Taenia divergence as specialize human parasite has been directly associated with earlier hominids and prior to the existence of modern Homo sapiens. Direct predator-prey relationship between humans and the original definitive and intermediate host of Taenia resulted host shift change. Ecological evidence showed that early hominids heavily preyed on one of the intermediate host, antelope, of Taenia spp. and thus resulting an earlier colonization by the parasite prior to mammalian domestication. The early hominids have an omnivorous diet. They are hunters and scavengers. The abundance of antelopes in sub-Saharan Africa savannah in the Late Pliocene resulted a vast food resource for hominids and other carnivorous animals such as felids, canids and hyaenids (Hoberg et al. 2001). Hominids who hunted antelopes or scavenged killed antelopes bridged the beginning of a host-parasite relationship between hominids and Taenia. It is possibly that during this time period, hominids may not have the means of cooking their food. This would have greatly increased their chances of catching the infectious, cesticerci, as a result of eating uncooked meat. Also, transmission of the parasite may have been enhanced by directly consuming the definitive host. Parasitological data supports the foraging of antelope by Homo spp, during the Late Pliocene and Pleistocene period. This corresponds to the initial contact of the ancestral Taenia and specialize into T. solium and T. saginata + T. asiatica, thus resulting in colonization of the early homonids as definitive host.
Host switching for Teania is most prevalent among carnivores and less prevalent among herbivores via co-speciation. An excess of 50-60% of Taenia colonization occurs among guild of carnivores; hyaenids, felids and homonids. (Hoberg 2006). Acquisition of the parasite occurs frequently among definitive host than among intermediate host (Hoberg 2006). Therefore host-switching is likely could not have come from non-guild, cattle and pigs. The establishment of cattle and pigs as intermediate host by Taenia spp. is consequently due to the synanthropic relationship with humans. During the past 8000-10,000 years, it is recognized that between these time frames the colonization of respective Taenia spp. from humans to cattle and to swine was established (Hoberg 2006). In contrast, the colonization of ancestral Taenia onto the early hominids was established during the past 1-2.5 MYBP (Hoberg 2006). It clearly shows that colonization of human Taenia predates the domestication of mammals.
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