Public Health Concerns of Taenidae and Their Metacestodes

Abstract

Tapeworms of the family Taeniidae are transmitted from the definitive hosts such as carnivores to the intermediated hosts including herbivores or omnivores and human beings via oral-fecal cycle [1,2]. This family includes two major genera namely Taenia and Echinococcus. The Taenia species include T. saginata, T. solium, T. asiatica, T. hydatigena, T. ovis, T. multiceps, T. serialis, T. pisiformis, T. taeniaeformis, and T. crassiceps [2,3]. In many endemic areas the diseases caused by the genus Taenia in humans are often categorised as neglected tropical diseases [3]. In general, the larval stages or metacestodes belonging to these tapeworms include hydatid cyts, cysticerci and coenuri [2]. All these species except for T. hydatigena (C. tenuicollis), T. ovis (Cysticercus ovis) and T. pisiformis (C. pisiformis), cause zoonotic parasitic diseases and thereby are of public health importance [2-5]. Humans acquire infection by inadvertent consumption of ova or larval stages (metacestode) present in undercooked meat [3]. Cysticerus bovis, the metacestode of T. saginata, occurs only in beef and humans are only the definitive hosts and receive the infection by ingestion of the raw meat containing the cysticeri [1,6,7]. Neurocysticercosis is regarded as the most common zoonotic parasitic disease of the central nervous system (CNS) caused by T. solium metacestodes called Cysticercus cellulosae affecting human beings as the definitive and also aberrant hosts [7,8]. Indeed, human acquire this metacestode via ingestion of undercooked pork infected with cysticerci. On the other hand, the eggs hatched in the human intestine can finally produce the cysts in areas with high blood flow such as CNS (neurocysticercosis), the muscles (human cysticercosis), the subcutaneous tissue and the eye (subcutaneous and ocular cysticercosis, respectively) [5,8]. Taenia asiatica, also known as Asian Taenia, similar to T. saginata is responsible for intestinal teniasis and has been identified in several Asian countries [3,9]. Cysticercus fasciolaris is the larval stage of the cestode T. taeniaeformis in wide variety of small rodents, and occasionally birds and humans, as intermediate hosts with cats as the definitive hosts [10]. Adult forms of T. taeniaeformis and C. fasciolaris have been recognized from the intestine and liver of humans, respectively. Nonetheless, it seems that these cestodes are associated with low health risk to humans [10]. Coenurosis occurs in cerebral and non-cerebral forms. The former is caused by the larval stage of T. multiceps which possesses a metacestode stage namely Coenurus cerebralis [2,11,12]. T. gaigeri with its metacestode termed C. gaigeri causes non-cerebral coenurosis with cysts in the muscles of the intermediate hosts including herbivores as well humans [12-15]. It is unclear and debatable that C. cerebralis and C. gaigeri are the same or different species and the researches are focusing and performing detailed studies to answer this question [11,12]. Coenurus serialis, the larval stage of the cestode T. serialis are found as fluid-filled cystic masses in the muscles and subcutis of rodents and rarely humans as the intermediate hosts. This cestode, in many characteristics, is similar to T. multiceps [16]. T. crassiceps is intestinal tapeworm of carnivores forming the cyst-like larvae or metacestodes (cysticerci) in the body cavities and subcutaneous tissues of rodents as the intermediate hosts [3,17]. The muscles and subcutis of the immunosuppressed humans and the eye and cerebellum in immunocompetent ones are involved by the cestode larvae as tumor-like masses [17]. Four Echinococcus species are known to infect the human hosts during their larval stage including E. granulosus, E. multilocularis, E. oligarthus, and E. vogeli that the latter two species are associated with neotropical echinococcosis [18].