**2. Characteristics of the parasites**

The genus *Taenia* belongs to the Phylum—Platyhelminthes; Class—Cestoda; Order—Cyclophyllidea; Family—Taeniidae; Genus—*Taenia*. The genus *Taenia* is a highly diverse, paraphyletic group with 45 species identified to date, among which *T. solium*, *T. saginata*, and *T. asiatica* are infectious to humans.

*T. asiatica* differs from *T. solium* and *T. saginata* by morphological characteristics such as the scolex, mature and gravid proglottids of adults, and the scolex and

#### *Epidemiology of Taeniosis/Cysticercosis in Humans and Animals DOI: http://dx.doi.org/10.5772/intechopen.110727*

bladder surface in the larval stage [6]. Today, the characterization of *T. saginata*, *T. solium,* and *T. asiatica* is based on molecular identification and determinations of the mitochondrial genome sequence. The complete mitochondrial genome sequence is available. The difference in the genome between *T. asiatica* and *T. saginata* is 4.6%, while *T. solium* differs by 11% [7]. *T. asiatica* is a relatively recently described species and received much attention in past decades. Discovery came from the paradoxical observation of a high prevalence of taeniosis caused by a *T. saginata*-like tapeworm in humans who did not consume beef, and the consequent description of the complete mitochondrial genome of "new" species. Experimental studies conducted in the 1980s and 1990s clearly demonstrated that the life cycle of *T. asiatica* is comparable to that of *T. saginata*, except that pigs are the preferred intermediate hosts and the liver is the preferred larval site.

Mitochondrial DNA data suggest that *T. saginata* and *T. asiatica* are very closely related [8–10]. *T. saginata* shows high genetic polymorphism (0.2–0.8%), while the genetic diversity of *T. asiatica* appears to be minimal, indicating that this parasite may be on the verge of extinction. However, research has shown the existence of hybrids between *T. asiatica* and *T. saginata*, reviving the issue of *T. asiatica*'s genetic diversity and its position as a distinct species. The identification of *T. saginata* and *T. asiatica* is by morphological and molecular characteristics and observed differences in the life cycle [6]. Genetic variation within the species *T. solium* was until recently almost unknown, even though investigations are ongoing. DNA analysis of *T. solium* isolates from around the world indicate variations between two geographic regions, Asia and Africa/South America, where *T. solium* appears as two genotypes and second is the Asian and the American/African genotype [11]. Analysis of the mitochondrial DNA of *T. saginata* shows great variation indicating greater complexity of this parasite species compared to *T. solium* [12].

### **2.1 Morphology of the parasites**

The three morphological forms of the parasites are adults, eggs, and larvae. Tapeworms of the genus *Taenia* are flat, white or yellowish, and very long-segmented parasites. *T. solium* has a length of 2–8 m, while *T. saginata* and *T. asiatica* are thicker and wider and can be longer. The head or scolex (attachment organ) has four shoots and a rostellum that may contain hooks (*T. solium*), be unhooked and depressed (*T. saginata*), or with rudimentary hooks (*T. asiatica*). Hooks of *T. solium* make a crown with two rows of 22–32 hooks, whose size is 159–173 μm. The scolex is the size of a pinhead, followed by a short and undivided region, the neck. In *T. saginata* the neck is longer and thinner, and in *T. solium* thicker and shorter. The neck continues with a long chain of proglottids or segments (called strobilas). The strobila resembles a ribbon and may consist of several thousand proglottids. Proglottids have different developmental stages: proximal are immature, followed by mature proglottids and distal proglottids containing eggs and gradually increasing in size. The posterior end of the tapeworm has the widest, longest, and oldest proglottids [13–15]. Mature proglottids have both male and female reproductive organs. The female reproductive apparatus includes an ovary, a closed uterus with branches—an ootype, a single mass of bile glands, and a lateral genital pore. Male organs consist of testes (follicles), vas deferens, and cirrus. The uterus in the gravid proglottid of *T. saginata* has 15–30 lateral branches compared to *T. solium* with 7–13 branches. On average, adult *T. solium* has about 1000 proglottids, each containing 50,000 eggs; adult *T. saginata* has about 1000–2000 proglottids, each with 100,000 eggs; while adult *T. asiatica* has 700–900 proglottids, with 80,000 eggs each.

The eggs are spherical, 20–50 μm in size, and morphologically indistinguishable between species of the Taeniidae family. Each egg contains a multicellular embryo with six hooks, hence called a hexanth embryo or oncosphere. Manny eggs released from the definitive host are fully embryonated and infectious. *T. saginata* eggs are infectious to cattle, *T. solium* to both pigs and humans, and *T. asiatica* eggs are infectious to pigs. Because of their resistance to desiccation, eggs can survive for days or months in the environment, in soil, or in water.

Cysticercus *T. solium* (*Cysticercus cellulosae*) is found in the liver, brain, and skeletal muscles of pigs 6 days after infection, while in humans they can be located in the nervous system, eye, heart, muscles, and subcutaneous tissue. A mature cysticercus is usually spherical or oval, white or yellow, 0.5 and 1.5 cm in size, and has a transparent wall, through which the scolex is visible. Cysticerci have two chambers: an inner one containing the scolex and a spiral canal surrounding an outer fluid-containing chamber. The racemous form of cysticercus appears as a large, round, lobulated bladder, limited by a delicate wall or resembles a cluster 10–20 cm in size containing 60 ml of fluid. The most important characteristic of the racemous form is that the scolex cannot be clearly seen. Young cysticerci cause mild inflammation in the surrounding tissue, while mature lead to a stronger immune reaction [16, 17]. Following ingestion by the final host, the pores of the bladder wall expand, resulting in the release of the scolex [18].

Cysticercus *T. saginata* (*Cysticercus bovis*), is a 1 cm wide oval bladder, filled with fluid and containing an invaginated scolex without hooks. Cysticercus are localized in the skeletal muscles of cattle, however, there are sporadic reports of cysticercosis in llamas, pronghorn, oryx, topi and other antelopes, gazelles, wildebeests, and giraffes [19]. These intermediate hosts are infected by grazing.

Cysticercus of *T. asiatica* (*Cysticercus viscerotropica*) is smaller than *T. saginata* with a diameter of approximately 2–3 mm. Both have a scolex with a round rostellum surrounded by four symmetrically placed suckers, while *T. asiatica* has two rows of rudimentary hooks that usually do not develop into morphologically recognizable hooks. Cysticerci of *T. asiatica* are found in domestic and wild pigs and develop in the liver and extrahepatic organs, but not in the muscles [20, 21].

Animals carrying cysticerci are usually asymptomatic while muscle stiffness occurs in extremely severe infections. *C. bovis* and *C. cellulosae* do not develop severe pathology in cattle and pigs unless vital organs, such as the heart, are massively infested.

### **2.2 The life cycle of the parasites**

Parasites of the family Taeniidae uniquely require two obligate hosts (mammalians): a carnivorous/omnivorous definitive host for the adult tapeworm stage and an herbivorous/omnivorous intermediate host for the larval (cysticercus) stage. The life cycle begins when herbivores/omnivores as intermediate hosts ingest plants, food, or water contaminated with taeniid eggs (or gravid proglottids). In the intestines, oncospheres hatch, activate, invade the intestinal wall, and migrate to tissues and organs, where they develop into cysticerci.

When the definitive host carnivore/omnivore eats raw or undercooked pork (*T. solium*, *T. asiatica*) and beef (*T. saginata*) containing fluid-filled cysticerci, the cyst (bladder) gets dissolved and the inverted scolex evacuates under the stimulus of the host's digestive enzymes. The scolex embeds in the intestinal wall, and the neck buds and the strobila is formed. As the adult parasite grows, the release of gravid

proglottids begins, and the first shedding occurs between 8 and 12 days after infection. New eggs may appear in the feces of the definitive host within 6–9 weeks, initiating the next cycle of infection [12].

People are infected in three ways:

