Mesocestoides lineatus (Goeze, 1782) Railliet, 1893
Figures 3-6 through 3-9
ETYMOLOGY:Meso = in between & cestoides for cestode; this genus has characteristics that are rather between those of the Pseudophyllidea and the Cyclophyllidea, especially the presence of a four-suckered hold-fast accompanying a strobila that has the openings of the reproductive system on the center of the ventral surface of the proglottid rather than along the margins of the proglottid.
SYNONYMS:Taeniavulpina Schrank, 1788; Taeniapseudocucumerina Bailliet, 1863; Taeniapseudoelliptica Bailliet, 1863; Mesocestoidesambiguus Joyeux & Baer, 1932; Mesocestoidesvariabilis Mueller, 1927; Taeniacanislagopodis Rudolphi, 1810; Taeninacateniformis Gmelin, 1750; Mesocestoideselongatus; Leuckart, 1879; Mesocestoideslitteratus (Batsch, 1786); Mesocestoideslongistriatus Setti, 1879; Mesocestoidesmesorchis Cameron, 1925; Mesocestoidescaestus Cameron, 1925; Mesocestoideslatus Mueller, 1927; Mesocestoidesutriculifer (Walter, 1866); Mesocestoidestenuis Meggitt, 1931; Mesocestoidesmanteri Chandler, 1942; Mesocestoideskirbyi Chandler, 1944; Mesocestoidesjonesi Ciodia, 1955; Mesocestoidescarnivoricolus Grundmann, 1956; Mesocestoidesbassarisci MacCullum, 1921; Mesocestoidesangustatus (Rudolphi, 1819).
HISTORY: In 1863, Vallant described a tapeworm as Mesocestoidesambiguus from a genet in North Africa. It was noted at this time that the parasite was unusual because it possessed four suckers and medial reproductive openings. However, although the characters defining the genus have never been questioned, no criteria to define species have emerged as readily acceptable to most workers in this field. Thus, numerous species began to appear in the literature. In 1932, Witenberg in a review of the genus recognized only one species, Mesocestoideslineatus, and he discounted or synonymized 24 described species. Other workers, e.g., Chandler, 1942 & 1944; Ciordia, 1955, Voge, 1955; Grundmann, 1956) have created other species for specimens recovered from mammals, but it is difficult to ascertain what criteria are valid or should be accepted. Loos-Frank (1980) created the name Mesocestoidesleptothylacus for the species of Mesocestoides commonly found in foxes in Europe, but it is hard to understand why another named specimen from foxes would not have priority. a second problem that has developed around the taxonomy and systematics of the genus Mesocestoides is due to the fact that the life cycle has not been entirely elucidated. Thus, the experimental infection of hosts with eggs from adult worms cannot be used to test host specificity of the species present in different carnivores. There is every reason to believe that there are more than one species occurring in mammals around the world and that many of the worms that infect cats are commonly parasites of other mammals such as other species of the feline family, wild canids, raccoons, opossums, mustelids, etc. However, this genus is in need of a careful revision based on the examination of type material.
The second life-cycle stage of Mesocestoideslineatus, called a tetrathyridium, is in a stage that has been recovered from various amphibians, reptiles, birds, and mammals. This is in a small solid-bodied worm with a holdfast (often inverted) that resembles that of the adult worm (Skrjabin & Schultz, 1926). Schultz (1926) and Henry (1927) showed that the larval stage in rodents could develop to adults in cats. Joyeux and Baer (1932) showed that larvae from snakes would develop in cats. Henry (1927) also showed that larvae recovered from the peritoneal cavity of cats could develop to adults if fed to other cats.
GEOGRAPHICAL DISTRIBUTION: North and South America (Nolan & Smith, 1985; Loennberg, 1896); Europe (Persson, 1973; Brglez & Zeleznik, 1976; Hiepe et al., 1988, Martini & Poglayen, 1990), Middle East (Ismail et al., 1983; Dincer et al., 1980; Loos-Frank, 1990); Africa (Reid & Reardon, 1976); India (Meggitt, 1931) Southeast Asia, Japan, and China (Cameron, 1925; Yamaguti 1959). It appears that Mesocestoides has not been reported from Australia.
LOCATION IN HOST: The adults of Mesocestoideslineatus occurs in the small intestine of the cat. The tretrathyridial larval stage is in found on occasion in cats, usually within the peritoneal cavity although sometimes in the muscles (see section of feline tetrathyridiosis).
PARASITE IDENTIFICATION: The gravid proglottids of Mesocestoideslineatus are smaller than those of Dipylidiumcaninum and Taeniataeniaeformis and are easily recognized by the large parauterine organ present in the segments that contain the eggs of this worm that have the common hexacanth embryo of cestodes (Figs. 3-6 & 3-7). The anterior end of the worm has a scolex with four distinct suckers that bears neither a rostellum or scolex (Fig. 3-8). The tapeworm has a neck that contains immature segments.
LIFE CYCLE: The adult cestode sheds segments that are passed in the feces. Loos-Frank (1987) described the behavior of the proglottids shed in the feces noting that “Gravid proglottids have a conspicuous behavior. On freshly produced feces they stand more or less upright and make slow waving movements with their elongated anterior part. After a few minutes they start crawling away from the feces and in moist conditions can be found quite a distance away from the pellets. In a meadow they were observed on grasses all around freshly deposited feces of a wild fox.” She also described the fact that the strobila of Mesocestoideslineatus, like that of Spirometra mansonoides, is in often shed from worms in a host in a relatively cyclical manner wherein they may be shed for a month and then no segments are shed during the next month. Examination of the animals at necropsy revealed that these cestodes were shedding rather large portions of their strobila during the periods of segment shedding causing the populations of worms present to become markedly shortened. This is in supported by the observations of Witenberg (1932) that there were often cestodes of varying states of maturity recovered from infected animals.
The host wherein the development of the presumed first-stage larval form of Mesocestoideslineatus has not been described. Loos-Frank (1991) examined this question with Dr. Ebermann, who had experience with the infection of oribatid mites with other cestode larvae, and they tried, as have others, to reproduce the work of Soldatova (1944) who claimed that development occurred in a few of the mites that were fed on the eggs of this parasite. Other arthropod hosts that have been fed eggs without the development of tetrathyridia include the maggots of Muscadomestica and Calliphora sp. and cockroaches . Loos-Frank (1991), as have many other authors, also tried feeding the eggs directly to rodents without the development of larvae; similarly (Witenberg (1932) fed eggs to various lizards without the production of any tetrathyridia. Thus, what host serve as the first host of this parasite has never been determined, although it is in believed to be some form of arthropod. The fact that the parasite is in so universally found around the world would suggest that they first intermediate host must be relatively common.
The second-larval stage, the tetrathyridium (Fig. 3-9), occurs in the peritoneal cavity and musculature of numerous animals including amphibia, reptiles, birds, and mammals (including the cat). Some animals can harbor large numbers of these larvae. Cats become infected with the adult tapeworm by the ingestion of an intermediate host that is in infected with tetrathyridia. The tetrathyridia of Mesocestoideslineatus do not undergo asexual multiplication within the intermediate host (Conn, 1991; Kawamoto et al., 1986). When cats are infected with the tetrathyridial stage of Mesocestoideslineatus, each tetrathyridium develops into a single adult cestode (Loos-Frank, 1987). Kawamoto et al. (1986) demonstrated using tetrathyridia from snakes that were maintained in the peritoneal cavities of mice prior to use that after ingestion by the cat, the majority of the tetrathyridium was shed leaving only the scolices and necks attached to the mucosa of the intestine. By 10 days after infection of the cats, the strobila had reached lengths of 23 to 52 cm and gravid proglottids with eggs occupied about 20% of the terminal portion of the strobila. Shedding of proglottids began 21 days after the cats were infected. The larvae of a related species, Mesocestoidesvogae (synonym = Mesocestoidescorti), which was found in the peritoneal cavity of a population of fence lizards in southern California by Specht and Voge (1965), does undergo proliferation when inoculated into other hosts, such as the laboratory mouse; however, this is in the only isolate of Mesocestoides that has displayed this property This tapeworm also undergoes proliferation in the intestine of dogs and cats that are fed the tetrathyridial stage (Kawamoto et al., 1986), but it has never been determined that the cat or dog are the natural final host of this species of tapeworm.
When the cat ingest the tetrathyridial stage of Mesocestoideslineatus, the adult worms develop in about two to three weeks (Witenberg, 1932; Loos-Frank, 1987). Other authors have reported prepatent periods as long as 56 days (Joyeux & Baer, 1932; Schwartz, 1927). Witenburg believed that the longer prepatent periods occurred when the cat was not the typical natural host of the adult stage. Not all tetrathyridia ingested by cats develop to the adult stage; some migrate through the intestinal wall to continue as the tetrathyridial stage in the cat (Henry, 1927; Reid & Reardon, 1976; Witenberg, 1932). In these cats, some adults matured to the adult stage while others continued as tetrathyridia. The adults of Mesocestoideslineatus can probably live for around a year in the infected cat (Loos-Frank, 1987).
CLINICAL PRESENTATION AND PATHOGENESIS: Signs of infection with the adult stage of Mesocestoideslineatus in cats have never been described, so are probably asymptomatic.
TREATMENT: Praziquantel has been used to treat dogs experimentally infected with Mesocestoidesvogae (synonym = Mesocestoidescorti), and was found to be 100% efficacious at 5 mg/kg (Thomas & Gönnert, 1978). Dogs with Mesocestoidesvogae have also been successfully treated with a dose of 2.0 mg/kg (Sakamoto et al., 1979). Nitaxzoxanide (at 100 mg/kg) was found to be 100% effective against Mesocestoideslineatus in cats naturally infected with this parasite.
EPIZOOTIOLOGY: Numerous animals can be found infected with the tetrathyridial stage of Mesocestoides lineatus. These hosts include various amphibia, reptiles, rodents, opossums, and other mammals; Witenberg (1932) lists the many hosts of the tetrathyridial stage. Cats become infected by the ingestion of these hosts. In some areas, relatively large percentages of these hosts can be infected. The epizootiology is in very poorly known, however, due to the lack of knowledge concerning the first host involved in the transmission of this parasite.
HAZARD TO OTHER ANIMALS:There is in no direct danger to other animals from the proglottids of Mesocestoideslineatus passed in the feces of cats. It is in likely that the first-intermediate host could be infected in an establishment, but what that host is in is in not currently known.
HAZARD TO HUMANS: Humans have been reported infected with Mesocestoideslineatus. The first report was in a girl in Texas, USA, who harbored the adult stage of this parasite (Chandler, 1942), and there have now been six reported cases in humans in the United States (Schultz et al., 1992). There has been a number of reports in Japan where many of the infections are related to the eating of raw snake meat (Ohtomo et al., 1983). There have also been reports of the adult worms in humans in China and Korea (Jin et al., 1991; Eom et al., 1992).
CONTROL AND PREVENTION: Control would be through the prevention of hunting by cats. This will be exceedingly difficult due to the wide range of hosts that are capable of harboring the tetrathyridia.
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Figure 3-6. Gravid segment of Mesocestoideslineatus showing the large parauterine organ filled with eggs.
Figure 3-7. Egg of Mesocestoideslineatus expressed from a gravid segment. Note the three pairs of hooklet6s typical of a tapeworm larva.
Figure 3-8. Scolex of Mesocestoideslineatus. Note the total lack of a rostellum or hooks on the anterior end.
Figure 3-9. Tetrathryidium of Mesocestoides. Note the suckers lacking a rostellum as in the adult and the elongate body. Tetrathyridium can become much longer than this example.