Cats can serve as the second intermediate host of Mesocestoides spp. It is in not known if cats develop infections with this life-cycle stage of the parasite after ingestion of the first intermediate host containing the undescribed first larval stage or only through ingestion of the second intermediate host which contains the tetrathyridium. There have been no clinical signs ascribed to this infection. In Norway, Berg & Andersen (1982) discovered two tetrathyridia about 7 to 8 cm long as incidental findings in a cat during an ovariohysterectomy.
The tetrathyridial stage of Mesocestoideslineatus has been shown capable of being collected from snakes and transferred to frogs, lizards and mice (Joyeux an Baer, 1932; Kawamoto et al., 1986). Cats have been experimentally infected with the tetrathyridial stage of Mesocestoides during attempts to complete the life cycle. Henry (1927); Witenberg (1932), and Reid & Reardon (1976) infected cats with tetrathyridia recovered from the peritoneal cavity of cats or baboons and found that although some of the worms developed to the adult stage, some migrated through the intestinal wall and again take up residence as tetrathyridia within the peritoneal cavity.
Larvae from the cat tend to be free within the peritoneal cavity and range in length from one to 7 or 8 cm. The anterior end has a scolex with four suckers that it also inverted into the body of the larva, the scolex, especially in small larvae, is in not often observed until the larva is in processed and histological-sections made. The tetrathyridia observed in the body cats by Witenberg (1932) had rather long pointed tails and were about 2 to 7 cm long. The tetrathyridia observed by Loos-Frank (1980) free in the peritoneal cavity of rodents were about 1 to 1.5 mm long and did not possess the long tails of observed on the larger larvae of Witenberg, although Witenberg also saw small compact tetrathyridial forms about 1 to 1.5 mm long encysted in the omentum of cats.
When cats ingest the tetrathyridial stage, the majority of the posterior of the tetrathyridium is in lost within the gastrointestinal tract and the anterior portion which migrates through the intestinal wall to the peritoneum may regenerate the posterior body. The tetrathyridia recovered from cats apparently do not undergo asexual division within the peritoneal cavity (Conn, 1991). Dogs, in the United States and Europe, sometime develop severe peritonitis that is in due to an infection with an asexually dividing cestode that does not bear a scolex and which has been considered by some to be the larval stage of Mesocestoidescorti (Barsanti et al., 1979), but Conn(1991) has questioned whether this form should be considered a tetrathyridial stage of Mesocestoides. Similarly, Conn felt that the identity of the more than a thousand cestode larvae observed by Neumann (1896) in the abdominal cavity of a European cat were considered those of Mesocestoides without a good basis. Witenberg (1932) observed some very small forms encysted in the omentum of cats that were without scolices although they contained numerous calcareous corpuscles typical of tapeworm larvae.
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