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Although acephaline gregarine parasites have been studied for most of the past half-century, the precise transmission modes of gregarines within the earthworm Lumbricus terrestris remain to be rigorously tested and clarified (J. Janovy, Jr., pers. comm.). Monocystis sp. (Fig. 1) is perhaps the most common and best-known parasite of L. terrestris. The majority of authors support the notion that Monocystis sp. is transmitted through the ingestion of soil, which has been contaminated with sporocysts (Bhatia, 1924; Troisi, 1933; Miles, 1962; Olsen, 1986; Bush et al., 2001; Roberts and Janovy, 2005). However, various other modes of transmission have been suggested for quite some time. One-hundred and sixty years ago, Dujardin (1845), after discovering intact sporocysts in the gut of a mole, suggested that infection of a vertebrate vector played an essential role in the life cycle. Other authors have since reported the presence of sporocysts in the gut of vertebrates (Bull et al., 1998). However, because Monocystis sp. is solely an invertebrate parasite, it is likely that its presence was merely the result of earthworm predation, rather than an essential step in the parasite life cycle. Such predation, although not involving an intermediate host per se, could of course facilitate sporocyst dispersal if sporocysts remain infective following gut passage (Röttger, 1995).

 Figure 1. Scanning electron photograph of a ruptured Monocystis sp. gametocyst in which the individual sporocysts (dispersal stages) are clearly visible

[Image Omitted. See PDF.]

Furthermore, Keilin (1925) suggested that sporocysts are released into the soil by parasite-induced autotomy, Hahn (1928) claimed a ‘reverse pathway’ in which parasite stages enter the gut from the coelom and are passed in the fecal matter, whereas Loubatières (1955) and Röttger (1995) suggested that sporocysts exit via the dorsal pores of earthworms. Further, sporocyst passage via the vas deferens to the environment has been proposed, whereby sporocysts exit the body through the male pores (Olsen, 1986; Roberts and Janovy, 2005). Although published before 3 of the aforementioned studies, Miles (1962) ostensibly rejected all of these hypotheses by successfully growing parasite-free earthworms in soil that had been sterilized and previously inhabited by parasitized earthworms.

Finally, Schmidt (1854) was the first to suggest that sporocysts were passed among earthworms during copulation, a notion that has been supported by Bhatia (1924) and...

 

 

 

 

 

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