Biology (life cycles)

Alternation of generations in the life cycle of Rhabdias bufonis was discovered by Mecznikow (1865) and first published by Leukart (1865). The comprehensive information on the development of this species is expanded in the papers of Schaake (1931) and Spieler and Schierenberg (1995). In this species, eggs of parasitic hermaphrodites pass from the host lungs into the intestine, accumulate in colon and, thereafter, are cast out of the host organism in faeces. The free-living stages develop in the droppings. Larvae from the eggs of hermaphrodites reach maturity as males and females of gonochoristic generation. The latter give rise to the hermaphroditic generation larvae which develop in droppings up to the third stage. At this stage the larvae become infective and are able to continue development only after the penetration into the host.

Fig. 1. Rhabdiasoid life cycle: strict alternation of two generations.

Railliet (1899) and Goodey (1924 a) failed to observe the gonochoristic generation in R. fuscovenosa. This species was considered to develop directly, in contrast to other species of the family. However, Chu (1936 b), after thorough examination of the development of R. fuscovenosa in laboratory cultures and, with original cultivation method applied, found out that the smaller part of hermaphroditic generation progeny developed indirectly. The progeny of the latter underwent the metamorphosis during the second moult and the third-stage larvae had filaroid morphology, in contrast to rhabditoid homogonic infective larvae and infective larvae in other species. These filariform larvae failed to infect the hosts. The author believed that both homogony and heterogony were characteristic for the life cycles of all Rhabdiasidae (as they were in Strongyloididae) but the former way predominated in amphibian parasites and the latter one predominated in the species from reptiles. The hypothesis of Chu, however, has not been confirmed by the other authors, who investigated the development of various rhabdiasid species, even when Chu's method of cultivation has been applied (see, for example, the works of Baker (1978) and Kuzmin (1997, 2000). Homogony was not found in the life cycles of amphibian parasites, nor it was in the life cycles of species from chameleons and lizards (i. e. R. gemellipara, R. chameleonis, Pneumonema tiliquae, Entomelas spp.). The only two species that have been found to possess the life cycles similar to that in R. fuscovenosa and Strongyloides spp. were R. elaphe and R. agkistrodonis (Kuzmin 1999; Kuzmin and Miskov 1999). Both species are the parasites of snakes. Heterogonic infective larvae in these species are rhabditoid and morphologically similar to the homogonic ones.

Fig. 2. Strongyloid life cycle, combination of heterogony and homogony.