Ecological issues of the nematode Rhabdias bufonis (Nematoda: Rhabdiasidae) in the steppe of the Republic of Kazakhstan

One of the candidates for the role of a model species in the study of the biology and evolution of helminthes is the nematode R. bufonis. For this status, insufficient information has been accumulated on the nematode infection of the hosts in biotopes of different landscapes, on the long-term, seasonal and gender-age dynamics of the helminth abundance, on the peculiarities of the development cycle in natural conditions and under anthropogenic pollution. In this work, data are presented on the infection of the moor frog from 14 biotopes of the steppe zone of the Republic of Kazakhstan. Data were obtained from the autopsy of 2637 specimens of R. arvalis. Almost half of the hosts were caught at the experimental site (0.5-0.7-500 m) in the floodplain of the Irtysh River, where helminthological monitoring of R. arvalis was conducted for 14 years. It was established that the distribution of the number of R. bufonis in the populations of the moor frog belongs to the over scattered distribution. According to the level of host infection extensiveness, the abundance index, and the ratio of the actual and theoretical number of nematodes, three groups of biotopic samples were distinguished: those with high, medium, and low values of infection parameters. The infestation of the amphibian R. bufonis was determined by the presence and size of zones in the biotopes with a moistened and oxygen-enriched substrate. Part L4 of R. bufonis on the way to the lungs of the moor frogs get stuck in small vessels, enter the body cavity and accumulate. Helminthes in the body cavity grow in length, but their genitals do not develop. The nematode infestation of frogs depends on the time of sampling, the age, and to a lesser extent, the sex of the caught individuals. The highest values of the extent of invasion of the host's lungs and body cavity and the number of helminthes in the natural conditions of the Irtysh River floodplain and in anthropogenic biotopes was observed in May and September, the lowest - in July. The minimum abundance of R. bufonis and the extent of lung invasion of R. arvalis is typical for younger age groups (body length 13.0-30.0 mm), the maximum - for older groups (body length > 36.1 mm). The host body cavity is more infected in young individuals (body length 20.1-30.0 mm). Large R. arvalis (body length > 36.1 mm) are less infected. Homing, or the proportion of R. bufonis, which successfully populate the lungs of the youngest frogs (body length ≤ 20.0 mm) is 42.3, and the largest amphibians (body length ≥ 44.1 mm) is 87.7 % In the Irtysh River floodplain and in the anthropogenic biotopes of the city of Pavlodar, the nematode twice implements a development cycle during one season. The first, spring-summer, is carried out in May - July of each current season. Post-invasive larvae of the spring generation that have penetrated the body cavity of R. arvalis remain there for 2.5-3 months. The spring generation of parasitic female nematodes lasts longer in the lungs, at that some of the individuals merge with the seasonal flow of autumn helminth generation. The development of the latter occurs in two stages. During the first one, all free-living stages of the nematode develop. It ends infecting of the host's lungs and body cavity. Further development is interrupted by the winter dormancy of the frog and its parasites. The last part of the autumn cycle of nematode development in warm autumn (August - September) occurs within one month. The second stage of the life cycle of R. bufonis: the release of eggs by females into the lungs of frogs ends at the beginning of the next season. The second cycle of helminth development, including winter dormancy, lasts 11-13 months. R. bufonis is a species with opportunistic populations. Long-term waves of nematode abundance and infection of the lungs and body cavity of R. arvalis occur in the floodplain of the river Irtysh synchronously, with a period of 3-4 years.