Hemoparasite invasion of Pelophylax ridibundus (Amphibia: Ranidae) in the Middle Volga region

   The hemoparasite invasion and immunophysiological status were assessed in 63 Pelophylax ridibundus individuals from three aquatic habitats in the Middle Volga region. Hydrochemical analysis of the frog habitats revealed that the maximum permissible concentrations of manganese, copper, chromium, zinc and cobalt compounds were exceeded in the fisheries-designated waterbodies. According to the Specific Combinatorial Water Pollution Index, the water quality classes corresponded to Class 4 (very polluted, Category G) and Class 5 (extremely polluted). High hemoparasite prevalence was established (58.7 % of individuals). Intracellular parasites of the class Conoidasida (representatives of the genus Dactylosoma from the family Dactylosomotidae, representatives of the genus Hepatozoon from the family Hepatozoidae) and extracellular parasites (class Nematoda, family Onchocercidae, microfilaria larvae) were identified in blood smears. Representatives of the genus Dactylosoma were found in the most infected individuals. Sex-specific differences in the infection indices were observed across the habitats. Hemoparasite infection was associated with decreased levels of large circulating immune complexes (CIC) and reduced large-to-small CIC ratio (aggregation index). Infected frogs exhibited elevated eosinophil counts and altered comprehensive leukocyte indices, namely: karyopyknotic index, lymphocyte-granulocyte ratio, lymphocyte-to-eosinophil ratio, and leukocyte shift index, indicating activation of their antiparasitic defense. Infected females showed a differential relative mass of the internal organs (heart, liver, kidneys, gonads – but not spleen) compared to uninfected individuals. In males, hemoparasite infection reduced their cardiosomatic and renosomatic indices, while exposure to extremely polluted water increased the hepatosomatic and gonadosomatic indices.

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