Nematode Infestation in Rana arvalis Nilsson, 1842: The Role of Age and Geographic Location

Ten parasite species have been identified in populations of Rana arvalis Nilsson, 1842 (from the western macroslope of the Middle Urals to the Ishim Plain), with the nematodes Oswaldocruzia filiformis (Goeze, 1782) and Rhabdias bufonis (Schrank, 1788) being dominant. A two-component (hurdle) model combining binomial regression (infection probability) and gamma regression (infection intensity) provides the best biological and statistical fit for nematode infestation patterns. Both infection probability and intensity increase with host age, peaking at 3–4 years: each unit increase in log2-transformed age (log2(Age + 1)) raises the odds of infestation 6.2-fold (95% CI: 4.9–7.8) and intensity 1.4-fold (1.2–1.5). Intensity also increases 2.1-fold (1.8–2.5) per additional nematode species. Geographic trends reveal latitudinal and longitudinal variability: infestation odds decrease 2.1-fold (1.5–2.7) per degree northward, while infection intensity increases 1.14-fold (1.1–1.2) per degree eastward. Neither host morph (striata/maculata) nor sex significantly influence nematode infestation parameters.