Hydroclimatic variability is associated with gastrointestinal parasite dynamics in black-horned capuchins (Sapajus nigritus cucullatus) in the Atlantic Forest of Argentina.

Understanding the environmental drivers of parasite dynamics in wildlife is critical for predicting disease risk under changing climatic conditions. We investigated the relationship between climatic variables and patterns of gastrointestinal parasitism in wild black-horned capuchin monkeys (Sapajus nigritus cucullatus) inhabiting the Atlantic Forest (Misiones, Argentina). A total of 131 fecal samples were collected from two study sites (COR: 25°51'S, 53°57'W; YL: 25°34'S, 54°4'W) between May 2019 and November 2021 and analyzed using flotation and sedimentation techniques. Environmental data (temperature, humidity, and precipitation) were obtained from NASA MERRA-2, and the Standardized Precipitation Evapotranspiration Index (SPEI, 3-month scale) was used to characterize moisture availability. Generalized linear mixed models indicated that SPEI was the most informative environmental predictor: both parasite frequency and richness were positively associated with wetter conditions (higher SPEI values), whereas seasonal categories showed no significant effects. At the taxon level, Strongyloides sp. occurrence increased with SPEI, while rhabditoid larvae increased with relative humidity and Hymenolepididae decreased with humidity. These findings suggest that continuous variation in moisture availability, rather than discrete seasonal categories, better captures parasite dynamics in this subtropical system. SPEI emerges as a valuable integrative index for capturing environmentally driven variation in parasite communities, particularly under pronounced climatic anomalies such as the 2019-2021 regional drought associated with the onset of La Niña conditions in 2020. Incorporating such indices into wildlife disease ecology may improve the prediction and monitoring of parasite dynamics under ongoing climate change.