Long-term dynamics of gastrointestinal parasite egg shedding in a semi-feral Misaki horse population in Japan.

This study investigated long-term gastrointestinal parasite egg shedding in a semi-feral Misaki horse population using 3200 fecal samples collected from 158 individually identified horses during five fiscal years (FY2019-FY2023; April 2019-March 2024). Strongyle-type eggs were detected in 91.7% of samples (2935/3200), with a mean count of 1352 eggs per gram (EPG). Repeated-measures analyses showed marked variation associated with fiscal year, season, age, and management history. Strongyle-type EPG values were generally higher in spring and summer than in autumn and winter, with the month-level pattern reaching its highest level around September. In an exact-date-of-birth analysis of 181 samples from 49 foals, EPG increased markedly after 3 months of age; this age association remained evident after adjustment for season, although age and seasonal exposure were strongly imbalanced. Among females with an identifiable previous treatment date, EPG differed across treatment intervals, but treatment timing was structurally linked to the annual late-September/early-October roundup and season and was therefore interpreted as a management-associated pattern rather than an independent treatment effect. Population-level climatic analyses showed that contemporaneous temperature was associated with strongyle-type EPG, although this association could not be distinguished from the recurring seasonal pattern in egg shedding; precipitation was not significant after joint adjustment and two-way clustering by horse and month-year. Parascaris spp. and Strongyloides spp. eggs were detected in 2.6% and 0.3% of samples, respectively, and were concentrated in younger horses. These findings provide a longitudinal characterization of parasite egg-shedding dynamics in an individually identified semi-feral horse population while highlighting the importance of age, season, repeated sampling, and management structure in interpretation.