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

Overview

• This study examined the long-term patterns of gastrointestinal parasite egg shedding in a semi-feral Misaki horse population in Japan over five years.
• It analyzed fecal samples from individually identified horses to understand how factors like age, season, management practices, and climate influence parasite egg counts.

Study Design and Data Collection

• Researchers collected a total of 3200 fecal samples from 158 semi-feral Misaki horses over five fiscal years (April 2019 to March 2024).
• Each horse was individually identified, allowing for repeated measures analysis to assess changes in parasite egg shedding over time within the same individuals.

Parasite Egg Detection and Counts

• Strongyle-type parasite eggs were the dominant findings, detected in 91.7% of samples (2935 out of 3200).
• The mean egg count was relatively high, averaging 1352 eggs per gram (EPG) of feces.
• Other parasite eggs found included Parascaris spp. (2.6% of samples) and Strongyloides spp. (0.3%), primarily found in younger horses.

Factors Affecting Strongyle-Type Egg Shedding

• Seasonality: Egg counts were higher in the spring and summer months compared to autumn and winter, with a peak around September.
• Age: Analysis of 181 samples from 49 foals showed that EPG increased considerably after 3 months of age, suggesting that young horses become more heavily infected as they age.
• This age effect remained significant even after accounting for season, despite an imbalance in combined age and seasonal exposure.

Impact of Management Practices

• The study assessed the effect of treatment intervals among females with known previous treatment dates.
• Differences in EPG were observed depending on how recently the horses had been treated.
• However, the timing of treatments was closely tied to annual roundups in late September or early October, making it difficult to separate treatment effects from seasonal patterns.
• This points to the importance of considering management schedules when interpreting parasite egg shedding data.

Climatic Influences

• Population-level climate data showed a correlation between temperature and strongyle-type egg shedding.
• Because temperature follows seasonal trends, it was challenging to distinguish whether temperature itself or seasonal factors drove the pattern.
• Precipitation did not significantly impact egg shedding once adjustments were made for both individual horses and time (month-year clustering).

Significance and Contributions

• The study provides a detailed longitudinal characterization of gastrointestinal parasite egg shedding in a naturalistic semi-feral horse population.
• It highlights the complex interplay of biological (age), environmental (season, temperature), and anthropogenic (management/treatment) factors affecting parasite dynamics.
• The findings emphasize the necessity of repeated sampling and careful consideration of management timing to accurately interpret parasite infection patterns in free-ranging equids.