Stable microclimates and autonomic modulation in horses housed in different stable architectures during monsoon in a tropical environment.

Overview

• This study investigated how different stable designs and the resulting microclimates during the tropical monsoon season affect the autonomic nervous system regulation, as measured by heart rate variability (HRV), in horses housed individually.
• It aimed to understand the relationship between stable architecture, environmental conditions, and stress levels in horses to improve their welfare in tropical climates.

Introduction and Background

• Horses housed in individual stalls are influenced by their surrounding climate conditions, which can impact their health and welfare.
• Tropical climates, especially during the monsoon season, present unique environmental challenges such as high humidity, variable temperatures, and presence of noxious gases.
• Stable designs affect the microclimate inside the stalls by modifying factors such as airflow, temperature regulation, and pollutant accumulation.
• There is limited research focused on how these factors interplay during monsoon periods, a critical gap especially in tropical environments.

Study Objective

• To assess how different stable architectures influence microclimate parameters — such as relative humidity, air temperature, and concentrations of noxious gases — during the monsoon season.
• To evaluate autonomic modulation in horses via heart rate variability (HRV) metrics in varied stable environments, thereby measuring physiological stress.

Methodology

• Twenty-two horses were randomly assigned to three different stable designs, each with distinct architectural features:
• Stables with solid external walls and lower volume-to-horse ratio
• Stables with solid external walls and higher volume-to-horse ratio
• Stables without solid external walls (likely more open or ventilated)
• Microclimate variables were monitored, including:
• Relative humidity
• Air temperature
• Levels of noxious gases (e.g., ammonia)
• The autonomic nervous system function was examined by measuring multiple HRV parameters:
• HRV was recorded every hour over a continuous 24-hour period to capture both day and night variations
• Heart rate variability serves as a non-invasive indicator of stress and autonomic balance (parasympathetic vs sympathetic activity)

Results

• Significant differences were found in microclimate conditions depending on stable architecture:
• Day versus night differences were notable across all designs
• Stables with solid walls and lower air volume per horse tended to have less favorable microclimates
• HRV indicated varying stress levels according to stable type:
• Horses in stables with solid external walls and lower volume-to-horse ratio showed decreased HRV values (statistically significant, p < 0.05 to 0.0001)
• Lower HRV corresponds to higher stress and reduced autonomic flexibility
• Horses in stables with higher volume-to-horse or without solid external walls had higher HRV, indicating better stress adaptation
• A clear correlation was demonstrated between microclimate parameters and HRV metrics, confirming that environmental conditions inside stables influence horses’ physiological stress responses.

Conclusions and Implications

• Stable architecture directly affects the microclimate experienced by horses during the tropical monsoon, which in turn affects their autonomic nervous system function.
• Lower air volume per horse and solid walls may contribute to poorer ventilation, higher humidity, or gas accumulation, increasing physiological stress indicated by reduced HRV.
• Stable design alterations, such as increasing ventilation or volume per horse, can mitigate stress and improve welfare in tropical monsoon conditions.
• These findings provide key insights for horse caretakers, stable designers, and animal welfare advocates to optimize housing strategies specific to tropical monsoon climates.
• Implementing well-ventilated designs and ensuring adequate space per horse can enhance comfort and reduce health risks during monsoon season in tropical environments.