Seasonal patterns of oxidative stress markers in captive Asian elephants in Thailand and relationships to elephant endotheliotropic herpesvirus shedding

Abstract

Introduction: Oxidative stress refers to an imbalance between oxidant and antioxidant activity and accumulation of reactive oxygen species, which can have detrimental effects on animal health. Annual fluctuations in oxidative stress status can occur, increasing disease susceptibility during certain time periods. However, a full understanding of factors related to oxidative stress in Asian elephants and how to mitigate the negative consequences is lacking.

Methods: This study measured six serum oxidative stress markers [reactive oxygen species (ROS), malondialdehyde (MDA), 8-hydroxydeoxyguanosine (8-OHdG), albumin, glutathione peroxidase (GPx), and catalase] and two stress markers [serum cortisol and fecal glucocorticoid metabolites (fGCM)] in 23 captive Asian elephants in Thailand over a 12 months period to examine relationships with age and season.

Results: Seasonal variations were observed, with several markers exhibiting significantly higher concentrations in the summer (ROS, MDA, 8-OHdG, albumin) and lower values during the rainy/winter seasons (MDA, 8-OHdG, albumin, catalase). By contrast, GPx was the only marker to be highest during the rainy season. For the stress markers, higher fGCM concentrations were noted during the rainy season, which contrasts with earlier studies showing more activity in the winter (tourist season). Positive correlations were found between the temperature-humidity index and ROS, GPx, and fGCM, while a negative correlation was observed with serum albumin. Elephant endotheliotropic herpesvirus (EEHV) shedding events were associated with higher concentrations of ROS and MDA. A moderate negative correlation was observed between 8-OHdG and the PCR threshold cycle of EEHV shedding (Ct), indicating DNA damage may be involved in EEHV shedding in elephants.

Discussion: Results revealed significant age and seasonal effects on several oxidative stress markers, indicating those factors should be considered in study design and data interpretation. There also may be physiological adaptations in oxidative stress conditions in relation to environmental changes that could impact health outcomes.

Keywords: Asian elephants; elephant endotheliotropic herpesvirus shedding; oxidative stress markers; seasonal pattern; stress markers.

Figure 1

Box and line plots display monthly patterns of serum oxidant marker concentrations in calves (≤8 years old, N = 12) and adult elephants (>8 years, N = 11): reactive oxygen species, ROS (A, B); malondialdehyde; MDA (C, D); 8-hydroxydeoxyguanosine; 8-OHdG (E, F). Boxplots represent median, quartiles, and the 25th/75th percentiles, error bars represent the 10th/90th percentiles, and open circles indicate outliers. Different superscripts show a significant month effect (p < 0.05) for calves (a,b,c,d) and adults (w,x,y,z). Bold black lines represent the overall mean trend line, and the shaded area is the 95% confidence interval. Pink solid lines represent the trendline for calves (N = 12), solid blue lines represent the trendline for adults (N = 11), and thin lines with dots represent monthly means (±SD) in each age group (pink: calves, blue: adults).

Figure 2

Box and line plots display monthly patterns of serum antioxidant marker concentrations in calves (≤8 years old, N = 12) and adult elephants (>8 years, N = 11): albumin (A, B); glutathione peroxidase; GPx (C, D); and catalase (E, F). Boxplots represent median, quartiles, and the 25th/75th percentiles, error bars represent the 10th/90th percentiles, and open circles indicate outliers. Different superscripts show a significant month variation at p < 0.05 within group; calves (a,b,c,d) and adults (w,x,y,z). Bold black lines represent the overall mean trend line, and the shaded area is the 95% confidence interval. Pink solid lines represent the trendline for calves (N = 12), solid blue lines represent the trendline for adults (N = 11), and thin lines with dots represent monthly means (±SD) in each age group (pink: calves, blue: adults).

Figure 3

Box and line plots display monthly patterns of stress biomarkers in calves (≤8 years old, N = 12) and adult elephants (>8 years, N = 11): serum cortisol (A, B) and fecal glucocorticoid metabolites (C, D). Boxplots represent median, quartiles, and the 25th/75th percentiles, error bars represent the 10th/90th percentiles, and open circles indicate outliers. Different superscripts show a significant month variation at p < 0.05 within group; calves (a,b,c,d) and adults (w,x,y,z). Bold black lines represent the overall mean trend line, and the shaded area is the 95% confidence interval. Pink solid lines represent the trendline for calves (N = 12), solid blue lines represent the trendline for adults (N = 11), and thin lines with dots represent monthly means (±SD) in each age group (pink: calves, blue: adults).