Untersuchung zur Stressbelastung von Robben (Pinnipedia) und Rindern

A prolonged or rapidly recurring stress exposure may lead to a chronic activation of the hypothalamic-pituitary-adrenal-(HPA) axis. Such a chronic stress exposure, for example due to environmental or anthropogenic stressors, may result in a misguided stress response as well as an overload of the adaptive mechanisms. As a consequence, physiological and also pathological changes may occur, which can also be associated with an increased susceptibility to disease (Besedovsky und Rey 1996, McEwen und Wingfield 2003). Determination of cortisol is considered to be the traditional method of choice to assess a stress exposure. However, levels of cortisol might be influenced by handling while gaining samples, especially in marine wildlife and livestock animals (Cook et al. 2000, Mormede et al. 2007). Thus, in human medicine the assessment of the steroid hormone dehydroepiandrosterone (DHEA) has become more and more important to differentiate between an acute and chronic stress response. It could be shown that a chronic hyperresponsivness of the HPA-axis causes on the one hand a decrease of the DHEA concentrations, whereas an increased production of cortisol from the common precursor hormone pregnenolone occurs (Parker et al. 1985, Guilliams and Edwards 2010). Therefore, possible differences regarding the cortisol-, DHEA- and DHEAS- concentrations were assessed in two different mammalian species in the present study. The aim was to measure the cortisol-, DHEA-, DHEAS-concentrations as well as the cortisol/DHEA-ratio in diseased seals and dairy cows in order to assess a potentially occuring chronic activations of the stress system. Moreover, these parameters were evaluated as part of a multi-species stress index for the evaluation of the impact of a chronic stress exposure on the organism. In the first part of this study, cortisol was determined in lachrymal fluid and saliva as alternative sampling materials in wild seals as well as seals being habituated to human handling. In earlier studies in humans and cattle the use of saliva and lachrymal fluid was already evaluated as a possible alternative sampling material (Banbury 2009, Khraim 2011, Hernandez et al. 2014). According to these studies the rise of cortisol shows a time lag in saliva respectively lachrymal fluid compared to serum. Therefore it was tested whether similar conditions can also be found in harbor and gray seals. Regression analyses revealed a reliable correlation between serum cortisol and the cortisol concentration in lachrymal fluid (R2=0.82, P< 0.01), whereas concentrations in saliva and tears (R2=0.68, P< 0.05) as well as between saliva and serum (R2=0.60, P< 0.05) displayed moderate correlations. Based on these findings cortisol concentrations in lachrymal fluid may rather represent increased cortisol values influenced by restraint and handling. However, the moderate correlation between saliva and serum cortisol concentrations may indicate a time-delayed increase of cortisol in the saliva samples of these pinnipeds. Thus, sampling of saliva may present a situation independent alternative for the assessment of cortisol in different seal species. In the second part of the present study, possible differences of the cortisol-, DHEAS- and DHEA-concentrations were examined in seals living in human care as well as healthy and diseased, free-ranging seals. This investigation was based upon the hypothesis that habituated seals do not experience stress due to the sampling procedure in contrast to healthy, free-ranging seals. Moreover, it was assumed that diseased, free-ranging seals were exposed to chronic stress before the sampling took place. At the beginning, the health status was assessed based on a general respectively postmortem examination and the determination of blood cell counts. In addition, cortisol-, DHEAS- and DHEA-concentrations were analyzed. The habituated seals showed significantly lower cortisol levels (P< 0.01) compared to both free-ranging seal groups. However, cortisol concentrations of the healthy free-ranging seals were similar to those of the diseased, free-ranging harbor seals. The DHEAS values were also similar between the three study groups. The free-ranging, diseased harbor seals showed the lowest DHEA concentrations compared to the healthy, free-ranging seal group (P< 0.05). In addition, the diseased, free-ranging seals displayed a higher cortisol/DHEA-ratio compared to the habituated seals (P< 0.001). Also regarding the calculated stress index, based on the relationship between the nutritional status and cortisol values compared to the DHEA- and IGF-I-concentrations, the group of the diseased, free-ranging harbor seals revealed the highest value. It may therefore either be assumed that a chronic stress exposure may possibly have influenced the health status of the diseased, free-ranging seals or that the poor health status may have triggered a stress response itself. However, the steroid hormone DHEA and the cortisol/DHEA-ratio seem to represent reliable tools for the assessment of a chronic stress response in harbor and gray seal. Based on the assumption that a chronic stress exposure causes a higher incidence of inflammatory reproduction diseases in the postpartal period (Ingvartsen 2006, Mulligan und Doherty 2008) differences of the cortisol-, DHEAS- and DHEA-concentrations were analyzed comparing healthy cows and those suffering from a postpartal metritis. For this purpose each cow had to undergo a clinical and gynecological examination of the uterus and ovaries as well as the evaluation of possible vaginal discharge. Furthermore, blood samples were gained for endocrinological analyses. Also a determination of leukocyte number was conducted in order to assess the activity of the cellular immunity. According to the results of leukocyte counts the examined cows were subcategorized as healthy, suffering from metritis with normal number of leukocytes (Mrange, 5-10 x 106/μl) or with leucopenia (Mlow,< 5 x 106/μl). As a result, concentrations of cortisol and DHEAS could be compared between the three study groups. Nevertheless, leucopenic cows suffering from metritis showed higher DHEA concentrations than the healthy cows (P< 0.05). Furthermore, the leucopenic, diseased cows revealed the lowest cortisol/DHEA-ratio. It can therefore be speculated that the higher DHEA values of the Mlow cows may possibly reflect the immunomodulatory and -protective characteristics of DHEA which has been described in the literature. Higher DHEA concentrations may therefore function as a hormonal counter regulation in order to maintain a balance between the opposite effects of cortisol and DHEA on the immune system. This conclusion is also supported by the low stress index of the leucopenic, diseased cows calculated from the relationship between the body condition score and cortisol concentrations compared to the DHEA- and IGF-I- values. Based on the potential antagonizing effects of the assessed adrenal hormone, the cortisol/DHEA-ratio may represent a prognostic tool for the evaluation of the course of disease. In the present study several evidence could be found for the different effects and interactions of the steroid hormone DHEA with the stress and immune system regarding the two investigated mammalian species. As a consequence, the establishment of a multi-species stress index based on these diverging and not completely comparable results does not seem to be an appropriate tool for an objective and reliable assessment of a potential chronic stress exposure. The cortisol/DHEA-ratio though may help to evaluate the impact of the HPA-axis on the function of the neuro-immune-endocrine network.