Mercury and Selenium: A contaminant and nutrient interaction assessment
By Carla Willetto, DVM, MSc, VETS, Inc., Victoria Woshner, DVM, PhD, Katrina Knott, MSc, and Todd O’Hara, DVM, PhD, University of Alaska, Fairbanks

Veterinary Environmental Toxicology Services (VETS) and the new Wildlife Toxicology Laboratory at the Institute of Arctic Biology (IAB) at the University of Alaska Fairbanks (UAF) have teamed up with Dr. Victoria Woshner to address selenium (Se) and mercury (Hg) from a “functional” perspective in bottlenose dolphins sampled in the Sarasota Bay area. Selenium and Hg interact in a yet unknown way that likely alters the toxicity of Hg and alters the nutritional and toxic properties of Se in cetaceans. For this reason we conduct functional assays in the suite of indicators to address Hg and Se status in Tursiops in concert with the current sampling for Hg and Se concentrations in blood compartments and epidermis. These functional assays include glutathione (GSH) peroxidase (Px) measurements, or GSH-Px assays. Field sampling protocols for this project were developed to fit in with overall capture operations and necropsy. Since Se forms an integral part of the GSH-Px enzyme the nutritional requirement of Se is important. Selenium in the form of selenocysteine is incorporated at the four active sites of the enzyme GSH-Px. This enzyme assumes a critical role in protecting against free-radical and oxidative damage associated with oxidative stress.  We are also evaluating tissues from capture-release and stranded dolphins using histologic (light microscopy) techniques to assess the functional state of the tissues. We will determine the presence and absence of specific tissue changes known to occur for Hg toxicosis and Se deficiency. Histological examinations are underway to detect possible indications of disease or toxicant effects such as ceroid deposits, neutrophilic infiltrate, fat cell necrosis or other associated changes. In a few animal and human studies it was observed that deficiency of Se could cause pregnancy complications (Zachera et al., 2001). Therefore, we propose to evaluate the blood and plasma levels of Hg and Se, histologic features, and glutathione peroxidase in blood to better develop a “functional” understanding of this Hg and Se interaction and the health status of the Sarasota dolphin population.

Preliminary data (Knott et al., 2005) indicated that the circulating blood concentration of total Hg in 38 bottlenose dolphins from Sarasota Bay, Florida was 543.3 ppm (or µg/L ww). This concentration is 100 fold higher than the recommended threshold level established for humans (5.8 ppm; US EPA), below which exposures are considered to be without adverse effects. It remains to be seen, however, whether this threshold level is meaningful to a fish-eating small cetacean. Blood and epidermal (outer skin) biopsies were collected from 38 bottlenose dolphins in Sarasota Bay, Florida, during summer (June) and winter (February) 2004-2005, as part of the Dolphin Health Assessment Project. Mean circulating levels of total Hg, whole blood total Se, serum total Se and GSH-Px activity were 543 µg/kg, 0.77 µg/g, 0.40 µg/g and 98.8 mU/mg Hb, respectively, and did not differ by season (winter v. summer). Total Hg levels in blood increased linearly with age in both male and female dolphins (expected finding). Serum total Se made up half of the concentration of total Se in blood and both were only marginally related to the increases of total Hg with age. GSH-Px activity was linearly related to blood total Se; but neither to serum total Se, nor total Hg. This may indicate measures of blood Se are more important “functionally” for GSH-Px but further assessment of this relationship is underway.

Sarasota Bay dolphins showed the typical response of increased Hg exposure and accumulation with age. High Hg levels in the blood of bottlenose dolphins, however, seem to have a minimal effect on the Se concentrations and enzyme activity that would signify detoxification. Total Hg levels, however, provide an incomplete picture of the effect of more deleterious forms of Hg, such as methylmercury (MeHg), that can be at high levels in fish and biomagnify to the dolphin. We are examining the amount of MeHg in the circulation of dolphins and the relation to levels of Se and GSH-Px activity. Because nutritional deficiencies are not expected in these robust free ranging animals (captured), these data will be important when cases of suspected inadequate nutrition (stranded) or Hg toxicosis are reported. Data gathered from the dolphin population in Sarasota Bay will also be compared to other studies of cetacean species, especially those residing in Arctic climes (e.g., beluga whales).

Literature Cited:

Knott, K., V. Woshner, R. Wells, C. Willetto, R. Swor, and T. O’Hara. 2005. Mercury and Selenium: An assessment of contaminant and nutrient interactions in bottlenose dolphins (Tursiops truncatus). American Association for the Advancement of Science (AAAS) Arctic Division Annual Meeting in Kodiak, Alaska in September 23-26, 2005.

Zachera B.A, W. Dobrzynski, U. Trafikowaska, and W. Sysmanski. 2001. Blood selenium and gluathione peroxidase level in women experiencing abortion. British Journal of Obstetrics and Gynecology, 108: 244-7.