Mercury immunotoxicity in the brown watersnake (Nerodia taxispilota): An in vitro study

Abstract

Mercury (Hg) is a heavy metal that enters the environment through natural and anthropogenic means. Once in the environment, Hg can biomagnify in food webs and is known to cause immunotoxic effects to wildlife. Compared with other vertebrates, knowledge of the reptilian immune system is lacking, especially in snakes. Further, even less is known about the impact of environmental contaminants on snake immunity. This gap in knowledge is largely due to an absence of established immune‐based assays or specific reagents for these species. In this study, brown watersnakes (Nerodia taxispilota; n = 23) were captured on the Savannah River (Augusta, Georgia, USA), weighed, measured, bled, and released. Peripheral blood leukocytes (24 h old) were enriched and evaluated with an established mammalian in vitro lymphocyte proliferation assay. Enriched leukocytes were then exposed to mercury chloride (HgCl2) at 3.75, 37.5, and 75 μM. Total mercury (THg) in whole blood was also quantified. Snake peripheral blood leukocyte enrichment yielded >90% lymphocytes with viabilities averaging >70%. Exposure to HgCl2 resulted in significant dose‐dependent suppression of proliferative responses relative to spontaneous proliferation at 37.5 and 75 μM (both p ≤ 0.01) but not 3.75 μM (p = 0.99). Mean ± 1 SE concentration of THg in whole blood was 0.127 ± 0.027 mg/kg (wet weight). Based on the in vitro findings with HgCl2, snakes in systems with heavy Hg pollution may be at risk of immunosuppression, but N. taxispilota at the site in this study appear to be at low risk.

Factors Predicting Apparent Ophidiomycosis in Wild Brown Watersnakes (Nerodia Taxispilota)

“…Contrary to our hypothesis, we did not observe any association between Hg exposure and likelihood of apparent ophidiomycosis in N. taxispilota from our study population. However, it is important to note that Hg concentrations measured in N. taxispilota (0.003-1.14 mg/kg, wet weight) were far below immunotoxicity thresholds documented in our previous in vitro study (approximately 10 mg/kg, wet weight; Haskins et al 2022). Further studies are warranted to evaluate how higher Hg exposures in the wild (e.g., Drewett et al 2013) may be associated with snake health and disease outcomes.…”

“…Exposure to environmental contaminants, such as Hg, can influence wildlife health by modulating the prevalence or severity of disease (Provencher et al 2016; Teitelbaum et al 2022). In vertebrates, Hg exposure is known to significantly alter immune function (Fallacara et al 2011; Lewis et al 2013; Haskins et al 2022), potentially resulting in dampened immune responses to pathogens. Contrary to our hypothesis, we did not observe any association between Hg exposure and likelihood of apparent ophidiomycosis in N. taxispilota from our study population.…”

“…Total Hg (THg) was measured in whole blood as reported previously (Haskins et al 2021a, c, 2022). In brief, THg was quantified on a Milestone DMA-80 Tri-Cell Hg analyzer (Milestone Inc., Shelton, Connecticut, USA).…”

“…In 2016, we initiated a project to evaluate the utility of using N. taxispilota to monitor legacy pollutants on the Savannah River downstream of a former chlor-alkali plant near Augusta, Georgia, and adjacent to the Savannah River Site (SRS; Aiken, South Carolina, US). Specifically, we aimed to determine whether the species could be used to monitor mercury (Hg) contamination and how Hg may impact their health (Haskins et al 2021a, b, c, 2022), including probability of detection of O. ophidiicola . Nerodia taxispilota are common, large-bodied snakes that occur at high densities and are conspicuous baskers, making them relatively easy to sample (Mills 2002).…”

Mercury-Modulated Immune Responses in Arctic Barnacle Goslings (Branta leucopsis) upon a Viral-Like Immune Challenge

“…For example, abnormally high levels of heterophils were found in American kestrels (Falco sparverius) exposed to Hg, while reduced CD4+ lymphocyte counts were reported in Hg-exposed workers . In vitro exposure to HgCl 2 inhibited the proliferation of immune cell lines from both chicken ( Gallus gallus domesticus ) and murine species, as well as primary lymphocytes from mice (Mus musculus), beluga whales, snakes (Nerodia taxispilota), and humans (Homo sapiens) . Different sensitivities of immune cell types to Hg exposure may explain the change in immune cell composition …”