Compromised immune competence in free-living tree swallows exposed to mercury

Abstract

Mercury is a pervasive environmental contaminant and a well-documented immunosuppressor. However, little is known about the effects of mercury contamination on health of free-living vertebrate populations. The South River in Virginia, USA was heavily contaminated with industrial mercury from 1929 to 1950, and recent studies have documented high levels of circulating mercury in riparian songbirds breeding below the site of contamination. Here we used two standardized immune assays, mitogen-induced swelling in response to phytohaemagglutinin (PHA) and antibody response to sheep red blood cells (SRBCs), to test for effects of mercury toxicity on the immune system of female tree swallows (Tachycineta bicolor) which feed on terrestrial and aquatic insects along the contaminated waterway. We found that females breeding at mercury-contaminated sites mounted significantly weaker PHA-induced swelling responses than those at reference sites in both years of study. However, among females on the contaminated sites, individual bloodstream mercury concentration did not predict the extent of mitogen-induced swelling. We did not detect any differences between reference and contaminated females in the strength of antibody responses to SRBCs, but sample sizes for this assay were significantly smaller. Overall, our results suggest that mercury toxicity can exert sub-lethal immunosuppression in free-living, insectivorous songbirds. The potential fitness consequences of the detected differences in immunocompetence caused by mercury toxicity warrant further study.

Like mother, like offspring: maternal and offspring wound healing correlate in snakes

“…Despite the fact that individuals collected from the contaminated site had very high tissue concentrations of Hg (female: 5.66±0.48mgkg ; neonate: 0.20±0.11mgkg 1 drymass), we detected no effects of Hg on healing ability of female N. sipedon or their offspring. This is contrary to previous studies that have shown that Hg affects vertebrate immune function (Spalding et al, 2000;Finkelstein et al, 2007;Hawley et al, 2009). However, two recent studies found few adverse sublethal effects of maternally transferred Hg in neonatal N. sipedon (Chin et al, 2013a;Chin et al, 2013b), suggesting that our study species may be more tolerant of Hg than many other species.…”

“…For example, many immunosuppressive contaminants can be maternally transferred from a female to her offspring during sensitive stages of development (Eisenreich et al, 2009;Bergeron et al, 2010). Exposure to contaminants could provide a source of variation in female and offspring immunocompetence, resulting in adverse effects such as reduced antibody responses and impaired T-cell function (Hawley et al, 2009;Pölkki et al, 2012). Because both parental and environmental sources can have multi-generational effects on phenotype, it is important to understand the contribution that each of these factors has on immunity.…”

“…We sought to understand the influence of mercury (Hg), a common pollutant thought to have immunosuppressive properties (Spalding et al, 2000;Finkelstein et al, 2007;Hawley et al, 2009), on maternal and offspring healing rates. We collected gravid female watersnakes along reference river sites located in central Virginia, USA, and at the South River, a site historically contaminated with Hg.…”

Relationships among plumage coloration, blood selenium concentrations, and immune responses of adult and nestling tree swallows

“…We also quantified the response of nestlings to PHA (Sigma-Aldrich) by injecting the patagium (wing web) of nestlings with 0.15 mg of PHA dissolved in 30 μl phosphate-buffered saline (after Smits et al, 1999;Hawley et al, 2009). The injection of PHA leads to localized swelling due to the influx and proliferation of leukocytes and T-cells at the injection site (Martin et al, 2006) and a build-up of free radicals that are produced during phagocytosis by leukocytes (Peretz, 1989).…”

Mercury exposure affects the reproductive success of a free-living terrestrial songbird, the Carolina Wren (Thryothorus ludovicianus)

“…Although less is known about the effects of mercury in avian invertivores, dosing studies indicate that this foraging guild is at greater risk of exposure than avian piscivores (Heinz et al 2009). Reduced reproductive success and suppressed immune function has been related to mercury exposure in the Tree Swallow (Tachycineta bicolor), an invertivore songbird (Brasso and Cristol 2008, Hawley et al 2009, Hallinger and Cristol 2011. Additionally, one of the more insightful field-based studies on an avian invertivore was conducted on the Carolina Wren (Thryothorus ludovicianus) at 2 contaminated river floodplains in Virginia, where adult reproductive impairment was quantified with known mercury body burdens (Jackson et al 2011a).…”