July 25, 2014
By Marie Zhuikov
Researchers already know that dioxin (a byproduct of burned plastics and industrial processes) is toxic and can contribute to cancer and heart problems in humans, but now they know it can impact a fish’s nose tissue. Why should we care? Because fish use their noses to navigate through water and to find streams for spawning.
Wisconsin Sea Grant-funded researcher, Michael Carvan, Shaw associate scientist at the University of Wisconsin-Milwaukee School of Freshwater Sciences, has been studying the impacts of dioxin on rainbow trout (Oncorhynchus mykiss) and zebrafish (Danio rerio). Although zebrafish are not a Great Lakes species like the trout, researchers use them because they are a hardy lab fish about which much is known.
Carvan and his research team fed both young rainbow trout and zebrafish food containing different levels of TCDD (tetrachlorodibenzo-p-dioxin) over a six-week period. They chose to work on juvenile fish because few studies for dioxin exposure exist on this critical life-stage. Most concentrate on either embryos or adult fish. Carvan said their technique “more closely mimics actual environmental exposure to dioxin.”
They found that 28 days of exposure to high doses of TCDD (100 parts per billion) caused lesions in the zebrafishes’ livers, kidneys, intestines and ovaries. Another surprising finding besides the impact on the nose tissue was that dioxin acted as an endocrine disrupter. “It had a dramatic effect on the development of the female zebrafish reproductive tract,” said Carvan. “At higher dosage levels, we couldn’t identify any female zebrafish. They all looked male.” To identify which fish were male and female, the team had to resort to analyzing a gender-specific biomarker.
Carvan said that rainbow trout are more sensitive to dioxin than are zebrafish. Because rainbow trout mature more slowly, the researchers were not able to determine TCDD’s impact on that species’ reproductive tract, but Carvan suspects that in areas where trout are exposed to significant amounts of dioxin or dioxin-like chemicals, their reproductive tracts would be impacted.
And as for the trouts’ noses, testing whether their sense of smell was actually impaired by dioxin exposure was not part of Carvan’s research, but he’d like to follow up to see if it leads to behavioral problems and perhaps impeded spawning success.
Carvan said the amounts of dioxin they exposed the fish to were, “high for the average fish in Lake Michigan, but they’re not necessarily that high for a fish found in a polluted environment.”
Carvan hopes his research is useful for state and federal regulators who are looking at the impacts of dioxin on wildlife populations. “They’ll look at the pathologies and will start analyzing fish from their particular geographic area and narrow down the endpoints. If I were looking at the effects of low levels of dioxin, I would absolutely investigate the nasal epithelium and gonad development,” Carvan said.
For more information, see the article and comments for, “Histopathic Alterations Associated with Global Gene Expression Due to Chronic Dietary TCDD Exposure in Juvenile Zebrafish,” which were published earlier this month in the open-access journal, PLOS ONE; and “Gene Expression and Pathologic Alterations in Juvenile Rainbow Trout Due to Chronic Dietary TCDD Exposure,” which was published in 2013 in Aquatic Toxicology.