Toxic chemicals weaken frogs' immune systems

Threatened populations of northern leopard frogs further imperilled by environmentally persistent flame retardants called PBDEs
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The following article was originally published by Environmental Health News.

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Young frogs exposed to flame retardants have weakened immune systems, which could leave them more susceptible to diseases that are ravaging amphibians worldwide.

A new laboratory experiment is the first to link flame retardants to immune-system problems in frogs and adds to evidence that pollutants may contribute to global declines of their populations.

Tadpoles of northern leopard frogs were exposed to polybrominated diphenyl ethers in their food from the time they could swim until they turned into frogs. Then scientists injected the young frogs with a foreign protein and found that they produced up to 92 percent fewer antibodies than nonexposed frogs.

One-third of world's amphibians extinct or nearly so

“Making antibodies to get rid of pathogens is vital to frogs’ ability to fend off disease,” said Tawnya Cary, a postdoctoral scholar at the Institute for Biology Education at the University of Wisconsin, Madison, and lead author of the study.

Nearly one-third of the world’s amphibians—more than 1,800 species of frogs, toads, salamanders and newts—are threatened with extinction or already extinct. Chytrid fungus is to blame for devastating many populations, although other threats include habitat destruction, ultraviolet radiation, and parasites.

One theory for the spread of the deadly fungus is that “pollutants alter immune function of the animals, and then they’re not able to fend off disease pathogens properly,” Cary said.

Metamorphizing tadpoles most vulnerable

The period when tadpoles are turning into frogs—called metamorphosis—is a “very vulnerable time”, said Louise Rollins-Smith, an associate professor of pathology, microbiology, and immunology at Vanderbilt University who did not participate in the study.

“Almost every organ reorganizes, including the immune system,” she said.

“They have these little immune systems during metamorphosis, and they’re already having to deal with diseases and viruses. Not producing antibodies would be just one more handicap during this critical time.”

Antibody levels reduced

The researchers found reduced antibodies in the frogs at levels of flame retardants commonly found in the environment. It’s likely that such immune suppression would make the frogs more vulnerable to disease, but Cary said the researchers did not have funding to subject them to infectious agents to test that theory.

“Our findings demonstrate that environmentally realistic concentrations of PBDEs are able to alter immune function in frogs; however, further research is needed to determine how these alterations impact disease susceptibility,” the authors wrote in their study published in the journal Environmental Science & Technology.

However, the highest levels of the chemicals did not correspond with the biggest decrease in antibodies.

Frogs exposed as tadpoles to the most PBDEs—634 nanograms per gram of food—produced 66-percent fewer antibodies than the nonexposed frogs. But those exposed to much lower concentrations—1.1 and 71.4 nanograms—showed even more effects: declines of 89 and 92 percent.

Flame retardants could affect hormones

According to other animal research, hormone-disrupting contaminants can have more or different effects at low levels than at high levels.

Cary said it’s plausible that flame retardants interfere with the tadpoles’ hormones, which could cause their immune system to develop abnormally. PBDEs bind to a receptor in frogs that is responsible for triggering the immune system, according to previous research.

The same chemical mixture that suppressed antibodies in the new study’s frogs also has weakened immune systems of mice, mink, and, possibly, people.

Frogs ingest PBDEs with food

Frogs are exposed to flame retardants through dirt, plants, and insects they feed upon.

PBDEs were used for decades in furniture-cushion foam, plastics, and electronics. The mixture that the researchers used was phased out in the United States 10 years ago, but the chemicals still enter the environment through long-lived products such as furniture, as well as from wastewater and sewage sludge, said Robert Hale, a professor of marine science at the Virginia Institute of Marine Science.

[Canada banned the use of the most toxic PBDE compound (penta-PBDE) in 2005, according to the Ministry of Aboriginal Affairs and Northern Development Canada.—GS]

Northern leopard frogs are found in the northern and western United States and southern Canada. The species has experienced some population declines and habitat loss—especially in the West—but was denied endangered species protection in 2011.

[The British Columbia population of northern leopard frogs is listed as endangered by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) and is on the provincial Red List.—GS]

Frog-disease increases still a mystery

It remains a mystery why frog diseases have worsened, but some experts suspect contaminants and climate change. A lab study released last year found that frogs exposed to the herbicide atrazine early in life were more likely to die when faced with the chytrid fungus. However, other studies have not found that pesticides increase susceptibility to the fungus, said Carlos Davidson, a biologist and professor of environmental studies at San Francisco State University.

While it’s possible that “climate change has changed the playing field by altering temperatures that now favor the pathogen,” research hasn’t proved that, he said.

Amphibians are particularly vulnerable to environmental factors such as contaminants because they have thin, porous skins that absorb water.

“This study is definitely another piece to the puzzle of us trying to understand exactly how these toxics affect the development of frogs,” Rollins-Smith said.

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