Pesticides have wreaked havoc on global amphibian populations, and now new evidence suggests that the problem may be even worse than thought. One of the most common herbicides—atrazine—appears to have an added deadly effect: It makes tiger salamanders more vulnerable to certain infections. Because atrazine also has deleterious effects on humans and other mammals, researchers fear this new effect on amphibians may be a harbinger of a larger problem for other species.
Atrazine is the second most popular agricultural pesticide in the United States, used primarily to control weeds on corn, sugar cane, and residential lawns. The compound mimics the hormone estrogen, and over the past 10 years, scientists have noticed that frogs and other amphibians exposed to the herbicide develop deformed genitalia and smaller voice boxes, making mating calls softer and reproduction impossible. Atrazine was introduced in 1958, and scientists started noticing upticks in fungal and viral diseases in exposed animals in the late 1990s. Studies confirmed that atrazine suppresses the immune system, but whether this led to increased infection rates remained a mystery.
To solve the conundrum, biologists Diane Denise Forson and Andrew Storfer of Washington State University in Pullman took a closer look at the tiger salamander, a small amphibian commonly found in marshes and ponds around the globe. Tiger salamanders tend to live in areas exposed to atrazine, and over the past few years biologists have noted a rise in their susceptibility to the Ambystoma tigrinum virus (ATV), which causes internal hemorrhaging and death. In the lab, the researchers exposed 384 tiger salamander larvae to levels of atrazine similar to those found in nature, and then introduced ATV for 3 days at the 12-week larval stage. (Atrazine is commonly sprayed in the spring, during this stage of development.)
Tiger salamanders exposed to atrazine were twice as likely to become infected with ATV than were those not exposed to the herbicide, the team reports in this month's issue of Ecological Applications. In addition, when combined with sodium nitrate, a type of fertilizer, atrazine lowered the levels of white blood cells that fight disease by nearly 20%. "Because amphibian skin is permeable, toxins can be absorbed directly into the bloodstream," says Storfer. "Their eggs are also permeable to environmental toxins, which makes them indicators of environmental contamination," adds Forson.
"This is one of the few, rare examples of a toxin causing an indirect, increased susceptibility to infection," says Andrew Blaustein, an ecologist at Oregon State University in Corvallis. However, the specific mechanism by which immunosuppression occurs still needs to be established, says Tyrone Hayes, a biologist at the University of California, Berkeley. This is especially important because atrazine has been associated with breast and prostate cancer in humans, he says.