Restrictions placed on the use of rat poisons in 2011 may have eased the problem, but the jury is still out.
(CN) — Researchers found that 82% of eagles examined in the United States had detectible levels of rat poison in their system — and 11% died as a result.
Anticoagulant rodenticides are used for a range of agricultural and conservation or habitat restoration purposes. They’re sold under catchy brand names like d-Con, Hot Shot, Generation, Talon and Havoc, and are used to control non-native rat populations that could otherwise displace native wildlife and contaminate human food supplies.
The problem is these rats don’t live in a bubble — they’re a major food source for other, more beloved creatures, like raptors (and their prey) — and there comes the rub.
Researchers from the University of Georgia examined 303 eagles in all, including the livers of 116 bald eagles and 17 golden eagles, to determine the extent of the damage caused by these poisons. The team published their findings Wednesday in a new study in the journal PLOS.
Of the 133 eagles autopsied, they found 109 had been exposed to rat poison, including 96 bald eagles and 13 golden eagles. Of those exposed, 12 died as a result, including 11 bald eagles and 1 golden eagle.
“The most concerning finding was the common and widespread exposure of eagles to these compounds despite regulatory efforts to mitigate such exposure,” said Dr. Mark Ruder, an assistant professor at the College of Veterinary Medicine at the University of Georgia, in an email. “In particular, brodifacoum, a second-generation anticoagulant rodenticide. Of the 133 eagles we tested for anticoagulant rodenticides, 81% were exposed to one or more anticoagulant rodenticide compounds. We also confirmed anticoagulant rodenticide intoxication in 12 eagles (11 bald eagles and 1 golden eagle), meaning the exposure was implicated in the cause of death in those birds.”
Bald eagles have made a remarkable recovery since the 1970s, when their numbers dwindled to dangerous levels, while golden eagle populations have largely remained stable. In 1972, the federal government banned the use of a once-popular insecticide known as DDT, which at one time wreaked havoc on raptor populations. Unfortunately, rat poisons now threaten to reverse any progress.
First and second-generation rat poisons work by interfering with the activation of clotting factors in an animal’s liver, which can lead to life threatening bleeding after a minor injury. The second-generation variety in vogue today has a longer half-life than its predecessor, increasing an animal’s chances of intoxication after a single exposure. That improves its effectiveness against rodents, but also against any unwary predator that eats said rodent.
“Unfortunately, the results were not totally unexpected,” Ruder said in the email. “Many disease problems in wildlife are largely created through human activity, or are greatly exacerbated. Anticoagulant rodenticides are used by people to control rodent populations in residential, commercial, and agricultural settings. Over concerns for animal and human health, in 2011 the U.S. Environmental Protection Agency began restricting the sale and use of second-generation anticoagulant rodenticides, like brodifacoum. However, this study clearly shows eagles are still being exposed.”
It’s impossible to tell from the existing data whether the situation has improved since many of the restrictions from the EPA’s 2011 decision were implemented in 2015. Eagles are obviously still being poisoned in sobering numbers, however a separate study on barn owls found their exposure to these poisons has been declining since restrictions were passed.
It’s also thus far impossible to tell exactly how these eagles are being exposed. Most signs point to their indirect poisoning by eating affected rodents — a favorite prey animal for golden eagles but not so much for bald eagles — though more research is needed to be certain.
When asked if there was some common factor linking the 4% of eagle deaths attributed to rat poison, Ruder said the opportunistic nature of his team’s data collection doesn’t lend itself to drawing such conclusions.
This study relied on donated eagle carcasses, which means the animals they examined were found by volunteers in (relatively) accessible places, in certain areas of the country, and thus excluded a large number of affected eagles from more remote locations, which could alter the results if introduced. He said that future “targeted studies in specific populations may better reveal such risk factors.”
“This study highlights a potential threat to bald and golden eagles that is worthy of further study,” Ruder concluded, in an email. “We do not know the full extent of intoxication of eagles with these compounds, nor do we know if sublethal impacts exist. Importantly, this potential problem is one that we can mitigate through changes in human behavior, as well as regulatory and management actions.”