Owls, hawks and other birds of prey who eat poisoned rats can themselves be poisoned and die. This finding, reported by John Elliott, an ecotoxicologist at Environment Canada, is leading both the Canadian and U.S. governments to restrict the use of rat poisons based on blood thinners and to urge the public to be a lot more careful when using these toxic substances.
Pesticides such as DDT have long been known to build up in the systems of animals that prey on rats and other such creatures. But similar, lethal effects can also occur from anticoagulant rodenticides (ARs). These function like the human blood-thinning drug warfarin, which is itself used as a rodent poison; they interfere with the blood’s ability to clot.
According to Nature News, environmental toxicologists consider warfarin a “first-generation AR,” which is less toxic than second-generation ones. But from studying 130 dead birds of prey around Vancouver, Elliott says that “virtually” all the owls and a majority of the hawks were found to have residues of at least one second-generation AR in their livers. This discovery led Elliott to state that
From a regulatory point of view [second-generation ARs] are ‘PBT.’ Persistent, bioaccumulative and toxic.
Especially alarming is Elliott’s noting that rat bait is also consumed by insects that the birds eat and, possibly, by the birds themselves. When he put sparrows in a cage with rat bait pellets, the birds went “straight for the bait.”
Fortunately, governments are seeking to address the poisoning of animals other than the intended targets. In Canada, authorities plan to reduce the outdoor use of ARs and to require that, when used outdoors, they be placed in tamper-resistant bait stations or in locations that owls and other non-target wildlife cannot access.
In the U.S., the Environmental Protection Agency is actually thinking of banning all second-generation ARs from the consumer market. The caveat is, such a policy may have limited benefits. Most of the rodenticides of concern are used by professional pest control operators who tend to leave the rat poison outside and for extended periods of time. In place of ARs, a “more portent” neurotoxin, bromethalin, is coming into use but there are significant dangers with it for children and pets as this drug has no antidote whereas ARs can be treated using vitamin K.
It goes without saying that leaving toxic substances outdoors is likely to have detrimental effects on wildlife. Far better to have pest-control operators use ARs far more sparingly and even to ban permanent bait stations, says Anne Fairbrother, the director of ecosciences at a science and engineering consultancy, Exponent. As long as there are powerful poisons sitting outside, it is inevitable that wild animals may encounter and consume them, with disastrous consequences.
Even more, the public needs to know that ARs can build up in other animals even if they have not actually eaten the poisons themselves. Elliot’s findings about the owls and hawks in Vancouver is a powerful reminder, and even a warning, that the chemicals — the toxins — we use can be dangerous whether or not an animal actually encounters them or not. Once such potent substances are in the environment, and into the food chain, there is no telling how far their effects might reach.
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