Issue Date: January 10, 2011
In the legend of the Pied Piper of Hamelin, the piper rids a 13th-century German town of rats by using a magic pipe to lure them to drown in a river. Then, as now, rats were seen as a scourge. They and other rodents are a public health threat, capable of transmitting bacterial, viral, or parasitic diseases through urine, fecal matter, or saliva. They can also be an ecological menace—when introduced to islands, they can drive native plants and wildlife to extinction.
But because magic, rat-luring flutes are in short supply, rodenticides are the modern tool of choice to control pest rodents. Rodenticide compounds, however, can find their way into other animals, including species that government agencies are trying to save. Rodents also reproduce quickly enough that they can develop resistance to the poisons through enzymatic mutations. At a symposium on rodenticides at the 6th International Chemical Congress of Pacific Basin Societies, or Pacifichem, held in Honolulu last month, researchers discussed efforts to track and evaluate the effects of the compounds on nontarget species.
Rodenticides used today are anticoagulant compounds that inhibit an enzyme called vitamin K 2,3-epoxide reductase (VKOR). Vitamin K promotes blood clotting by working with another enzyme, γ-glutamyl carboxylase, to add a carboxyl group to glutamic acid residues on several clotting-factor proteins. During the process, vitamin K is oxidized, and VKOR reduces the compound back to its active form. Anticoagulant VKOR inhibitors such as warfarin, diphacinone, and bromadiolone therefore block vitamin K regeneration and hinder processing of clotting factors. Animals dosed with such rodenticides die either through external bleeding after injury or through internal hemorrhaging after high doses of the compounds damage blood vessels.
One of the challenges of rodent control, said William Pitt, a research wildlife biologist with the U.S. Department of Agriculture’s Wildlife Services National Wildlife Research Center, is simply tracking whether and how much of a rodenticide is being consumed by rodents or by other animals.
Pitt, based at a field station in Hilo, Hawaii, is experimenting with marker compounds to try to figure out how much bait to put out to control invasive mice on small islands or islets with vulnerable bird colonies or endangered plants. Right now, he is working with rodenticide-free bait laced with the fluorescent dyes rhodamine B, tetracycline, or pyranine. After rodents or other creatures consume the bait, researchers capture the animals and check for the presence of the dye by looking for fluorescence under ultraviolet light to track how much rodenticide a population is ingesting.
In an ideal world, the dyes would show up quickly in an animal and last a long time in its body, characteristics known as initiation and persistence. The target species would also not be able to detect the dyes. “If you put something out and the animals won’t eat it, that affects your whole study,” Pitt said.
So far, Pitt and colleagues have found that rhodamine B and tetracycline score well for initiation and persistence, but the wild mice they were testing refused to eat it. The mice will eat pyranine, but it doesn’t persist. The researchers are now varying the amounts of the dyes in the bait to see whether they can find concentrations that work better.
Pitt is also studying wild pigs’ exposure to the rodenticide, which can happen when the animals get into bait stations or simply when they eat bait that is broadcast over an area. Although the pigs are also a species that has been introduced to the Hawaiian and other islands in the Pacific Ocean, hunting and eating the animals has become an integral part of cultural traditions.
Pitt and colleagues fed pigs as much diphacinone-laced bait as they would eat over two days and then examined the residues in the pigs’ tissues to see what risks the compound poses to dogs or humans who might eat the meat, liver, or entrails. “We found that the risk was fairly low,” Pitt said. A pregnant woman, for example, would have to eat more than 5 oz of liver from the highest residue pigs for 21 days straight before she would accumulate enough of the compound in her body to start to affect blood clotting. The scenario “is possible, but not probable,” Pitt said.
In contrast, birds that prey on rodents might be more at risk than previously realized, said Barnett A. Rattner, a wildlife toxicologist at the U.S. Geological Service’s Patuxent Wildlife Research Center, in Beltsville, Md. Rattner has been studying exposure of the American kestrel, a small falcon, and the eastern screech owl to diphacinone. Until recently, the compound had been tested only in bobwhite quail and mallard ducks, which are exposed through water or soil contamination or possibly by eating bait directly.
Rattner and colleagues dosed the kestrels and owls with small amounts of diphacinone, both a single time and repeatedly over several days. Such conditions are more likely to mimic actual wildlife exposure than a single high dose, Rattner said, noting that compounds similar to diphacinone are used therapeutically at small daily doses in humans to prevent blood clots, such as those that could cause a stroke.
Both kestrels and owls are significantly more sensitive to diphacinone than quail or ducks, the researchers found. The effects show up in increased blood-clotting time and smaller or fewer red blood cells, which can indicate blood loss. The consequences for the birds range from poor health to death, Rattner said. Even a seemingly small effect could alter a wild animal’s ability to forage for food, Rattner said. Furthermore, he added, animals can be coexposed to lead or other types of environmental contaminants that also affect blood formation, putting further stress on the animals’ health.
Philippe Berny, a professor of pharmacy and toxicology at the French veterinary school VetAgro Sup, is tracing the exposure routes from rodenticide bait to both target and nontarget species. In French villages, for example, rodenticides are supposed to be used only in houses, but the compounds are turning up in predators and scavengers such as weasels, foxes, owls, and seagulls.
Berny and coworkers are trapping rodents and looking at fecal samples from predators to try to identify contamination pathways. There are “probably interactions between different rodent populations—field rodents coming into houses or a house mouse goes outside and brings some bait with it,” Berny said. “But we don’t really know what’s going on.” He has developed a liquid chromatography/mass spectrometry method that, in one shot, looks for more than a dozen different anticoagulants, including both rodenticides and natural anticoagulants found in some plants. The goal is to trace residues in animals to identify exposure routes and patterns.
Berny is also trying to track rodent resistance to anticoagulants through mutations in the VKOR gene. In France, 25 to 50% of rodents are resistant to anticoagulants, Berny said. One mutation, a change in the genetic code that swaps a tyrosine for a phenylalanine in the enzyme backbone, appears to have no biological cost to rats. Rats with the mutation do not appear to be generally deficient in vitamin K and have normal reproductive cycles, and so the mutation persists and leads to a resistant population.
To detect resistant strains, Berny and colleagues use methods based on the polymerase chain reaction to analyze fecal samples, which are much easier to collect than the rodents themselves, Berny said. In the lab, the researchers reproduce the mutations in cell cultures, use enzyme assays to determine how effective the mutations are, and try to work out the best rodenticide to use against specific resistant populations—both to effectively manage the rodents and to limit exposure of other species to the compounds.
Overall, the talks at Pacifichem indicate the need for further research into rodenticides and their effects, said Katie Swift, a predator control specialist for the U.S. Fish & Wildlife Service, who coorganized the rodenticide symposium with John Johnston, a scientific liaison with USDA’s Food Safety & Inspection Service Office of Public Health Science.
“We know that sublethal residues are showing up in nontarget wildlife, but we don’t know whether they’re affecting the animals’ immune systems or reproductive success or making them more vulnerable to other causes of mortality,” Swift said. She pointed to unexpected mortality rates of nontarget species as a point of concern in island rodent eradication efforts. Wherever rodenticides are being used, Swift said, “we need to do more to assess the risk.”
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