Herbicidal activity generally consists of interference with plant-specific biochemical reactions. Thus, mammalian toxicity is generally low and not predictable from the mechanism of herbicidal action. In contrast, rodenticide target selectivity is not based on differences in biochemistry between humans and rodents but rather on differences in physiology or behavior, especially feeding behavior. For example, an emetic may be included in a rodenticide formulation to promote vomiting in humans who accidentally consume the product; rodents do not have a vomit reflex.
The chlorophenoxy herbicides, 2,4-dichlorophenoxyacetic acid (2,4-D) and 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), were used in defoliating operations in Vietnam, and the adverse health effects of the contaminant 2,3,7,8-tetrachlorodibenzodioxin (dioxin) continue to be controversial.
The bipyridyl herbicides paraquat and diquat are broad-spectrum herbicides. As little as 10 mL of paraquat concentrate is lethal in adults. Paraquat damages the lungs and may result in the appearance of a respiratory distress syndrome appearing 1 or 2 weeks after poisoning. In contrast, diquat causes minimal lung damage because it does not selectively accumulate in the lung. Acute renal failure, liver toxicity, and gastrointestinal damage are sequelae to diquat poisoning.
Warfarin, a coumarin anticoagulant, is incorporated into cornmeal for use as a rat poison. Repeated exposure results in sufficient inhibition of prothrombin synthesis to cause fatal internal hemorrhage.
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