Dioxin intoxication from chronic exposure of horses to pentachlorophenol-contaminated wood shavings.

Investigations into the cause of health problems on a horse-breeding farm led to the discovery of high concentrations (630 to 9,810 mg/kg of bedding) of pentachlorophenol in wood shavings used as bedding for horses over a period of 2 to 4 years. Toxicologic signs in the horses were characteristic of toxic effects associated with exposure of polychlorinated dibenzo-p-dioxins and dibenzofurans. Tissue residue analysis confirmed presence of toxic polychlorinated dibenzo-p-dioxin and dibenzofuran isomers known to be in pentachlorophenol, substantiating the bioavailability of polychlorinated dibenzo-p-dioxins and dibenzofurans in the wood shavings. The findings provide evidence that residue concentrations in the range of 2 ng/g of toxic polychlorinated dibenzo-p-dioxin isomers in liver or fat correlate with toxicologic effects in horses.