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

entachlorophenol (pcp) is a widely used wood preservative that causes a myriad of toxic effects in animals. 1 Aside from the acute toxicity of pCP associated with its ability to uncouple mitochondrial oxidative phosphorylation, some of the toxic effects associated with PCP exposure may be attributable to the chlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) isomers that are found as contaminants of commercial grades of pcp 2 -5 lmmunotoxicity associated with exposure of laboratory animals to technical grade PCp6-8 or its PC DO and PCDF contaminants 9 -11 is also well established.An ll-year-old Arabian mare (horse 1) was referred to the teaching hospital with a I-month history of appetite depression, weight loss, ventral body and limb edema, severe hair loss, and anemia.The edema had been unresponsive to topical administration of dimethyl sulfoxide, antihistamines, diuretics, and walking.Oral prednisone trea tment was effec tive in reducing the edema, but the condition worsened eac h time treatment intervals decreased or stopped .The referring veterinarian reported mild increases in bilirubin and blood urea nitrogen concentrations and increased alkaline phosphatase activity.Dermatophytes or Der- matophilus organisms were not found in fungal cultures of skin and impression smears.The horse was seronegative for equine infectious anemia and equine viral arteritis.Clinical findings at admission included alopecia and crusty, scaly dermatitis around the eyes, face , muzzle, axilla, inguinal region, and neck (Fig 1).The hair epilated easily.Cracks or fissures in the skin exuded a clear serum-like fluid.The horse had grade 2 pitting edema of the ventral portion of the