THE MERCK VETERINARY MANUAL
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Overview of Pentachlorophenol Poisoning

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Pentachlorophenol (PCP), commonly known as penta, has been used as a fungicide, molluscicide, insecticide, and wood preservative. Its use is now permitted only for industrial purposes; agricultural and domestic uses are prohibited, because it is classified as a highly hazardous pesticide.

The oral LD50 of penta in rats is 150–210 mg/kg body wt. Common signs of poisoning include nervousness, rapid pulse and respiratory rate, weakness, fever, muscle tremors, convulsions, loss in righting reflexes, and asphyxial spasms followed by death. Corneal injury may result from splashes or vapor overexposure. Chronic poisoning results in emaciation, fatty liver, nephrosis, and weight loss.

The persistence of penta in soil and water and apparent widespread use has resulted in significant exposure to animals. Young swine have died after dermal exposure to freshly penta-treated wood used in farrowing crates or farrowing houses. In vivo studies in swine demonstrated that exposure to penta-contaminated soil can result in significant dermal absorption of the pesticide. Penta can be absorbed through intact skin and lungs and is an intense irritant to the skin and mucous membranes. Penta absorption in skin was greater in water or water-based mixtures than in 100% ethanol. Because animals typically have access to water at all times, this hydrophilic characteristic of penta suggests enhanced dermal absorption.

When absorbed, penta increases metabolism by uncoupling cellular phosphorylation. Animals fed in troughs made of lumber treated with PCP may salivate and have irritated oral mucosa. Both penta and its major metabolite, tetrachlorohydroquinone (TCHQ), can induce epidermal hyperplasia in mice.

Poultry have been exposed to sawdust and shavings from penta-treated wood. Associated adverse effects include reduced growth rates, kidney hypertrophy, and decreased humoral immune response. Penta exposure can also result in an off-taste to eggs and meat as a result of degradation of chlorophenols to chloroanisols. Vaporization or leaching of penta in pens, enclosures, homes, and barns has caused illness and death.

Cattle and pigs exposed to wood treated with commercial grade penta had increased mortality, possibly decreased fertility in boars, and reduced productivity (milk, meat, etc). The lethal dose in cattle and sheep is ~120–140 mg/kg body wt.

Commercial lots of technical-grade penta contain small but biologically significant amounts of highly toxic impurities such as chlorinated dioxins and dibenzofurans, tetrachlorophenols, and hydroxychlorodiphenyl ethers; these compounds can exert their own effects such as early fetotoxicity. Commercial-grade penta causes hepatic porphyria, increased microsomal monooxygenase activity, and increased liver weight. Pure penta was not teratogenic in rats.

Penta can cause residues in animal tissues. Also, a significant amount of hexachlorobenzene is metabolized in animal tissues to penta. Pentachlorophenol is considered to be a carcinogen and a tumor promoter, although studies have shown that the pure material does not increase the incidence of tumors in rats and mice. The technical-grade material has also been shown to be immunotoxic in laboratory studies. Penta must be handled very carefully and kept away from animal contact.

Whole blood analysis for penta may aid in the diagnosis of poisoning; diagnosis is usually made on the basis of the signs and the proximity of treated lumber in the animal's environment.

There is no known antidote. Termination of exposure, bathing dermally exposed animals, oral administration of activated charcoal, and supportive therapy may be indicated. Bathing should be done gently with cold water and detergent so as not to cause vasodilation and increased absorption. Antipyretics, eg, aspirin and acetaminophen, should not be used. Treatment involves cooling the animal and administering fluids, electrolytes, and anticonvulsants.

Last full review/revision May 2014 by P. K. Gupta, PhD, Post Doc (USA), PGDCA, MSc VM & AH BVSc, FNA VSc, FASc, AW, FST, FAEB, FACVT (USA), Gold Medalist

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