Pentachlorophenol (PCP), commonly known as penta, has been used as a fungicide, molluscicide, and insecticide and as a wood preservative. Product names include Dowicide EC-7®, Penchlorol®, Pentacon®, Penwar®, Priltox®, Sinituho®, and Weedone®. PCP is defined by the US EPA as a heavy-duty wood preservative. It is used primarily to treat wooden utility structures including poles, crossarms, and log anchors. The migration of PCP beyond the immediate vicinity of treated utility poles has been documented nationally. PCP is commonly observed and measured in soil immediately adjacent to utility poles. PCP is a stable, persistent compound, has a restricted use, and is no longer available to the general public. It is banned in many countries and its use severely restricted in others, but it may be purchased and used by certified applicators. It is now only permitted 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 weight. It is readily absorbed from all routes of exposure and is excreted unchanged or as a glucuronide conjugate. The half-life in blood is 15–78 hours. PCP is an uncoupler of oxidative phosphorylation and therefore produces fever in poisoned animals. It is an irritant and may produce eye and skin irritation, hair loss, throat or respiratory irritation, and abdominal pain. High-level exposure can lead to CNS effects, such as excitation or seizures. Chronic exposures cause decreases in body weight. Except for signs of irritation, gross lesions are not remarkable. The liver, kidneys, and CNS are the major target organs. 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 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.
Animals fed in troughs made of lumber treated with PCP may salivate and have irritated oral mucosa. Both penta and its major metabolite, tetrachlorohydroquinone, 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 weight.
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.
Penta can cause residues in animal tissues. Also, a significant amount of hexachlorobenzene is metabolized in animal tissues to penta. PCP 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. Oral animal studies suggest that exposure to PCP decreases the survival of offspring in rats. There is evidence that PCP produces maternal toxicity (depressed maternal body weight) but does not cause birth defects. Increases in liver tumors and two uncommon tumor types have been reported.
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.
Ending exposure, supportive care, and bathing dermally exposed animals
There is no known antidote for PCP poisoning. Treatment involves termination of exposure, bathing dermally exposed animals, cooling the animal, and administering fluids, electrolytes, and anticonvulsants. 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.