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Zinc phosphide (Zn3P2) is a heavy, finely ground, crystalline gray-black powder that is practically insoluble in water and alcohol. It was first synthesized in 1740 and was first used as a rodenticide in 1911-12 by the Italians. It was not used in the U.S. as a rodenticide until 1939 when it was used in place of strychnine, which was in short supply because of rodent control in Europe during World War II. Zinc phosphide was popular as a rodenticide because the rats tended to die in open areas, which was psychologically rewarding to animal control personnel. The popularity of zinc phosphide decreased during the late 1940's and early 1950's when sodium monofluoroacetate (1080) and the anti-coagulant rodenticides first appeared. Due to the recognition of problems associated with 1080 and strychnine usage, interest in zinc phosphide again increased. Currently, zinc phosphide is recognized as the slowest acting of the commonly used acute rodenticides.
Zinc phosphide is often recommended as the rodenticide of choice because it is fairly specific for rodents and there is no true secondary poisoning, except possibly in dogs and cats. Most animals that feed on rodents are unaffected because the zinc phosphide does not accumulate in the rodent's muscles or other tissues. Experimentally, several predators and scavengers have been exposed on a secondary nature, but only dogs and cats have been affected.
Nationwide, there have been poisonings of all species of domestic livestock, dogs and cats but these are usually accidental exposures and are few in number. All species of animals are subject to zinc phosphide poisoning, but avian species, specifically gallinaceous birds, are the most seriously affected. In Michigan, wild turkeys, ring-necked pheasants, black and gray squirrels, Canada geese, and possibly white-tailed deer have died from zinc phosphide poisoning. There have not been any reported cases of secondary poisoning in the state to date.
Through the years across the U.S., various types of baits have been treated with zinc phosphide. These baits include fruits (apples), vegetables (sweet potatoes), meat (hamburger, damp sausage rusks, canned dog or cat food), seeds, grains (oats, corn, wheat), cereal, and bread (bread mash). Concentrations of zinc phosphide of 0.75% to 2.0% have been used in the past.
In Michigan, technical grade (94% purity) zinc phosphide (2.0% concentration) is used for rodent control, usually using laced grains for bait. Zinc phosphide laced grains (corn, wheat, and oats) can be purchased from Pocatello Supply Depot in Pocatello, Idaho. The grains can be used for broadcast distribution. Another distribution method is through the use of T-feeders. T-feeders are pieces of PVC pipe glued together in the form of an inverted T. The feeders are attached to stakes and placed close to the ground. These feeders are filled with treated grains and are used throughout the pine plantations in the northern portion of the lower peninsula of the state. Zinc phosphide-laced baits have a strong, pungent phosphorus-like (garlic-like) odor that attracts rodents but makes the bait unattractive to other animals. Avian species, specifically wild turkeys, are not bothered by the odor and will feed on the grain.
Poisoning occurs by the liberation and rapid absorption of phosphine gas (PH3) into the bloodstream when the zinc phosphide comes into contact with the dilute acids in the stomach. This results in damage to the blood vessels and erythrocyte membranes and eventual cardiovascular collapse and irritation of the alimentary tract. Toxicosis usually is evident in 15 minutes to 4 hours following ingestion of a toxic dose.
There are no definitive clinical signs for zinc phosphide toxicity. Clinical signs include anorexia (loss of appetite), nausea, vomiting (often containing blood), abdominal pain, colic, diarrhea, prostration, lethargy, ataxia (loss of muscular coordination), chest tightness, dyspnea (frequent rapid breathing), salivation, excitement, convulsions, paralysis, rigor, and coma. In fatal cases there is liver, kidney, heart, and brain damage. Death is usually due to anoxia (decreased amount of oxygen in organs and tissues).
Gross lesions observed at necropsy are venous congestion, capillary breakdown, pulmonary congestion, interlobular lung edema, liver and kidney congestion, and gastroenteritis. When the stomach (true stomach, crop, or gizzard) is opened, an odor of carbide (acetylene) may be apparent. Yellow mottling can be seen in the liver in animals that live long enough for liver damage to occur.
Microscopic examination reveals congestion of the liver and kidney. Renal tubular necrosis can be seen in many instances and cloudy swelling and fatty degeneration can be seen in the liver.
In order to diagnose zinc phosphide poisoning in an animal, phosphine gas must be detected. There appear to be two fractions in zinc phosphide, one which degrades rapidly and one which degrades slowly in the alimentary tract. It is the slowly degrading portion that must be detected to confirm zinc phosphide poisoning. If a suspected case of zinc phosphide poisoning occurs, the unopened carcass should be sent to the laboratory as quickly as possible. If the carcass has been scavenged and the stomach contents are exposed, they should be removed immediately and placed in an airtight container and submitted to the laboratory.
At necropsy, the stomach contents should be examined and checked for the presence of an acetylene odor. The collected stomach contents should be submitted for zinc phosphide analysis at a 50-ppm detection level. Any values at or above the 50-ppm level in the stomach contents are significant and indicate a zinc phosphide poisoning. Significant values are not available for other tissues at this time.
In the event of a zinc phosphide poisoning, once clinical signs are observed, the prognosis is guarded at best. There are no specific antidotes for zinc phosphide, but several treatments can be attempted to offset the effects of the chemical. Sodium bicarbonate can be given orally to neutralize the stomach acidity, calcium gluconate and sodium lactate can be given intravenously to combat systemic acidosis, the stomach and intestinal tract can be evacuated, oxygen administered, and cardiac and circulatory stimulants given.
Zinc phosphide poisonings can be controlled if the applicators follow existing procedures for the use of the material. T-feeders appear to be the safest technique available to minimize the impact on wildlife. If the feeders are constructed properly, the bait will be available only to rodents. Zinc phosphide baits may remain toxic for several months until degradation by weathering or decomposition of the carrier, or removal of the grain by insects occurs. Loss of toxicity in the field proceeds at a rate of 6% per day due primarily to weathering. Dry baits must be considered to be toxic indefinitely.
The significance of zinc phosphide poisonings in Michigan is that accidental deaths of non-target species have occurred. Wild turkeys have been the most seriously affected. Human exposures are rare, being limited to the handling of bait material. If proper care is taken, the rodenticide can be safely used and its impact on non-target species minimized.
