Page 657 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

622 SECTION | VIII Rodenticides

VetBooks.ir ZINC PHOSPHIDE Following oral ingestion, zinc phosphide reacts in the
stomach and intestine with water and hydrochloric acid to
Introduction
liberate phosphine gas. Metabolism of zinc phosphide can
Zinc phosphide is an inorganic compound with the occur via oxidation of the phosphorus to various phospho-
appearance of gray crystalline powder and a decaying fish rus oxyacids or via reduction of the phosphorus to phos-
or garlic odor. It has the chemical formula Zn 3 P 2 , with a phine gas. Zinc phosphide excretes in the urine either as a
molecular weight of 258.1. Its structural formula is shown hypophosphite or as dissolved phosphine. The presence of
in Fig. 47.7. Zinc phosphide was first synthesized in strong reducing substance in the urine is a common fea-
ture of poisoning with zinc phosphide. Other metabolites
1740, and was first used as a rodenticide in 1911 12
include phosphoric acid and phosphate. Phosphine gas is
in Italy. It was not used in the United States until 1939. In
also exhaled from the lungs.
the 1970s, there was a renewed interest in zinc phosphide
for rodent control in agriculture. Currently, zinc phos-
phide is used worldwide for the control of a range of ani- Mechanism of Action
mals, including mice, rats, voles, ground squirrels, prairie
The overall toxicity of zinc phosphide is due to both zinc
dogs, moles, muskrats, rabbits, and gophers. Pelleted zinc
and phosphine gas, but it is primarily from phosphine. It
phosphide is sold extensively under brand names such as
needs to be pointed out that animals can have access to
Arrex, Blue-ox, Gopha-rid, Kil-rat, Mous-con, Phosvin,
phosphine gas from the feed/forage fumigated with phos-
Pollux, Ratol, Ridall, Rodenticide AG, Zinc-tox, and ZP.
It is also used on crop/noncrop areas, including lawns, phine or aluminum phosphide (Easterwood et al., 2010;
golf courses, and around wetlands. Nath et al., 2011). Phosphine can cause irritation of the
alimentary tract. It can enter the bloodstream and exert
toxicity through multiple mechanisms, including acetyl-
cholinesterase inactivation, mitochondrial disruption, and
oxidative stress (Nath et al., 2011). Phosphine adversely
affects the lungs, liver, kidneys, heart, and CNS. It causes
CNS depression, irritation of the lungs, damage to the
FIGURE 47.7 Structural formula of zinc phosphide.
blood vessels and erythrocyte membranes and, eventually,
Background cardiovascular collapse.
The history of the use of zinc phosphide as a rodenticide
Toxicity
dates back approximately a century. In the early years,
technical zinc phosphide and highly concentrated paste Zinc phosphide is an extremely toxic compound. The oral
were sold in shops and used by consumers to make their LD 50 in rats is 41 mg/kg body weight. In sheep, the LD 50
own baits for rodents. This led to use of the technical ranges from 60 to 70 mg/kg. It is also very toxic to cows,
material for suicide and to accidental deaths in small chil- sheep, goats, pigs, rabbits, and other species. Following
dren and pets. oral ingestion, both zinc phosphide and phosphine are
Zinc phosphide is recognized as the slowest acting of absorbed from the GI tract, although the majority of acute
the commonly used rodenticides. It can cause toxicity and effects are caused by phosphine. Zinc phosphide causes
death in rodents after one feeding if adequate dose is con- damage to the liver and kidney. Phosphine causes CNS
sumed. However, bait must be continuously available to depression, irritation of the lungs, and damage to the
be effective. In fact, zinc phosphide is recommended as liver, kidney, heart, and CNS. Death occurs as a result of
the rodenticide of choice because it is fairly specific for cardiac arrest or, more commonly, pulmonary edema. In
rodents and secondary poisoning is rare, except in dogs addition, there are reports of cardiac arrest accompanied
and cats. by kidney damage. Following a large dose, death usually
Rodenticide-grade zinc phosphide usually comes as a occurs within 1 h, whereas with smaller doses, death can
black powder containing 75% zinc phosphide and 25% occur between 4 and 72 h. Symptoms usually appear
antimony potassium tartrate, an emetic to cause vomiting 20 25 min after exposure. Animals are prostrated with
if the material is accidentally ingested by humans or deep, slow respiration, finally terminating in convulsions.
domestic animals. Following repeated exposure, cumulative effects occur in
the liver, kidney, and lungs.
Zinc phosphide is highly toxic to wild animals, birds,
Toxicokinetics
and to freshwater fish. The most sensitive bird species that
Zinc phosphide can be absorbed into the body by oral has been evaluated is geese (LD 50 of 7.5 mg/kg).
ingestion, inhalation, and through damaged skin. Pheasants, mourning doves, quails, mallard ducks, and