Page 658 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Non-Anticoagulant Rodenticides Chapter | 47 623

VetBooks.ir horned larks are also very sensitive to this rodenticide. The many countries, but it is still widely available in develop-
ing countries as a rodenticide. The possibility also exists
fish species that have been found sensitive to zinc phos-
for Tl to be used as a chemical warfare agent (Thompson,
phide include bluegill sunfish, rainbow trout, and carp.
2015). In general, Tl poisoning cases are on the decline,
but diagnostic labs still receive suspected baits and tissues
Diagnosis
from poisoned animals, especially dogs, on a regular
Diagnosis of zinc phosphide in animals is based on detec- basis.
tion of zinc phosphide, phosphine, and zinc in body tis-
sues and fluids. At necropsy, stomach content smells like Background
acetylene. A zinc phosphide level of 50 ppm or higher in
stomach content is considered significant, and is indica- Thallium is a toxic heavy metal that was discovered by
tive of zinc phosphide poisoning. Sir William Crookes in 1961 by burning the dust from a
sulfuric acid industrial plant. It forms two kinds of com-
pounds: monovalent thallo- and trivalent thalli-
Treatment
compounds. The monovalent Tl resembles potassium, and
There is no specific antidote, and treatment is mainly trivalent Tl resembles aluminum. Tl tends to form
symptomatic. Vomiting should be induced as soon as pos- stable complexes with soft ligand donors, such as sulfur-
sible after ingestion, followed by gastric lavage or admin- containing compounds. Inorganic TI (I) compounds are
istration of activated charcoal. Sodium bicarbonate can be more stable than Tl (III) analogs in aqueous solution at
given orally to stop liberation of phosphine gas. Calcium neutral pH. In contrast, organothallium compounds are
gluconate and sodium lactate can be given intravenously stable only in the trivalent form. Tl is particularly toxic in
to combat acidosis. its Tl (I) compounds, such as sulfate (Tl 2 SO 4 ), acetate
(CH 3 COOTl), and carbonate (Tl 2 CO 3 ). The sulfide (Tl 2 S)
and iodide (Tl I) are both poorly soluble and, therefore,
Conclusion much less toxic. Today, Tl is recognized as one of the

Zinc phosphide is a slow-acting but highly toxic rodenti- most toxic heavy metals, with an LD 50 of 30 mg/kg in
cide. At acidic pH in stomach, zinc phosphide generates rats and 8 12 mg/kg in humans.
phosphine gas, which is responsible for the majority of
toxic effects. There is no specific antidote, and treatment Toxicokinetics
is symptomatic (emesis, gastric lavage or administration
of activated charcoal). The water-soluble Tl compounds are rapidly absorbed
following oral, inhalation, and dermal exposure. After
absorption, Tl compounds are widely distributed to the
THALLIUM body tissues, including brain, heart, kidney, skeletal mus-
cle, and testes. Both monovalent and divalent Tl appear to
Introduction
distribute in tissues in a similar manner. In blood, Tl is
Thallium (Tl) is a bluish-white heavy metal that occurs found slightly more within erythrocytes than in plasma.
naturally in the earth’s crust. The word thallium derives Following an acute exposure, the maximal concentration
from the Greek word thallos, which means a young twig of Tl is found in the kidneys, preferentially in the
or shoot. Tl enters the environment from natural and medulla. Studies conducted in rats show lower concentra-
anthropogenic sources. Natural sources of Tl are less bio- tions of Tl in renal tissue of young compared to adult.
available, and therefore, of less toxicological concern Tl 1 and K 1 are monovalent cations with similar ionic
than anthropogenic sources. The largest anthropogenic radii, but Tl 1 accumulates intracellularly more than K 1
sources of Tl are related to coal combustion and heavy because of its greater affinity for certain enzymes and
metal (primarily zinc and cadmium) smelting and refin- protein. Because of its large volume of distribution and
ing. Tl salts were introduced as pesticides in Germany in low free plasma concentration, renal excretion of Tl is
1920. The sulfate salt is most common, and has been slow, and its residue can be detected in the tissues for
widely used as a rodenticide and ant killer. In the past, TI months. Organic Tl compounds, such as thallous malo-
was also used for medicinal purpose against dysentery nate, show a higher elimination rate constant, but are sim-
with violent or persistent diarrhea, syphilis, gonorrhea, ilar in toxicity and distribution pattern compared to the
gout, mycosis of the scalp, and as an inhibitor of sweat inorganic Tl compounds, such as thallous sulfate
secretion in tuberculosis patients. Approximately 50 years (Aoyama, 1989). The elimination half-life is between 8
ago, Tl was frequently referred to as the poison of choice and 30 days. Tl is excreted in urine, bile, feces, saliva,
because Tl is as toxic as arsenic or lead. Tl is banned in tears, and milk. In mammals, Tl excretion via the GI tract