Page 522 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

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VetBooks.ir Chapter 36

Zinc

Tam Garland

INTRODUCTION Once zinc has been absorbed it is bound to plasma
albumin and to macroglobulins, and transported to the
Zinc (Zn) is a transitional metal in group XII on the peri-
liver. Zinc is extracted by the liver and returned to the
odic chart and is the fourth most commonly used metal
bloodstream for distribution to the liver, pancreas, kidney,
today. It is a moderately reactive metal with a common
and spleen, all of which rapidly accumulate the zinc.
valence state of 12. It is also an essential element in
These tissues, as well as muscle and prostate, are induced
mammals and birds and is a component of approximately
to synthesize metallothionein in the cells.
200 metalloenzymes. Although zinc has an essential role
Under normal dietary conditions, excess zinc is
in nutrition and consequences of nutritional deficiency
excreted through the feces. Nevertheless, excretion of
(Chasapis et al., 2012; Oteiza, 2012; Marchan et al.,
zinc, especially in toxic situations, is limited. Excretion
2012; Marger et al., 2014; Mayward and Rink, 2015), this
occurs through the bile and feces but may also occur
chapter will focus on its toxicity.
through urine (Abdel-Mageed and Oehme, 1990) and
saliva.
BACKGROUND
MECHANISM OF ACTION
As the fourth most commonly used metal it is expected to
be found in a variety of places with a multitude of uses. The mechanism of action producing clinical signs is
Zinc is an economical metal to use and is relatively non- not well defined or understood. The characteristic
toxic. Table 36.1 details some of those uses. Since zinc clinical signs are more easily recognized. The most
has nutritional qualities, it is not uncommon to find it recognized abnormality observed is severe intravascu-
added as a supplement to feed. Problems with toxicity lar hemolysis and gastroenteritis. This may be a
generally occur when the levels are 1000 parts per million result of excess zinc interfering with copper and iron
(ppm) or greater. storage and utilization, resulting in a suppression of
hematopoiesis.
Diets high in zinc interfere with hepatic copper storage
PHARMACOKINETICS/TOXICOKINETICS and may compete with calcium for intestinal absorption.
Ingested zinc is primarily absorbed from the duodenum
and the intestine by a carrier-mediated mechanism. TOXICITY
Approximately 25% 50% of ingested zinc is absorbed.
However, absorption of zinc in influenced by many fac- The different forms of zinc have different toxicities. The
tors, including whether or not food is in the stomach. zinc salts have a median lethal dose (LD 50 ) of approxi-
Plant phytates can bind zinc and in an alkaline pH envi- mately 100 mg/kg body weight. Zinc oxides are less
ronment can form insoluble complexes. Similarly, zinc toxic. Zinc oxides are frequently found in ointments, such
absorption is decreased in the presence of phosphates and as for preventing sunburn or treating diaper rash. Dogs
calcium in the diet. However, the presence of some pep- often ingest it when it is applied topically to them or to
tides, amino acids, and ethylenediamine tetra acetic acid someone they can lick it off. Dogs also ingest the oint-
disodium (EDTA) may cause an increase in absorption. ment by chewing on the tube container. It has been esti-
Generally, the stomach acid provides for rapid release of mated that the toxic dose is approximately 108 g of zinc
zinc from ingested metallic objects. for dogs (Breitschwerdt et al., 1986).

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