THE MERCK VETERINARY MANUAL
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Overview of Toxicology

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For a discussion of common poisonings in poultry, see Poisonings.

Veterinary toxicology involves the evaluation of toxicosis and deficiencies, identification and characterization of toxins and determination of their fate in the body, and treatment of toxicosis. Toxicology has been receiving even more attention in the general public with the widespread interest in crime scene investigator (CSI) television shows. The recent worldwide melamine contamination in pet and swine feed, pet jerky treats causing illness and death, and concerns with use of β-agonists in food animals indicates the relevancy of veterinary toxicology to current animal health and food safety. Veterinary toxicology can be challenging because of the low frequency of cases observed in a practice setting. When a toxicosis occurs, it often involves a large number of animals and may also involve litigation. A current reference book for veterinary toxicology is helpful to make sure the correct samples are obtained and submitted for diagnosis.

A toxic agent is referred to as a toxicant or poison. The term toxin refers to poisons produced by a biologic source (eg, venoms, plant toxins); the redundant term biotoxin is occasionally used. Toxicosis, poisoning, and intoxication are synonymous terms for the disease produced by a toxicant. Toxicity (sometimes incorrectly used instead of poisoning) refers to the amount of a toxicant necessary to produce a detrimental effect.

Acute toxicosis refers to effects during the first 24-hr period. Effects produced by prolonged exposure (≥3 mo) are referred to as chronic toxicosis. Terms such as subacute and subchronic are used to cover the large gap between acute and chronic.

All toxic effects are dose dependent. A dose may cause undetectable, therapeutic, toxic, or lethal effects. A dose is expressed as the amount of compound per unit of body weight, and toxicant concentration as part per million or part per billion. These quantitative expressions are also used for feedstuffs, water, and air as well as tissue levels.

LD50 is the dose that is lethal to 50% of a test sample. It is an estimator of lethality and the most common expression used to rate the potency of toxicants. Other terms used for prediction of illness or lethality include no observed effect level (NOEL), maximum nontoxic dose (MNTD), and maximum tolerated dose or minimum toxic dose (MTD).

Last full review/revision October 2013 by Steve M. Ensley, BS, DVM, MS, PhD

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