Veterinary toxicology

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  • 1.Copyright 2001 by ButterworthHeinemann A member of the Reed Elsevier group All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of the publisher. Every effort has been made to ensure that the drug dosage schedules within this text are accurate and conform to standards accepted at time of publication. However, as treatment recommendations vary in the light of continuing research and clinical experience, the reader is advised to verify drug dosage schedules herein with information found on product information sheets. This is especially true in cases of new or infrequently used drugs. Recognizing the importance of preserving what has been written, ButterworthHeinemann prints its books on acid-free paper whenever possible. Library of Congress Cataloging-in-Publication DataISBN 0-7506-7240-4British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. The publisher offers special discounts on bulk orders of this book. For information, please contact: Manager of Special Sales ButterworthHeinemann 225 Wildwood Avenue Woburn, MA 01801-2041 Tel: 781-904-2500 Fax: 781-904-2620 For information on all ButterworthHeinemann publications available, contact our World Wide Web home page at: 10 9 8 7 6 5 4 3 2 1 Printed in the United States of America

2. Series PrefaceThe Practical Veterinarian was developed to help veterinary students, veterinarians, and veterinary technicians quickly find answers to common questions. Unlike larger textbooks, which are filled with detailed information and meant to serve as reference books, all the books in The Practical Veterinarian series are designed to cut to the heart of the subject matter. Not meant to replace the reference texts, the guides in the series complement the larger books by serving as an introduction to each topic for those learning the subject for the first time or as a quick review for those who already have mastered the basics. The titles are selected to provide information about the most common subjects encountered in veterinary school and veterinary practice. The authors are experienced and established clinicians who can present the subject matter in an easy-to-understand format. This helps both the first-time student of the subject and the seasoned practitioner to assess information often difficult to comprehend. The editor and authors hope that the books in The Practical Veterinarian series will meet the needs of readers and serve as a constant source of practical and important information. We welcome comments and suggestions that will help improve future editions of the books in this series. Shawn P. Messonnier, D.V.M.vii 3. PrefaceThis book was written to provide the busy practitioner and the veterinary student a source of information concerning the more common intoxications in the United States. Veterinary toxicology is a very broadbased discipline with literally thousands of possible toxicants. The list was reduced using a core knowledge guidance paper written by the diplomates of the American Board of Veterinary Toxicology. Chapter 1 presents an initial discussion of the absorption, distribution, metabolism, and elimination of veterinary toxins and provides the reader with a framework for a rational therapeutic approach. It also provides the reader with information concerning calculations involved in veterinary toxicology. This begins the process of understanding the estimation of dose that is critical to differentiating between exposure and intoxication. Chapter 2 provides the reader with some reminders of possible toxins based upon the patients clinical signs. Chapter 3 discusses the pathophysiology of selected intoxications and gives the reader some deeper insight into the processes by which these poisons produce their clinical effects. It also provides a rational approach to symptomatic or antidotal therapy. Chapter 4 represents the bulk of this book, which is dedicated to individual monographs of specific toxins that are arranged alphabetically. This chapter should provide the reader with the requisite information to diagnose and treat a veterinary toxicosis. Chapter 5 concerns antidotal therapy and provides a quick access to the relatively limited number of antidotes that are available to the veterinarian. Chapter 6 discusses some of the basics of diagnostic toxicology as well as other sources of information that may be beneficial to the reader. It is my hope that this text provides the reader with a greater understanding of veterinary toxicology and most importantly the information necessary to diagnose and treat our veterinary patients. I would like to take this opportunity to thank some individuals who made this whole process possible. I first would like to thank my colleagues who are diplomates of the American Board of Veterinary Toxicology. I am greatly indebted to their original and clinical research (performed and published) that serve as the backbone of this text. I would also like to thank Leslie Kramer from ButterworthHeinemann ix 4. xPrefacewho patiently guided me through the process of putting these pages together. I am thankful that my sons, Adam, Alex, and Andrew, will soon see the fruits of this labor. Most especially, I am forever indebted to my loving wife, Karen, who supports me always in all things. J. D. R. 5. 1 Overview of Veterinary ToxicologyIntroduction The art and science of toxicology are only slightly younger than humankind. Early in the development of hunting and warfare, there is evidence of the use of poisoned arrows to gain tactical advantage. The principles of toxicology predate poison arrowsthey are as old as bacteria and rooted in plants. The vascular plants developed many successful chemical strategies to discourage or prevent predation by herbivorous insects and animals. Today tens of thousands of potential toxins can affect our veterinary patients, and there are fewer than two dozen specific antidotes. Imagine treating the entire spectrum of infectious diseases with only 24 antibiotics. In human medicine, the diagnosis and management of intoxication are simplified by the following: Toxidromes: clinical syndromes strongly associated with certain toxins Greater access to diagnostic tools Fewer financial restraints In veterinary medicine, the diagnosis and management of intoxication pose the following challenges: 1 6. 2Overview of Veterinary Toxicology Numerous species with differing presentations Malicious poisoning Treatment of herds of animals Maintaining the safety of the food supplyIt is paramount for veterinarians to understand the more commonly encountered toxins and to treat patients accordingly. The rewards are to return patients to their normal states and to prevent future cases of poisoning. Always rememberTreat the patient, not the poison.Toxicology Toxicology is the study of poisons and their effects on normal physiologic mechanisms. Information and concepts come from the following disciplines: pharmacology mathematics chemistry ecology zoology botanyDefinitions LD50: lethal dose 50 the dose of a toxin that causes the death of half of a group of animals generally only useful for an idea of the relative danger posed by an agent many LD50 data are obtained from observations of rats LC50: lethal concentration 50 the concentration of a toxin that causes the death of half of a group of animals generally used for toxins in air or water 7. Overview of Veterinary Toxicology3 Toxicity the quality of being poisonous the dose (e.g., mg/kg) of a poison that elicits a response often inappropriately used to equate toxicosis Toxicosis a clinical syndrome associated with exposure to a poison (e.g., acetaminophen toxicosis) the physiologic response to a toxin not the same as toxicityScope of Veterinary Toxicology The toxins that affect the more common domestic species are extremely diverse. The types of toxins often encountered include: Metals Mycotoxins Feed-related intoxicants Pharmaceutical agents Pesticides insecticides herbicides Biotoxins plants poisonous animals* (insects) venomous animals* (snakes, insects) bacterial toxins*A poisonous animal contains a toxin within its body and must be ingested to elicit toxicosis. A common veterinary example includes blister beetles (cantharidin toxicosis), which is discussed in Chapter 4. A venomous animal produces a toxin and has a delivery mechanism (e.g., fangs or stinger) to inject the toxin into the prey. Common examples would include bees, wasps, and rattlesnakes. 8. 4Overview of Veterinary ToxicologyThe Metabolic Fate of Toxins The Dose Makes the Poison This fundamental precept of toxicology has been attributed to Paracelsus. A dose-response relationship must exist (Figure 11). The greater the dose, the more likely that toxicosis will occur. Some toxins exhibit a steep dose-response relationship and are considered highly toxic.Exposure Is Not Equal to Intoxication To cause intoxication, a substance must be absorbed and delivered to the site of action at a concentration high enough to elicit a physiologic response. For example, the presence of poisonous plants in a pasture is not enough; there must be evidence of consumption of these plants.Figure 11 Example of dose-response relationship for three hypothetical toxins. The squares represent a toxin with a steep dose-response curve. If the response in this chart were mortality, the squares would present the greatest risk and the circles the least risk. 9. Overview of Veterinary Toxicology5 Different animals in a herd may consume different plants or forage. There are individual and species variations in susceptibility to toxins. The clinical signs observed must correlate with the suspect plant.Toxicokin