Chapter One

Introduction

We humans have an intimate relationship with the plants that surround us. We take them for granted as we use them for food, clothes, and shelter. We use them medicinally; indeed, more than one-third of our modern pharmacopoeia has its origins in plant products. We please our senses, decorate our living spaces, and express our feelings for one another with them. Plants are an essential part of many of our religious and social rites. Paradoxically, some of the plants we prize for these varied uses may also pose a threat to us or to our domesticated animals. Toxic plants are very much a part of our environment. Until their effects, ranging from mild irritation or discomfort to rapid death, become apparent, they are often ignored or simply overlooked. Because of their ubiquity, there is a need for a comprehensive treatment of toxic plants likely to be encountered in North America, north of the Tropic of Cancer, growing wild or cultivated. The first edition of this book was written in response to that need.

OBJECTIVE AND SCOPE OF THE FIRST EDITION

The objective of this undertaking was to write a comprehensive treatment of toxic plants that brought together the currently available information on (1) their morphology and distribution, (2) the disease problem or problems associated with them, (3) their toxicants and mechanisms of action, (4) the clinical signs and pathologic changes associated with their toxicity, and (5) the principal aspects of treatment. The perspective of the first edition was primarily veterinary science.

Compilation of the information presented in the first edition began in the 1980s as a series of articles for the Oklahoma Veterinarian and an agricultural extension publication, Poisonous Plants of Oklahoma and the Southern Plains. Well received, these publications dealt primarily with native plants and their toxicity for livestock. Initially, the present book was anticipated to do the same for the United States. Gradually, however, its scope and depth of coverage evolved—larger area, more plant families, and greater detail than first envisioned. These changes came about in part because of the increasing popularity of ornamental plants for both house and garden. There has been a corresponding increase in awareness of toxicity problems associated with some of them.

OBJECTIVE AND SCOPE OF THE SECOND EDITION

In the 11 years since publication of the first edition, a wealth of toxicologic information has been compiled—unknown toxicants identified, mechanisms of intoxication elucidated, and additional reports of problems published. In addition, there has been a corresponding increase in taxonomic knowledge with significant changes in the classification of plant families and genera and associated changes in nomenclature. Because of this almost exponential increase in our knowledge of toxic plants, work on a second edition was initiated in 2009.

In addition to compiling and presenting the literature of the last decade, we have also slightly altered the perspective of this edition. We have included information about four additional aspects of plant toxicology; they are summarized in the following subsections.

Intoxications in Humans

The first edition focused primarily on veterinary science because of our professional backgrounds and the need for such a book in the discipline. In this second edition we have attempted to place increased emphasis on human intoxications because the information acquired about both humans and other animals is often interrelated and supportive. For the most part, plant intoxications in humans, while not uncommon, do not pose the lethal risk (with the exception of Datura and Cicuta) seen with livestock and other animals, but they nevertheless may be numerous and sometimes serious as revealed in annual reports from Poison Control Centers (Litovitz et al. 2001; Bronstein et al. 2007). It may be expected that in most instances similar disease problems will occur in both humans and animals with a few exceptions.

For some taxa, we have included information about problems associated with herbal products as examples of their intoxication potential but a comprehensive discussion of adverse reactions to these products is beyond the scope of this book. In addition we have included some information about potential bioterrorism threats because of the serious problem presented by the extreme toxicity of some plants such as those possessing type 2 ribosome-inactivating proteins—most notably Ricinus communis and species of Adenia (Pelosi et al. 2005; Stirpe and Battelli 2006; Monti et al. 2007; Stirpe et al. 2007; Ng et al. 2010). Considerable information on the mechanisms of intoxication is emerging because of the interest in effects of plant toxicants as models for various human disease problems such as Huntington’s disease, ALS, Alzheimer’s disease, and Parkinson’s disease (Tukov et al. 2004; Bradley and Nash 2009; Cox 2009; Pablo et al. 2009; Tunez et al. 2010).

Treatments for humans are given in very general terms because physicians and medical institutions may have different treatment protocols. General references for specific procedures include Greene and coworkers (2008) and Lee (2008) for gastrointestinal decontamination and use of ipecac, and Froberg and coworkers (2007) for plant poisonings specifically in humans.

Intoxications in Wildlife and Captive Animals

In this edition, a special effort has also been made to document the effects of poisonous plants on wildlife, both free roaming and captive. References for specific information about particular genera and species are included throughout the book. General references to be consulted include Fowler (1981, 1999) and Van Saun (2006).

The reader should keep in mind that in general most wild herbivores respond similarly to plant toxicants as do our domesticated animals, with a few exceptions such as those compounds produced by Quercus (oak), Centaurea (star thistle), Acroptilon (knapweed), and Pinus (pine). Some plants are invariably toxic to most wild animal species, for example, cardiotoxic and cyanogenic plants as well as species of Lantana (lantana) and Nicotiana (tobacco) (Basson 1987). Other plants, however, may affect wild animal species quite differently as illustrated by responses to tannins, especially those produced by species of Quercus (the oaks).

With respect to toxic plants, species of wildlife are not necessarily immune to their effects, but avoid problems associated with their toxic secondary compounds by ignoring some plants, eating only small amounts, and/or exhibiting natural gastrointestinal/hepatic degradation/detoxication of these noxious compounds (Fowler 1981; Laycock 1978). Unfortunately, captive or domesticated wild species may have access to toxic plants with which they have not coevolved or which they have not encountered previously. In some instances, boredom of captive animals may lead to ingestion of toxic plants in their enclosures. Such problems have been reported in a variety of herbivores ranging from elephants to tortoises.

There are also other reasons for ingestion of toxic plants by wild animal species, including poorly nourished, hungry animals which may be nonselective in their eating habits or to seasonal variations in palatability or acceptability of otherwise noxious plants in their environment. Thus management plays a vital role in animal intoxications (Pfister et al. 2002). Additional reviews regarding the role of secondary plant compounds on nutritional toxicology of birds and herbivores are available (Cipollini and Levey 1997; Dearing et al. 2005; Torregrossa and Dearing 2009).

Role of Plant Secondary Compounds in Plant Intoxications

An additional problem given increased attention in this second edition is the role of secondary plant compounds as toxicants in honey and or their affect on bees. A number of general reviews on these subjects are available: Patwardhan and White (1973), White (1981), Detzel and Wink (1993), Faliu (1994), Adler (2000), and Kempf and coworkers (2010). Some attention has been given to the problem of milk and meat tainting but without exhaustive discussion. Reviews are available but this is a subject not given great coverage with respect to noxious noncultivated plant species (Richter 1964; Armitt 1968a,b). Methyl sulfide is clearly a factor in tainting and probably many plants that have sulfur-containing constituents are likely culprits (Patton et al. 1956; Gordon and Morgan 1972).

Role of Fungal Endophytes in Plant Intoxications

Great interest is now directed toward the role of fungal infections of plants as contributors to the synthesis of toxicants in host plants. The fungi involved in these infections may be endophytes or epiphytes. In some instances the toxins may be produced exclusively by the fungus, whereas in others the toxicants may be produced by both the plant and the fungus (Wink 2008). Examples of these situations are the presence of an endophytic fungus in Hypericum perforatum, which produces hypericin similar to the host plant, and an endophyte in Podophyllum peltatum, which produces podophyllotoxin again similar to the host plant. In contrast, an endophytic strain of the fungus Fusarium oxysporum also produces podophyllotoxin but in Juniperus recurva, a totally unrelated species (Eyberger et al. 2006; Kour et al. 2008; Kusari et al. 2008).

Because these fungi, especially the endophytes, are in many instances clearly beneficial to the host plant, there is good reason to expect...