Chapter 2.

GENERAL PRINCIPLES OF TOXICOLOGY

1.Instructional Objectives

The objective of this section is to present to the student information regarding the classification of poisons,types of poisoning,metabolism of toxicants, mode of action of toxicants and factors affecting the action of toxicants. In addition, common causes of toxicosis as well as common toxicological problems in domestic and farm animals are discussed in this section.


2. Performance Objectives

After studying material presented in this section, students should have a broad understanding of toxicants especially with regard to their physiological effect on the body. The students should also be well acquainted with commonly encountered toxicoses in selected domestic and farm animals including various symptoms associated with these toxicoses and specific lesions associated with various types of toxicoses.


3. Classification of Poisons

The graduate veterinarian's ability to serve his clientele is no better than his ability to accurately differentiate between the various possible causes of the illnesses presented to him. It should be noted that mere memorization of clinical signs is not the only way to recall a certain toxicological problem. Perhaps, a better method of recognition is to classify the common toxicoses according to their specific lesions. Since there are many plants,feed additives, insecticides,herbicides, fungicides,
and heavy metals that may fall under a specific lesion, each lesion could be subdivided and indexed to help with differential diagnosis. The following classification based on toxic effects of poisons is far from complete, but it may act as a starting point for future indices of diagnostic classifications.
 
3.1. Gastrointestinal Syndrome
This is seen in poisoning from oak, acute copper, E.coli, Salmonella, carbamate fungicides,ANTU,acute crotalaria, paraquat herbicides, thallium, bracken fern, warfarin, etc. The important clinical signs include: bloody diarrhea,hematuria, enteritis, sometimes GI irritation. Death may occur within one to a few days. In case of wild indigo, staphylococcus,peptides and amides,and borates deaths seldom occur. Severe gastrointestinal syndrome is caused by pokeweed,sneezweed, inorganic arsenic herbicides. Death may occur within 1-2 days with bloody diarrhea,hemolytic crisis (pokeweed), and excess salivation (sneezweed).
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3.2. Neuromuscular (paresis) Effects
Some poisons causing these effects act rapidly,some act slowly. Oleander,buttercup, buckeye, lead(in poultry)act rapidly causing death in 1-2 days. Others include: death comas, poison hemlock, organophosphates. All the above cause in coordination plus GI syndrome where as jimsonweed,2,4-D, organic tin, ticks cause only in coordination. Other examples of toxicants causing CNS effects include botulism,horse tail, bracken fern (in horses), pigweed, etc.
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3.3. Bone-Teeth-Hoof-Hair Deformities
Ergot - sloughing of tips of tail, ears, teats, etc.
Fluoride - bone and teeth lesions
Selenium (chronic), poisonvetch, copper deficiency,chronic thallium, lead nitrate, chlorinated naphthalene,are all associated with hair problem.
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3.4. Kidney Lesions
Depending on the types of renal damage caused by toxicants kidney lesions are categorized as follows:
A. Hematuria: This may be accompanied by fever as in the case of bracken fern or there may be no fever as seen in poisoning from oak, cassia, inorganic mercury or cadmium.
B. Hemoglobinuria: This is caused by crucifers(mustard) with photosensitization. Similar condition is seen in
chronic copper toxicity.
C. Oxalates: These are formed in poisoning from amaranthus (which also causes perirenal edema), halogeton,black greasewood, rhubarb, and ethylene glycol.
D. Perirenal Edema: Amaranthus and oak(with as cites); nightshade (no as cites).
E. Degenerative Changes in Kidneys: These can be caused by chronic organic mercury compounds, chronic thallium, sulfonamides and so forth.
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3.5. Liver Lesions
These are caused by several toxicants such as selenium (acute,)
aflatoxins,crotalaria,tannic acid,carbon tetrachloride,
phenothiazines, gossypol, tarweed, etc.
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3.6 Photosensitization
Phenolic fungicides (necrosis of contact tissue); vehicles for insecticides; blue-green algae(death within 1-2 days);
crucifers (with hemoglobinuria), snow-on-the-mountain (without
hemoglobinuria); phenothiazines, alfalfa; St. Johnswort,
horse brush.
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3.7 Hemorrhagic Syndrome
Bracken fern (with fever); those that do not have fevers include: sweet clover,warfarin,pindone,radiation, mycotoxins,
crotalaria.
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3.8 Fevers
Castorbean,oleander, have rapidly acting toxic materials and
death may occur within 1-2 days. On the other hand, milkweed,
bracken fern,buckeye, locust, etc. are known to be slow-acting.
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3.9 Abortions and/or Anomalies
Fusarium fungal metabolities, poisonvetch,lupine, broomweed,
ergot,2,4,5-T,nitrates. The following cause birth defects:
chronic selenium,veratrum,oak, locoweed, jimsonweed, hem-
lock, lead, and mercury.
The classification of poisons made in the previous section is based on the organ or system that is the 'target' site for the effect of the toxic chemical. Such classification that is based on the toxic effects of poisons on the body may be unsatisfactory because the same substance can have different effects on different organs of the body. It can also vary in its action between one species and another. Therefore a relatively simple classification can be made on the basis of the structural features of the toxic chemicals that are responsible for their toxic properties and their affinity for 'target' sites in the animal body. Various diverse chemical compounds can be thus divided into two groups, namely, inorganic and organic compounds.
Inorganic compounds include metals,metalloids, their salts and acids and alkalis. Organic compounds on the other hand include all carbon compounds other than carbonates,and the metallic carbides and cyanides. Furthermore, an analytical toxicologist endeavours to separate poisons into characteristic groups and several classifications can be made according to the analytical procedures involved. A typical subdivision is: (1) volatile poisons, (2) metallic poisons, (3) toxic anions,(4) non-volatile organic poisons isolated by solvent extractions, (5) miscellaneous poisons. Finally, poisons may often be classified conveniently by their origin (plant poisons) or use (pesticides).
 


4. Types of Poisoning

It has been customary to subdivide poisoning into acute, a sudden violent syndrome caused by a single large dose of poison and chronic, a persistent, lingering condition brought on by small repeated doses, with subacute poisoning somewhere in between. This subdivision, however, is not really tenable, as there are types of poisoning which would be difficult to fit into any of these categories. 'Chronic'copper poisoning in sheep only becomes manifesting an acute hemolytic crisis. Symptoms of bracken poisoning may not appear until months after the plant has been ingested. In addition to these types of poisoning, there are other unto- ward effects due to 'poisonous' substances. (1) Allergy, an immunological response due to sensitization of the subject by a previous dose, although less common in animals than in man,is still quite well known. (2) Carcinogenicity, in which the agent is responsible for the formation of neoplasia, bracken and the cycads are examples;(3) Teratogenicity, in which material ingested by the mother at some definite stage in pregnancy produces abnormalities in the off- spring. The classical example in veterinary toxicology is the cyclopean malformation in lambs due to Veratrum californicum.

5. Metabolism of Toxicants

 

1 Absorption
In order to exert their toxic effects most poisons must be absorbed into the blood stream. Under natural conditions toxicants may enter the body through the lungs, gut and skin. Substances maybe injected into the body subcutaneously, intramuscularly, intravenously and intraperitoneally.
1. Respiratory tract: The very extensive highly vascular pulmonary mucous membrane affords an excellent channel of absorption for gases and for solids and liquids,particularly when in a fine state of dispersal (as aerosols or dusts).
2. Alimentary tract: It is the most usual route of entry of a poison. In all species, much absorption occurs in the small intestine; in the dog it can also take place from the stomach, in the ruminant from the rumen and reticulum, and in all species, especially in non-ruminant herbivora, from the large gut. The contents of the alimentary tract can modify the action of poison.
The hydrochloric acid present in gastric juice may aid the solution of originally insoluble materials. A full stomach or rumen may delay symptoms of poisoning or may dilute the poison to such
an extent that it is relatively harmless.
3. Dermal exposure: The unbroken skin does not offer a favorable channel of absorption to most compounds. Nicotine is one of the exceptions which in aqueous solutions,is very efficiently absorbed through intact skin. Absorption takes place more readily from oily solutions or emulsions.
Absorption through damaged or abraded skin or a wound occurs
as through moist mucous membranes. Subcutaneous or intramuscular
injection is equally effective, while intravenous administration
is the fastest way of introducing a poison into the blood stream.

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2. Distribution and Accumulation

 

Hepatic storage: All foreign compounds entering the body pass to the liver which is the major detoxifying organ in the body and by virtue of this fact many poisons accumulate in this organ. It is therefore not unexpected to find hepatic lesions as a consequence of exposure to many toxicants.
 
Extra hepatic storage: Some poisons are selectively deposited in certain organs or tissues. Iodine is largely taken up by the thyroid glands; strontium, fluorine and lead are deposited in the bones. A knowledge of the distribution of a particular poison is of great help in the selection of organs for chemical analysis.

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3. Biotransformation and Detoxication
Many foreign compounds that are introduced into the body undergo chemical transformation, and this process is generally referred to as metabolic transformation or biotransformation.Very frequently this process is referred to as the detoxication mechanism. However, this term is misleading because the metabolic transformation of a foreign compound may result into increased or decreased toxicity of the product. Two categories of enzyme systems are known to exist in mammals that cause chemical transformation of foreign compounds.
One category consists of enzymes that normally occur in the tissues and are responsible for transformation of normal endogenous substances in tissues as well as foreign chemical e.g.,alcohol dehydrogenase. Second category consists of an enzymes system that is very important in toxicology. These are known as drug metabolizing enzymes, which are present in many tissues but are particularly in liver cells. These enzymes are located in small particles called microsomes which in turn are located in the smooth surface of endoplasmic reticulum. These microsomal enzymes are capable of catalyzing a variety of biotransformation reactions,the major ones being oxidation, reduction, and hydrolysis.
The drug metabolizing enzymes system is also called as mixed function oxidase (MFO) enzyme system for the obvious reason that is capable of catalyzing various types of reactions. The oxidase system requires the presence of a cofactor NADPH, and molecular oxygen for its activity. NADPH reduces a component of the microsomes which reacts with molecular oxygen to form an active oxygen intermediate which oxidizes the drug or the foreign chemical. The components of microsomes which is reduced by NADPH is a heme protein called cytochrome P-450.
R.T. Williams (1959) divided the biotransformation mechanism of foreign chemicals into two major types: (1)the non synthetic reactions involving oxidation,reduction and hydrolysis,and(2) the synthetic or conjugations involving production of a product that is biosynthesized from the chemical (or its metabolite) plus an endogenous metabolite such as glycine. If we call the non synthetic reactions as phase 1 of biotransformation mechanism, then conjugation reactions will be phase II, which are described below.
 
Conjugation Reactions: These are reactions in which a poisoner ametabolite is combined with some compound provided by the animal body. Following is the list of some of these processes.
 
Glycine Conjugations: Benzoic acid combines with glycine to form hippuric acid.
 
Glucuronic Acid: Conjugation with this occurs in may cases. e.g.,dinitrophenol, chloral hydrate.
 
Sulfate Conjugation: Some phenols conjugate with =SO4 to form ethereal sulfates.
 
Cysteine Conjugation: Arsenic trioxide and mainly benzene, polycyclic hydrocarbons, and certain halogenated hydrocarbons conjugate with cysteine to form mercapturic acids.
 
Acetylation Reactions: This takes place between CoASH and amino groups of aromatic compounds, e.g., sulfanilamide derivatives. Dog is deficient to acetylate aromatic amine groups.
 
Methylation Reactions: It is relatively uncommon form of detoxication and is confined to the heterocyclic nitrogen atoms in compound of the pyridine and quinoline type.
 
Thiocyanate Formations: Inorganic cyanides are converted by the enzymerhodanese to thiocyanate. It is a true detoxication, for sodium thiocyanate is nearly 200 times less toxic than sodium cyanide and is slowly excreted in the urine.
 
Glutamine and Ornithine Conjugation: Conjugation processes with glutamine and ornithine occur in man and bird, respectively.
 
Inhibition of Biotransformation Mechanisms
The microsomal enzyme systems can be inhibited by several compounds in concentrations which by themselves have little pharmacologic activity. The most common example of such inhibitor is SKF 525A (diethyl aminoethanol ester of diphenyl propyl acetic acid). Thus, the inhibition of the microsomal enzyme system would result into the inhibition of the metabolism of toxic foreign compound, the presence of which may cause increased toxicity. On the other hand, when the product of metabolic transformation is of greater toxicity than that of the parent compound,the inhibition of the microsomal enzymes by SKF525A would be expected to protect the animal from toxicity resulting from metabolism of the parent compound.
 
Induction of Biotransformation Mechanisms
The total quantity of microsomal drug metabolizing enzymes can be increased in humans and in animals by prior administration of large variety of chemical substances. Such substances include the anesthetics, such as nitrous oxide; the sedatives,such as barbiturates; the analgesics, such as phenylbutazone; and the insecticides, such as chlordane.
Induction of increased enzyme activity usually involves repeated exposure to the inducing agent. It is usually temporary and lasts for two to four weeks following the administration of the inducing chemical. Since metabolic transformation has been shown to result in the formation of more or less toxic products as compared to the parent compound, enzyme induction may be protective to the animal (when detoxication is involved) or detrimental to the animal (when toxication is involved).
 
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4. Elimination
Following are the major ways through which poisons and their metabolites are excreted.
1.Fecal excretion. Ingestation of a relatively insoluble poison (e.g., lead arsenate) is followed by excretion of the major part in the feces. Substances may also find their way into faeces via bile; metals stored in the liver are slowly excreted in this way.
2.Pulmonary excretion. Volatile poisons may be mainly excreted in the expired air, e.g., CS2, cyanide. In phosphorus poisoning the breath may smell of garlic odor and glow in the dark. Diagnosis of hemlock poisoning may be made from the characteristic "mouse-like" odor of coniine in the exhaled air (and also in urine). The lesions found in the lungs in paraffin poisoning are probably due to irritant effect caused by pulmonary excretion.
3. Urinary excretion. This is the most important pathway of the excretion of a poison. Irritant poisons cause damage to kidney. Urine is often a convenient material for diagnostic analysis. In veterinary field it is of great importance in detecting the pasture contamination with fluorides.
4. Milk and dermal excretion. Excretion can also take place through skin,e.g., arsenic, and in lactating animals in milk. Many of the insecticides are fat soluble and it has been shown that DDT, aldrin, and several other chlorinated hydrocarbons can be detected in minute amounts in cow's milk.
 

 

6.Toxicokinetics

 

Toxicokinetics involves the relationship between tissue concentration of a toxicant and time. There are two types of kinetics: 1. Zero-order kinetics and 2. First order kinetics.
In zero-order kinetics, a toxicant is eliminated from the body at a fixed amount per unit time. This occurs when the toxicant elimination process is saturated and a fixed amount of toxicant (e.g., mg or g) is excreted per unit time.
Many toxicants are eliminated from animal tissues in a fixed-order manner. Thus, in first-order kinetics a constant fraction of a toxicant/drug is eliminated per unit time. For example, if the initial level were 500 mg and it took ten days to reach 250 mg, then it will take additional 10 days to reach 125 mg. During the first 10 days a total of 250 mg toxicant was excreted and during the next 10 days only 125 mg. In each case, the amount excreted was a constant fraction (i.e., 50%)of the original amount. The ten-day period is called half-life of the toxicant. Various species handle excretion of a toxicant in different manner and therefore a given toxicant will have different half-lives in different species.

 


7. Mode of Action of Toxicants

 

In a broader sense, mode of action of poisons may be divided as physical and chemical disruption of the living process.
 
Physical Action: Because of their physical properties such as lipid solubility,some substances exert a non-specific inhibitory effect on enzyme systems by virtue of the fact that their physical nature is such as to bring about their accumulation in vital parts of cells where they depress cellular functions. Many compounds, including hypnotics and anesthetics such as hydrocarbons, chlorinated hydrocarbons, alcohols, ethers, ketones exert their inhibitory effects in this way.
 
Chemical Action: Majority of poisons produce their effect as a result of their chemical interactions with cell components. It is the enzymes concerned in cell oxidation and oxidative phosphorylation which seem to be most vulnerable to the action of poisons. Enzymes have active sites and the toxic compounds may occupy those active sites thus preventing normal substrates to combine.
The enzyme inhibition caused by a toxicant may be irreversible (asin the case of certain organophosphates) or reversible (as in the case of carbamates). Some of the factors that may affect the enzyme inhibition are: chemical structure of the inhibitor,cell permeability, enzyme inhibitor concentrations,the presence of an antagonist of the inhibitor, lethal synthesis, presence of an
antidote.
 


8. Factors Affecting Actions of Toxicants

Numerous factors influence the action of poisonous substances. In addition to those already described (e.g., route of absorption, biotransformation), they include (1) dosage, (2) the physical and chemical nature of the poison (3) the source of the poison (4) repeated exposure to the poison (5) species (6)size, age and sex (7) general state of health of the animal.
 
1. Dosage
Harmful effect of a toxic compound is largely dependent on the amount of that compound absorbed into the body.
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2. Physical and Chemical Nature
The physical state,e.g., whether solid,powder,orin solution will affect the dose of a poison. Coarsely crystalline arsenic trioxideis slowly absorbed and so has relatively low toxicity; finely powdered arsenic is highly toxic. Many substances are readily absorbed from oily than from aqueous solution,
e.g., insecticides.
Chemical nature is important in regard to toxicity, yellow phosphorus is a most poisonous substance,its allotrope red phosphorus is inert when taken in body, it is in soluble and is excreted unchanged. Compounds containing trivalent arsenic are much more toxic than those containing the pentavalent form. Barium carbonate is intensely toxic than barium sulfate.
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3. Sources of Poisons
Under certain circumstances poisoning from a particular compound may be enhanced or reduced. Some plant poisons are destroyed by drying or storage and hay contaminated by them is harmless(e.g., buttercups). Presence of oils in the diet will enhance the absorption and so toxicity of poison e.g., phosphorus. Accumulation of copper in the liver may be mobilized by administering molybdenum and vice versa.
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4. Repeated Exposure
It is logical that several doses of a poison will be effective than a single dose. The degree of harmfulness of repeated small doses also depends on whether the poison accumulates in body and whether its effects are cumulative (e.g., carcinogen). Carcinogens are the example of such chronic toxicity.
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5. Species
There are extraordinary wide variations in response to a particular poison between species.
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6. Size, Age and Sex
In general, the amount of a poison required to produce toxic symptoms is related to the weight of the animals.This relation-
ship between weight and dose may vary between species.
Very young and very old animals are usually more susceptible to poisons. There are few instances of sex difference in response to poisons in animals. For example, red squill has about twice the toxicity for female rats than for males.
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7. General State of Heath
Debilitated animals are more susceptible to poisons and drugs because their general resistance and detoxication mechanisms are defective. For example, hepatic or renal disease may enormously increase the susceptibility to poisons.

 


9. Common Causes of Toxicoses

 

Accidental poisoning occurs in animals and maybe divided roughly into: (1) poisoning by naturally-occurring toxicants and (2) poisoning by man-made chemicals. But there is no clear cut dividing line between these categories. Naturally occurring toxicological hazards include poisonous minerals and poisonous plants; man-made hazards include industrial contaminants,pesticides, domestic materials, unsuitable food and water,use of drugs, etc.

 


10. Common Toxicological Problem in Domestic and Farm Animals

 

1. Dogs and Cats
Pesticides,garbage, ethylene glycol,heavy metals, biotoxins (toads, snakes, ticks), phyto toxins, mycotoxins,drug reactions.
 
2. Poultry
Pesticides(very sensitive to insecticides), feed and water additives, fungi, bacterial toxins, gases and fumigants, heavy metals.
 
3. Zoo Animals
Largely malacious and quite variable situations, drug reactions, poisonous plants,accidental-organophosphates and warfarin baits.
 
4. Exotic Animals
Largely due to feed additives. Mink - Botulism,chronic lead, phenolic wood preservatives, stilbesterol, etc. Rabbits - milkweed, toxic plants, neck paralysis and in coordination common. Turtles - paint on shell produces lump-back deformities.
 
5. Cattle
Heavy metals,pesticides, dietary and environmental contaminants
(e.g., urea,nitrate, cyanide,mycotoxins), poisonous plants; snake
and insect bites,drug adverse reactions.
 
6. Sheep and Goats
Poisonous plants - photo sensitizers, cyanogenetic, selenium, oxalate, lupine, sneezeweed, laurels, white snake root, larkspur, etc; pesticides,anthelmintics,others--lead, nitrate,sulfur, fluoride.
 
7. Horses
Poisonous plants--oleander, bracken fern, castor bean, locoweed, lupine, selenium-containing,groundsel, crotalaria, cyanide; pesticides, drug adverse reactions, snake and insect bites, other aflatoxins, heavy metals, toxic gases.
 
8. Swine
Salt,coal-tar (pitch)and petroleum products, nitrates,wood preservatives, heavy metals, organic arsenicals, fungal toxins, poisonous plants,gossypol,insecticides, botulism, edema disease (endotoxins), rodenticides.


Review Questions

1. Classify various types of common toxicoses (syndromes) according to their specific lesions in target organs and give at least two examples of each toxicosis.
2. List different types of poisonings and point out whether allergic reactions, carcinogenicity, mutagenicity and teratogenicity will fit into those types of poisonings or not.
3. Which is the fastest way for absorption of poisons to cause poisoning?
4. List various pathways through which poisons can enter the body.
5. How poisons act on the components of living cells?
6. Define the following terms.
a. Biotransformation
b. Detoxication
c. Activation
d. Induction
7. What are phase I and phase II reactions? How are they involved in making a toxic substance more or less toxic?
8. List two types of modes of action of poisons.
9. List various factors that affect the action of poisons.


Reading Assignments

1. Clarke, E.G.C. and M.L. Clarke, (eds.) Veterinary Toxicology. 1st. ed. part 1, (pp. 1-26). The Williams and Wilkins Company, Baltimore, 1975.
2. Casarett, L.J. and J. Doull. (eds.) Toxicology: The Basic Science of Poisons. 2nd Ed. Ch. 3-5, 9-12, 14-15.
3. Barkan, B.A. and F.W. Oehme. 1975. A classification of Common Midwestern Animal Toxicosis. Vet. Tox. 17:37-49.
4. Buck, et. al., Clinical and Diagnostic Veterinary Toxicology. 2nd ed. 1976. pp. 25-37.