Toxicology Overview

Introduction

Toxicology is the study of chemicals or substances that can damage tissues or destroy life. Such chemicals and substances are commonly referred to as poisons, and can include plants, pharmacological agents, heavy metals, herbicides, insecticides, rodenticides, mycotoxins and snake bite venoms. The key to understanding the effects of potential poisons is to recognise that they all have a dose dependant adverse effect - even water can be toxic when administered or consumed in large quantities!

Exposure to a toxic substance does not always result in poisoning - the substance must be absorbed and react with tissues/organs for toxicity to occur. The study of this is known as toxicokinetics. The incidence of poisonings in animals in the UK is not specifically documented as there is no legal obligation to report cases, but the Veterinary Poisons Information Service do offer 24 hour advice and guidance on potential poisoning cases. Incidents associated with pesticide poisoning can be investigated via the Wildlife Incident Investigation Scheme.

Toxic Effects

The toxic effects of substances can be categorised in a number of different ways:

• Adverse effects that are unexpected
• Side Effects that are a normal but undesirable effect, particularly with drugs
• Idiosyncratic effects that are disproportionately large from what could be expected
• Allergic reactions which require a previous contact with that substance

In addition, toxic effects can be divided into:

• Immediate or delayed effects such as radiation poisoning and other carcinogens
• Reversible or irreversible effects such as damaged liver tissue which can rejuvinate

Toxic effects are usually considered in the light of the damage they do to the affected tissues. This process is considered in the light of the toxicokinetics of the individual substances.

Toxicokinetics

There are 4 aspects to toxicokinetics:

1. Absorption
2. Distribution
3. Metabolism
4. Excretion

Absorption

The rate of absorbtion is dependant on the route of administration and the bioavailability of a substance.

The route of administration is significant because of the natural barriers that exist which may prevent or lessen a toxic effect by reducing the dose absorbed. Gastric acid and the low pH of the stomach are significant barriers to the absorbtion of swallowed substances, but conversley, substances that cause local irritation to the intestinal lining, where most absorption will occur, can enhace uptake as a result of this disruption to the gastrointestinal barrier. In addition, some drugs can increase absorption by utilising transport carrier systems present in the GI tract, and lipid soluble compounds will be more readily absorbed across the GI epithelium. Dermal absorption is more readily achieved in people where there is no fur and an increased vascualarity in the dermis, but dermal absorption in animals is increased in areas where there is abrasions, waterlogged skin of exposure to organic solvents.