The uses and problems of species specificity in toxicology.

Introduction

In toxicology, species specificity is the ability of a toxin or toxicant to restrict its toxicity to specific species, whilst being relatively non-toxic to others.

Uses

The ability of toxicants to restrict its toxicity to specific species has been useful in the production of chemical biocides. A biocide can deter, render harmless, or exert a controlling effect on a harmful organism/s. Most biocides are selective; they are only toxic for a particular species/group of species. In this way, a biocide can be applied to a population including many different species, and exert its toxicity only to the harmful species. Biocides include pesticides (useful in agriculture) and antimicrobial compounds (useful in healthcare).

Problems

Species specificity can cause unexpected problems when a chemical, thought to be non-toxic, comes into contact and produces a toxic effect in a species not yet studied.

Chocolate
For example, much of the human population enjoys eating chocolate; sometimes in large amounts. However, many dog owners need to be aware that chocolate causes significant toxicity in dogs, sometimes leading to death. In fact, chocolate is the leading cause of canine poisoning in the UK. It was discovered that dogs are susceptible to the toxic effects of a chemical known as theobromine, found in chocolate. 

Theobromine belongs to a group of drugs known as alkaloid alkylxanthenes and is very similar to caffeine. It exerts its effects by acting as a phosphodiesterase (PDE) inhibitor and an adenosine receptor antagonist. Theobromine effects the nervous system, both centrally and peripherally. In modern medicine, theobromine is used as a vasodilator, a diuretic and as a heart stimulant, and has been used to treat high blood pressure. Its more useful in treatments than caffeine because it effects the CNS to a lesser degree, and hence has fewer addictive properties. Theobromine is also able to stimulate the heart to a greater degree and increases heart rate.

Theobromine is naturally found in the cocoa plant. The quantities found in chocolate are too small to have much of an effect on humans. Theobromine is rapidly and efficiently eliminated in humans and thus the half life of theobromine is 6-10 hours. We would need to eat ~10kg good quality dark chocolate to ingest a lethal dose. In dogs, however, its half life is much longer: ~17.5 hours. This means that 40-50g of good quality dark…