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An introduction to poisons

Poisoning can occur in an individual pig or in a group or even affect a whole herd. In the latter two cases a number of animals will be affected at the same time all showing similar clinical signs. A study of the history may indicate a common exposure to the poison by contact or ingestion.

How can poisoning occur?

A toxic compound may gain entry by mouth and the intestinal tract, the respiratory tract, the eye, the womb, through the skin or by injection. It is transported via the blood stream and deposited in various tissues throughout the body. It is important to know which of these are involved so that relevant tests can be carried out.

Water soluble poisons are excreted by the kidney in the urine. Other poisons are first detoxified in the liver into water soluble compounds and then excreted in the urine. Some poisons are detoxified in to non-water-soluble substances which may then be combined with other substances to neutralise them further. Fat soluble poisons are likely to accumulate in the liver. Poisons may also be excreted through the skin or through the milk with possible adverse effects on sucking piglets. Some may be absorbed across the placenta of the pregnant sow and affect the foetus. Tissues used for testing include liver, kidney, blood and stomach contents.

If fed to excess many dietary components including minerals and vitamins, can cause illness. Many medicines are highly toxic if used above their therapeutic levels.

It is a common fault with stock people when treating animals to assume that twice the dose will act twice as well. This is a fallacy. Overdosing may well have the opposite effect.

Most poisons and medicinal products are dependent upon the size of dose for their clinical effects.

These progress from:

  • No clinical signs.
  • A therapeutic effect.
  • A toxic effect.
  • A lethal effect.
  • The degree of toxicity is measured by a term LD50 which is the dose that will kill 50% of the exposed population.

Factors influencing the effects of a poison

  • The age of the pig.
  • Its weight and size. Generally the younger the pig the more severe the effect.
  • The nutritional status and feed intake.
  • The health status.
  • The droplet size of the toxin. The smaller the size the greater the absorption.
  • Sex. Gilts and sows for example are more susceptible to organophosphorus poisoning.

Common substances that may cause poisoning

  • Antibacterial medicines: carbadox, furazolidone, monensin, sulphadimidine.
  • Trace elements e.g. iron, copper, zinc, iodine, selenium, arsenic, mercury, lead, fluorine.
  • Coal tars.
  • Gases: ammonia, carbon monoxide, hydrogen sulphide.
  • Insecticides: organophosphorus, carbamates, lindane, dieldrin.
  • Nutrients: essential minerals - copper, iodine, iron, manganese, selenium, zinc.
  • Rat poison: warfarin.
  • Salt: if water is limited.
  • Toxic plants.