Take another line of reasoning. Animals used in trial work are usually bred specifically for this purpose and are healthy in all respects except the condition induced for the intention of the trials. The human patient, afflicted with the same condition as the trial rat, may also be infected with several other health problems which may substantially influence circumstances. The human patient may be confined to bed during the period of therapy whereas the animal model would probably be returned to its cage to carry on life in the norm way.

There is a difference of opinion amongst the profession; people involved in this field of work about the value animal trial results. Without doubt the results are valuable, but interpretation of animal trial information must take into account the limiting factors.

With reference to the second category of animal trials, those used on substances intended as veterinary products, we are probably approaching a more attainable ideal. At least here we are testing substances on animals for subsequent use with animals.

One of the main problems in this area is the attempt to scale up the results of work done on rats and mice to be applicable to, say, horses or cows. It is not always true that something which works well on a laboratory scale also does so on a full scale. Another factor which can be influential is the difference in activity of different groups. For instance, the effect of a particular drug could be quite different in an active working animal, such as a horse, to a relatively sedentary animal maintained in a laboratory cage.

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