Letter: Flawed animal models

From David Hammond

In his letter, Leigh Jackson makes the point that in specific cases animals can be useful models to study particular aspects of biological processes (1 April, p 24). He supports this by quoting the Nuffield Council report on animal testing that states there are evolutionary continuities in the form of behavioural, anatomical, physiological, neurological, biochemical and pharmacological similarities between animals and humans. This is a generalisation that hides the real truth. I illustrate this point using the example of the rat.

There are some anatomical or physiological peculiarities of rats that affect compound absorption, pharmacokinetics and metabolism, or cause unexpected reactions to a test compound. Rats are obligate nose breathers, for one; this can alter how and when a substance enters the blood stream. The placenta is considerably more porous in the rat than in the human. Owing to differences in the distribution of intestinal microflora within the rat gut, they are much more likely to metabolise an orally administered compound, possibly into a toxic or active metabolite.

The rat secretes stomach acid continuously, whereas the human stomach secretes acid only in response to food or other stimuli. Rats are nocturnal, prone to different diseases than humans, have different nutritional requirements, and cannot vomit. David E. Semler of G.D. Searle and Company, writing about these discrepancies in Animal Models of Toxicology, goes on to describe differences between male and female rats, between different strains of rats, and between the results of studies on rats conducted at different research institutions. Even when the same rats are used for the same experiments at different research institutions, the results are different.

Extrapolation is the sine qua non of vivisection, but given the above, the basis is flawed.

Hassocks, West Sussex, UK