Animal models can assist medicine in humans

Editor—The debate about whether we can learn from animals is far more subtle than the rapid responses to Roberts et al's systematic review of fluid resuscitation after haemorrhage suggest.¹,² It depends on the problem and on the species of the animal; the lessons of comparative medicine are most useful when the model has sufficient similarities to suggest its relevance and enough differences to make it informative. Blanket assertions that we cannot learn from animals are naive, but many generalisations about humans fail when we look at ethnic differences and genetic predisposition.

Several odd points arise in Roberts et al's systematic review.

Firstly, the authors confuse treatment aimed at increasing circulating volume (for example, with solutions based on isotonic sodium) with treatment with hypertonic sodium solutions, whose benefit in shock probably rests on pharmacological properties other than their minuscule and transient impact on circulating volume.

Secondly, the review contains no trials from species routinely treated for haemorrhage by veterinary surgeons. There is a substantial literature on the use of hypertonic saline to treat haemorrhage in animals, notably cats, dogs, and horses. Treatment of haemorrhage is rare in sheep and exceedingly so in pigs.

The review makes the usual assumption that research in animals is synonymous with studies in laboratory animals, mainly rats. The transected tail model in rats is particularly misleading as the transection of the major artery is likely to predispose to rebleeding once arterial pressure starts to rise. This may be relevant to some forms of clinical haemorrhage, but I doubt if it is a good model for most. I also doubt that studies with controls that have been denied any fluid resuscitation after haemorrhage are scientifically or ethically justifiable nowadays.

Comparative medicine reminds us that humans are not unique; they are animals of a particularly interesting type. Only when we realise this can we start to think sensibly about questions such as how much salt mammals need and how much is detrimental, and why some animals are susceptible to extremely high blood pressure yet resistant to its damaging effects. We can also capitalise on the fact that spontaneous tumours in animals, equivalent to those in humans, may be a useful intermediate step—between tumours induced in experimental rodents and tumours in human patients—in evaluating new treatments. Awareness of such opportunities is too low in the medical community.