Evidence for Tolerance
We believe the vast majority of scientists working on animal diseases would have little disagreement with the concept of tolerance (although they may disagree with the semantics; see Box 1).
This is because a considerable body of data is anecdotally consistent with tolerance in animals [7].
For instance, unlike sickle cell anaemia in humans, which is a classical resistance factor because it reduces malaria parasite densities, α+-thalassaemia (another heritable blood disorder) is very likely a tolerance factor.
It does not affect malaria parasite densities, but is nonetheless associated with reduced incidence of “severe” (life-threatening) malaria [8].
But to generate a science of tolerance, the phenotype needs to be clearly measurable, independently of resistance.
It is difficult yet to say whether α+-thalassaemia is definitely a tolerance factor, because there are several factors that may generate spurious variation for tolerance.