Inharmonious coexistence

Bernard Dixon¹ attempts to construct a thesis that is seriously flawed, as are several of the supporting arguments.

While biological systems may tend towards a dynamic equilibrium between species, such processes always have, and presumably always will, be punctuated by sudden and unexpected perturbations. In the case of infectious diseases in human beings the most obvious examples are when microbes successfully jump species–quite probably critical events in the emergence of HIV and, most recently, severe acute respiratory syndrome (SARS). There is no reason to expect that such events will stop happening in the future or that we will be able to predict or prevent them, whatever precautions we decide to take in food preparation and agriculture.

In choosing tuberculosis as one of his two selected infections that have been profoundly affected by the arrival of antibiotics and vaccines, Dixon makes a strange choice. The effect of BCG on tuberculosis incidence is, at best, modest and the disease worldwide goes from strength to strength despite best chemotherapeutic efforts. By contrast, vaccines have had major effects on the incidence of not only diphtheria, but also tetanus, pertussis, polio, haemophilus and meningococcus C meningitis, measles, mumps, and congenital rubella, whereas improving nutrition and living conditions has probably affected these diseases little or not at all–although these are probably the most effective means to combat tuberculosis. The implication that the achievements of immunisation pale into insignificance alongside the impact of improvements in nutrition and sanitation is seriously misleading. Benefits of childhood vaccines can and already have reached areas where efforts to improve living conditions have not.

If there is truth in the hypothesis that increased incidence of autoimmune and allergic disorders are the price of fewer infections, this is certainly the consequence of the very same socioeconomic improvements (known as the hygiene hypothesis) and not due to “the crusade to eliminate infections”. Indeed, there are both hypothetical reasons and direct evidence to suggest that increasing antigen exposure through immunisation in childhood might be protective against such cleanliness-associated problems.

The only assertion Dixon makes with which I agree is that there is a future for probiotics, but infectious diseases are not about to go away.