Long shadow of the cholera epidemic

Sir, – Writing about poets lost prematurely to cholera in the 19th century, Tom Delaney concludes with a salutary reminder that cholera is not merely of historical interest (An Irishman’s Diary, June 27th). Millions of cases still occur each year, in countries often beset by many other challenges.

An interesting curiosity of the disease in Ireland today is the genetic legacy of the survivors as much as the stories of those who succumbed to it in the past.

A useful analogy may be made with malaria. The causative pathogen of malaria lives within red blood cells. Individuals with abnormalities of red cell shape are both less likely to acquire that infection and, when they do, it tends to be milder.

Due to the cumulative effects of evolution, sickle-shaped blood cells became common in areas where malaria is endemic. Those with one gene for the condition fared well in such regions and are ubiquitous throughout Africa.

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Unfortunately those with two copies in the natural world do less well, as the cells tend to clump together during illness, often with disastrous consequences.

There are also genetic variations that protect against the worst ravages of cholera. They control the flux of sodium and chloride ions, along with water, across biological membranes. Abnormality in one inherited gene for this appears to give a better probability of surviving cholera, but if inherited from both parents the phenomenon can lead to cystic fibrosis. This illness is far more common in Ireland than most societies. It is most likely that the excessive frequency of CF in Ireland is a legacy of multiple outbreaks of such illnesses in past millennia.

Perhaps most encouragingly, sickle cell disease has recently become treatable using bone marrow transplants and gene-editing technologies. This is both a basis for hope to those affected globally but also a further illustration of the inequality of humanity when faced with illness as the treatments remain costly and available to very few. – Yours, etc,

BRIAN O’BRIEN,

Kinsale,

Co Cork.