Photo: Reuters
Photo: Reuters

A ride on the evolutionary treadmill

In a real sense then, public health is like a constant treadmill. So far, we have stayed one step ahead of disease by the constant discovery of new drugs

On a quick trip to Delhi not long ago, I found plenty of people talking about chikungunya. Not surprising, because there’s been what looks like a sudden upsurge in cases of the disease in that city. Almost everyone I spoke to knew of someone suffering from it—sometimes even themselves—ruminating ruefully about the severe joint pains it produces.

Chikungunya has been known for over 60 years, though it’s really in the last decade that we have seen large outbreaks in different corners of the world. Such outbreaks happen because the organism responsible either shows up where it was previously unknown, or evolves to defeat barriers to its spread. There is evidence that by 2005, the chikungunya virus had evolved to use a particular species of mosquito as a carrier. That species took the disease to new parts of the world.

I’m reminded of a dramatic video I saw recently, in which bacteria encounter ever higher levels of deadly (to them) antibiotics, but evolve to survive and spread.

I’m also reminded of my brother, who had an attack of malaria while working in rural Odisha some years ago. It was frightening because he developed very high fever and convulsions. The disease did not respond to treatment with chloroquine—then a standard prescription for malaria. So he used two other drugs, sulfadoxine and pyrimethamine, and eventually recovered.

But his experience was a glimpse into the perpetual battle modern medicine fights to control disease. Why was the malaria resistant to chloroquine?

The answer lies in Charles Darwin’s theory of evolution. In essence, the theory says that as species evolve, they retain and develop the characteristics which promote their survival and reproduction. This also means that they suppress the characteristics that retard survival and reproduction.

In India, malaria comes in two main strains, caused by two different microscopic parasites that mosquitoes carry: vivax and falciparum. The falciparum strain can affect the brain; when that happens, it is called cerebral malaria. This is what my brother suffered.

Chloroquine used to be an effective treatment against the falciparum parasite, and was thus heavily used across the world. But that use itself laid the seeds of its eventual ineffectiveness. Certainly, every use of chloroquine killed most of the falciparum parasites that were exposed to it. But some managed to survive. When these hardy survivors reproduced, they passed on to their descendants whatever characteristics had helped them survive. Over several generations, these characteristics got strengthened, and an ever greater fraction of the parasites had them. Eventually, a strain of falciparum—a mutant strain, if you like—appeared that was totally resistant to chloroquine.

And that’s the strain that worked its way, via a bite from a thirsty Odisha mosquito, into my brother’s body and brain.

What happened to falciparum is a perfectly natural process, with simple and inexorable logic. It’s called evolution, of course, and it happens to every species on the planet, including humans. For example, archaeological evidence shows that we are today a taller, stronger race than we were when we first stood upright and walked out of East Africa. For whatever reason, evolution has done that to us, working on us over many generations.

There is, however, one crucial difference between us and falciparum, and that takes us to the heart of the tussle between disease and medicine. Since each human generation lasts 25-30 years, evolution for us is a slow process. But falciparum lives and breeds in time scales measured in minutes (all micro-organisms do). Generation follows generation at breakneck speed. Naturally, evolution among these parasites also proceeds at breakneck speed. Traits that contribute to survival—here, the resistance to chloroquine—are passed on and reinforced swiftly. In some cases, it can be just a few weeks before resistance begins to appear.

Evolution has turned around to bite us—literally so, in the case of malaria and chikungunya, and the mosquitoes that carry their parasites. Whenever a new drug to treat a disease is discovered, it’s only a matter of time before the disease no longer responds to it (for that matter, following the same logic, it’s also only a matter of time before mosquitoes evolve to resist the pesticides we use to kill them today).

In a real sense then, public health is like a constant treadmill. So far, we have stayed one step ahead of disease by the constant discovery of new drugs. But it is a precarious tightrope we walk. Who knows when we will lose the advantage of being a step ahead? Here in India, we are already faced with a strain of tuberculosis which is resistant to every known drug. Why shouldn’t that happen with malaria?

All of which is why, in the long run, prevention and precaution are better bets than treatment. Among other things, that means the cleanliness that deprives mosquitoes of places to live and breed. Don’t fling garbage out unthinkingly, don’t let water accumulate, that sort of thing.

Come to think of it, it might serve us well to pass on those ideas to our descendants too.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. A Matter of Numbers explores the joy of mathematics, with occasional forays into other sciences.

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