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Can We Domesticate Diseases?

Some parasites, bacteria and viruses are worse than others – that much is obvious. But why are some worse? Why do some kill the very host that they rely on for survival while others just cause minor inconvenience? It’s not an easy question to answer, and each case is different.

The many cold viruses, for example, seem to have found a happy medium. They don’t kill us or even greatly incapacitate us. That means that we are free to be up and around and spreading the microbes. It’s a good strategy from the perspective of the cold virus.

On the other hand, there are microbes that do more harm. Malaria, for example, doesn’t rely on us moving around in order to infect other people. From the perspective of its survival and reproduction it's better that we are incapacitated. That allows mosquitoes to feed on our blood and spread the disease.

Flickr photo by Umberto Salvagnin

Cholera too finds advantages in incapacitating us. Specifically it benefits from making us very ill and prone to diarrhea. That allows it to spread by way of unprotected water supplies and sewage systems. Even the clothes of the ill person can spread the infectious microbes to water supplies when washed.

There are many means of transmission for the various disease organisms. But those with the best transmission systems are likely to be the most harmful, since they don’t need us to be moving around to infect others. So how do we deal with these virulent microbes?

The typical response has been stronger and stronger antibiotics and other such “direct attacks.” The problem with that is that we are helping these diseases become stronger and more resistant to the very drugs that are meant to save us. Is there another way?

Evolutionary biologist Paul Ewald thinks there is. He suggests that we might foster evolutionary changes in disease organisms to make them less harmful. To do so we have to create evolutionary pressure against harmfulness or virulence. How is that possible?

A clue comes from cholera outbreaks that spread across South and Central America for several years, starting in 1991. In countries that had poorly protected water systems the bacterium became more virulent and killed a higher percentage of infected people as it spread. In countries that had better water systems the disease actually became less dangerous and less deadly. This, Ewald’s research suggests, is evolution in action.

Those cholera bacterium that caused less harm were selected for as the disease spread in areas with protected water supplies. They survived because the infected person could more easily be up and around to spread the disease. The more virulent strains more quickly died off since they had no means of transmission.

Consider the implication of this. If we were able to block the normal transmission routes of some deadly diseases, only the less harmful strains would reproduce. Think of it this way: If you could have a cholera strain that was no worse than a cold you might not even know you had it, so you could freely spread it. Soon – and we’re talking about only a few years potentially due to the rate of reproduction and therefore evolution in microbes – the innocuous strain might replace the more virulent ones entirely.

What Ewald is suggesting is that we take control of the evolution of microbes. He says we would be “essentially domesticating those disease organisms,” and adds that “With a mild version, most people won’t even know they’re infected.” If for example, every malaria patient was protected (by netting or staying indoors) from mosquitoes, the protozoa that causes malaria (P. falciparum) would face severe evolutionary pressure. Specifically, to survive and thrive it would need to change to a less virulent form that allowed the host to remain mobile rather than incapacitated, so that there was opportunity for that host to spread the disease.

Attacking disease organisms directly has sometimes made them worse. We actually apply evolutionary pressure to make them more deadly. But if we take control in various ways to change them into more benign forms, formerly deadly diseases may join the ranks of the many microbes that happily exist in our bodies without doing us any real harm. Now there’s a great new idea.

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Domesticating Diseases