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? Its 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 dont kill us or
even greatly incapacitate us. That means that we are free to
be up and around and spreading the microbes. Its a good
strategy from the perspective of the cold virus.
On the other hand, there are microbes that do more harm. Malaria,
for example, doesnt 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
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 dont need us to be moving
around to infect others. So how do we deal with these virulent
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
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,
Ewalds 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 were 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 wont even know theyre 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 theres a great new idea.