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How to evolve towards harmlessness

What if we could convince pathogens to evolve to the point of harmlessness? Professor Paul Ewald thinks we can, and he’s got some interesting research to prove it.

The key is to think like a bacterium. If you’re not mobile on your own, you need your host to move you around… which makes it a very bad idea to kill your host. As a result, bacterial infections transmitted person to person have an evolutionary bias towards not killing off their hosts. Bacteria that can get around other ways – for instance, waterborne diseases like Cholera – don’t have this disincentive. It turns out there’s a strong correlation between waterborne pathogens and their chance of killing their host.

So… could we make Cholera less toxic? Turns out you can – if you can prevent it from spreading by water. Obviously this isn’t an experiment you can plan – but sometimes you stumble onto one. Peru had a cholera outbreak in 1991 – shortly after, cholera made it to Chile, which has an excellent sewage system. The toxicity of cholera in Chile fell sharply – it became more mild because it couldn’t spread via the water supply.

This might work with malaria as well – if malaria can’t transmit via a vector (mosquitoes), perhaps it becomes more mild. Ewald shows data from the Tenesse River authority pre-WWII. Damning rivers led to a malaria outbreak in northern Alabama. Without antimalarial drugs, the TVA responded by mosquito-proofing houses. Malaria died out entirely between 1934 and 1941. “If you have moderate biting densities, you eradicate mosquitos by mosquito-proofing houses.” This won’t work in a high-biting density like Nigeria, but could work in other countries. And it’s got benefits over introducing anti-malarial drugs, which lead to drug resistance. Anti-malarials are a short-term strategy – evolving away from toxicity is a long term solution.