Can We Domesticate Germs? Harnessing Evolution to Fight Disease

Imagine a world where we could control the very evolution of disease-causing organisms, turning them from deadly foes into harmless neighbors. Evolutionary biologist Paul Ewald explores this fascinating concept, suggesting that by understanding how germs evolve, we can manipulate their behavior to our advantage.

The Germ's-Eye View of Disease

At the heart of Ewald's argument is the idea that we need to consider disease from the perspective of the germ itself. Germs, like all living things, are driven by the need to reproduce and spread. Their harmfulness, or virulence, is often a consequence of how they achieve this goal.

• Germs need to move from one host to another.
• Sometimes, they rely on the host's well-being to facilitate this movement.
• Other times, they don't need the host at all.

When germs don't need a healthy, mobile host to spread, natural selection favors those that are more exploitative and damaging. Think of it as a predator-prey relationship – the germ benefits from taking advantage of the host, even if it causes harm. However, when germs do rely on host mobility for transmission, milder strains tend to thrive. This is because a healthy, active host is more likely to spread the germ to new individuals.

Diarrheal Diseases: A Case Study

Diarrheal diseases offer a compelling example of this principle. These diseases can spread in three main ways:

1. Person-to-person contact
2. Person-to-food-to-person contact
3. Waterborne transmission

Waterborne transmission is particularly interesting because it doesn't require the host to be healthy. A person sick in bed can still contaminate water sources, infecting many others. As a result, the theory predicts that diarrheal disease organisms transmitted through water should be more harmful.

Evidence in Action

Data supports this prediction. Studies of diarrheal bacteria show a strong positive correlation between the degree to which they are transmitted by water and how harmful they are. This suggests that by blocking waterborne transmission, we could potentially drive the evolution of milder strains.

Domesticating Germs: A Real-World Experiment

The key question is: how quickly can this evolutionary shift occur? If it takes thousands of years, it's not a practical solution. But if it can happen in just a few years, it could be a game-changer.

Ewald highlights the case of Vibrio cholerae, the bacterium responsible for cholera. This organism produces a toxin that causes severe diarrhea, flushing out competitors and facilitating its own spread. The idea is that if we could prevent waterborne transmission, we could force Vibrio cholerae to rely on person-to-person contact, favoring milder strains that don't incapacitate their hosts.

The Peru Cholera Outbreak

In 1991, a cholera outbreak struck Lima, Peru, and quickly spread to neighboring areas. This unfortunate event provided a natural experiment. Ewald and his team studied Vibrio cholerae strains from different countries in the region:

• Chile: Known for its well-protected water supplies.
• Ecuador: Characterized by less protected water supplies.
• Peru: Situated somewhere in between.

The results were striking. In Chile, Vibrio cholerae strains evolved to produce less toxin within just a few years. By 1995, cholera was largely controlled in Chile. In contrast, in Ecuador, the strains appeared to become more harmful, likely due to the prevalence of waterborne transmission. This provides strong evidence that controlling transmission routes can indeed influence the evolution of virulence.

The Antibiotic Resistance Connection

Controlling the evolution of virulence could also have a significant impact on antibiotic resistance. Highly virulent organisms are more likely to cause symptomatic infections, leading to increased antibiotic use and, consequently, increased antibiotic resistance. By promoting the evolution of milder strains, we could potentially break this vicious cycle.

A Tale of Two Countries

Again, the data from Chile and Ecuador support this idea. In the early 1990s, both countries showed variation in antibiotic sensitivity among Vibrio cholerae strains. However, by the end of the decade, Ecuador was experiencing a growing antibiotic resistance problem, while Chile maintained antibiotic sensitivity. This suggests that Chile not only dodged the bullet of increased virulence but also avoided the rise of antibiotic-resistant strains.

Malaria: A New Frontier

Ewald's principles extend beyond diarrheal diseases. He proposes a similar approach to tackling malaria, a mosquito-borne illness. The key intervention here is mosquito-proofing houses. By preventing mosquitoes from biting sick individuals confined to their homes, we can create an environment that favors milder strains of the malaria parasite.

The Alabama Experiment

A historical experiment in northern Alabama provides compelling evidence for this approach. In the 1930s, the Tennessee Valley Authority dammed the Tennessee River, creating stagnant water and a surge in mosquito populations. As a result, a significant portion of the population was infected with malaria. The solution? Mosquito-proof every house in the region.

The results were dramatic. Within three years, mosquito-proofing alone led to the eradication of malaria in northern Alabama. This demonstrates the power of controlling transmission routes to influence the evolution of disease.

A New Paradigm for Disease Control

Ewald's work suggests a paradigm shift in how we approach disease control. Instead of simply battling pathogens with drugs and vaccines, we can harness the power of evolution to make them less harmful. This involves understanding the transmission dynamics of each disease and implementing interventions that favor the survival and spread of milder strains.

This approach offers several advantages:

• Long-term solutions: By influencing the evolution of pathogens, we can create lasting changes that are less susceptible to resistance.
• Synergistic effects: Interventions that promote milder strains can also reduce the spread of disease and the development of antibiotic resistance.
• Justifiable investments: Many of the interventions that promote the evolution of milder strains, such as providing clean water and mosquito-proofing houses, are also beneficial for public health in general, making them easier to justify economically.

By embracing an evolutionary perspective, we can unlock new and powerful strategies for controlling infectious diseases and creating a healthier future for all.