The web of infectious diseases – 06/17/2023 – Reinaldo José Lopes

Colleague Claudinei Queiroz’s beautiful report on the relationship between environmental changes in São Paulo and the current outbreak of spotted fever in the Campinas region is one of the inspirations for this column. In his text, he shows how deforestation, the extermination of large carnivores and the intensive cultivation of sugarcane and maize near water courses led to the growth of the capybara population and, it seems, to an increased risk of transmission of the disease through the ticks that these large rodents carry.

At least for the time being, there is no reason to imagine that this process ends up transforming Rocky Mountain spotted fever into a generalized public health problem. The chances of direct contact with the invertebrates that carry the bacteria that cause the disease will continue to be relatively very low in the case of a population that is already as urbanized as ours. And it is to be expected that doctors and patients will be more alert to symptoms and risk situations, quickly resorting to antibiotics and avoiding the worst.

Even so, the tragic deaths of recent days are extreme cases of a much broader phenomenon. To try to explain it, I’m going to risk a mixture of Charles Darwin and Ariano Suassuna (although the late playwright from Paraíba wasn’t a big fan of the theory of evolution, unfortunately).

In his classic “O Auto da Compadecida”, Suassuna describes death as the force that “equalizes everything that is alive in a single herd of condemned people”. Well, the micro-organisms or viruses that cause infectious diseases often do not kill their victims—some even say, somewhat simplistically, that a “smart” microbe is one that learns not to kill its hosts. But it makes sense to apply Suassuna’s definition to them: whenever possible, they end up working like this web that connects different living beings into a single herd, jumping from branch to branch along the Tree of Life.

And it is here that Darwin takes the stage of our self. If we think that microbes — such as the Rocky Mountain spotted fever bacteria — are nothing more than reproductive machines (which, of course, is true for everything that is alive), the relationship they develop with their most common hosts, such as ticks and capybaras, involves finding the balance between continuing to reproduce and spread, on the one hand, and preventing this process from damaging its victims to the point of preventing the microbes from multiplying further.

However, sometimes it happens that an organism quite different from the usual hosts ends up presenting itself to the microbe or virus – a human being who has passed through the patch of bush where a flock of capybaras had previously lain down, say. In these cases, the line between an evolutionary opportunity (the chance to successfully colonize a new type of organism and be even more successful in reproducing) and a dead end is blurred.

In the case of Rocky Mountain spotted fever, the blind alley has prevailed. For tick bacteria, we are a mere deviation from the route, and this may explain in part the aggressiveness of the disease in our body: there was not enough coexistence for the bacteria to adapt to spending more time, with more subtle symptoms, inside the body of ticks. a human being.

It turns out that messing up the natural web of long-term relationships between disease-causing and hosts is one of the recipes for generating new pandemics, similar to what, by all indications, happened in the case of Covid-19. Here is yet another selfish, but at least sensible, argument to curb the destruction of the country’s remaining natural environments. Continuing like this could bring even more uncomfortable visitors into our cells, perhaps far more skilled in the art of adapting to their new home.

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