Many disease outbreaks caused by environment, not evolution

Many bacterial epidemics in human history might be due to environmental changes rather than the evolution of virulent pathogens, a new study suggested Monday.
The finding, published in the U.S. journal Proceedings of the National Academy of Sciences, was based on an analysis of 149 genomes of Salmonella enterica serovar Paratyphi A, which is a major cause of enteric fever.
Enteric fever is currently estimated at 27 million clinical cases each year, resulting in 200,000 deaths.
"When epidemics break out, many scientists suspect they have been driven by increased virulence or fitness, possibly associated with the gain of novel genes or mutations," lead author Zhemin Zhou from the University of Warwick in Britain said in a statement.
"We wanted to trace genetic changes in a prominent bacterial pathogen back to see if this was true," he said.
Using phylogenetic methods and statistical models, the researchers found Paratyphi A originated at least 450 years ago, and had not changed dramatically over the centuries.
This suggests that the pathogen had not become more efficient at causing enteric fever, they said.
"We found the pathogen formed seven distinct lineages that spread globally since the mid-19th century. Tracing the pathogen, we found there were genetic mutations that may have transiently improved drug resistance or improved the metabolic efficiency," Zhou said.
"However, most mutations were short-lived and removed by evolutionary forces."
The researchers interpreted the history of Paratyphi A as " reflecting drift rather than progressive evolution."
"Most recent increases in frequencies of bacterial diseases are due to environmental changes rather than the novel evolution of pathogenic bacteria," they wrote in their paper.