We poor humans are vulnerable to so many ills: Each year, for instance, an estimated 21.5 million people develop typhoid fever, a subset of enteric fever, which affects a total of 27 million people and results in about 200,000 deaths annually. Having analyzed the genome of the pathogen causing typhoid fever, University of Warwick researchers determined that epidemics are more likely to be caused by chance environmental changes, such as spread to a new geographic area, rather than by genetic mutations creating a more virulent strain of bacteria.

What is enteric fever?

Enteric fever is called typhoid fever when it is caused by one type of bacteria and called paratyphoid fever when caused by three other types of bacteria. Both fevers, which are common in the developing world, cause diarrhea, high temperatures, and abdominal cramps. The illness usually lasts four to seven days, and most people recover without treatment. To avoid typhoid fever, most travelers venturing into the third world will get a vaccination shot and avoid eating potentially contaminated foods, such as local water and raw or improperly cooked food. While typhoid fever is not particularly lethal, outbreaks occur from time to time, leading many scientists to wonder: What causes a sudden epidemic?

One theory says this is due to genetic changes in the bacterium itself. "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,” Zhou explained. To examine this hypothesis, Zhou and his colleagues decided to trace genetic changes in one bacterial pathogen all the way back to its origins. For the current study, then, the team of researchers isolated and analyzed a full 149 genomes of Salmonella enterica serovar Paratyphi A, a major cause of typhoid fever.

After reconstructing the genealogy, transmission history, and evolution of this bacterium, the team concluded Paratyphi A originated at least 450 years ago. The pathogen remained surprisingly similar over the centuries, the researchers discovered, which suggests it had not become more efficient at causing enteric fever over time. "This implies that many epidemics and pandemics of bacterial disease in human history reflected chance environmental events, including geographical spread and/or transmission to naïve hosts, rather than the recent evolution of particularly virulent organisms,” said Dr. Mark Achtman, Warwick Medical School and senior author of the study.

While a mutation may make the bacteria more resistant to a given drug for a brief spell, then, most mutations are "short-lived and removed by evolutionary forces,” Zhou said. In the end, there's no deeper science involved in epidemics and for those who like to travel far and wide, fever outbreaks be damned, the best way to avoid typhoid fever is simply by getting the proper shots and not eating risky foods.

Source: Zhou Z, McAnn A, Weill FX, et al. Transient Darwinian selection in Salmonella enterica serovar Paratyphi A during 450 years of global spread of enteric fever. PNAS. 2014.