14 DAY TRIAL // Typhoid fever, a disease that affects 21 million people and kills 200,000 worldwide annually, is provoked by a bacterium called Salmonella typhi (photo).
This disease should not be confused with typhus, which is provoked by bacteria from Rickettsia genus, and is transmitted to humans from animals by fleas, mites and lice.
Tracing the evolutionary history of Salmonella typhi, scientists found that asymptomatic carriers played a main role in the evolution and global transmission of Salmonella typhi.
Typhoid fever is a major health threat in the developing countries and still keeps a hold on in Europe and North America.
The genetic evolution of this bacterium was difficult to assess, as it presents little genetic diversity.
But, as more knowledge accumulate in the domain, with Yersinia pestis (plague bacterium), Escherichia coli (which sometimes inducts food poisoning), Helicobacter pylori (which provokes ulcers) and Neisseria meningitidis (which causes meningitis) genomes deeply investigated, scientists got the bases to investigate the evolution of this pathogen.
The international German-British-Vietnamese team managed to assemble for the first time 105 strains from the whole world and analyzed the sequence diversity within 90,000 base pairs in each strain.
This massive approach detected 88 differences in the DNA sequence, revealing a diverging population evolution over the last 10,000 to 43,000 years.
Surprisingly, the ancestral strain still exists, but intermediate genotypes to other current strains vanished due to selection's power.
The bacterial strains are not localized but distributed globally, proving that Salmonella typhi has spread inter-continentally on multiple occasions.
This is only because of the long-term transport by asymptomatic healthy carriers, which also allowed Salmonella typhi to resist in hunter-gatherers populations prior to the Neolithic expansion of city states.
Because of the healthy carriers, the bacterium's different strains persist for many decades within each country. Antibiotics have turned ineffective in recent decades in efforts of curing typhoid fever.
20 years of fluoroquinolones treatment, especially nalidixic acid, in Southeast Asia, led to the emergence of multiple resistant strains, of which one, H58, has multiplied dramatically and spread worldwide, especially across South Africa and Africa.
In Vietnam, up to 95% of Salmonella typhi strains are now immune to former antibiotics and the newer antibiotics are much more expensive than standard fluoroquinolones.
And most probably, the bacterium will turn resistant to these antibiotics as well.
In fact, indiscriminate antibiotic usage provokes the evolution of treatment resistant bacteria.
The healthy carriers ensure a safe reservoir for these bacteria, helping them to elude short-term antibiotic treatment and vaccination, indicating that typhoid fever will remain a major health problem for the foreseeable future, as simple treatment of the disease will not suffice to eradicate this bacterium.