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Deadly New Salmonella Spreads In Wake Of HIV In Africa

A new deadly form of Salmonella is spreading in sub-Saharan Africa. Now a new study suggests the rapidly evolving invasive intestinal disease may be following a wake created by other disease epidemics such as HIV and malaria, as it takes advantage of immune systems weakened by them.

The study authors report their findings in the 30 September online issue of Nature Genetics. Joint first author Chinyere Okoro, from the Wellcome Trust Sanger Institute in Cambridge, UK, says in a press statement:

"The immune system susceptibility provided by HIV, malaria and malnutrition at a young age, may provide a population in sub-Saharan Africa that is large enough for this detrimental pathogen to enter, adapt, circulate and thrive." Whole-Genome Sequencing Reveals More Typhoid-Like Strain In their paper, Okoro and colleagues report how they tracked the spread of human invasive Salmonella Typhimurium, in two waves from different focal hubs in Southern and Central Africa beginning about 52 and 35 years ago, respectively. The bacterium causes non-Typhoidal Salmonella (iNTS), a disease with a significant mortality rate of up to 45%.

"The HIV epidemic in sub-Saharan Africa is thought to have begun in a central region and underwent expansion eastwards, a strikingly similar dynamic to that observed for second iNTS wave," says Robert Kingsley, joint first author and also of the Sanger Institute.

In other parts of the world, iNTS causes acute inflammatory diarrhoea and is self-limiting, with a mortality of less than 1%. But in sub-Saharan Africa, the disease is more severe, probably because most of the people who catch it are malnourished or already ill with malaria or HIV, and because of the new strain.

The new Salmonella strain behind the disease is also more typhoid-like type, as Okoro explains:

"We used whole genome sequencing to define a novel lineage of Salmonella Typhimurium that is causing a previously unrecognised epidemic across the region. Its genetic makeup is evolving into a more typhoid-like bacteria, able to efficiently spread around the human body."

Typhoid fever is caused by a different strain of Salmonella called S. typhi.

This study is the first to use whole-genome sequencing on iNTS. Drug-Resistant Genes To define the new strain of Salmonella Typhimurium, the researchers applied sequencing and phylogenetic or "family-tree" methods to sub-Saharan African samples and compared them to samples from the rest of the world.

They suggest a factor in its successful spread is the bacterium has acquired genes that make it resistant to several front-line drugs.

For instance, one such gene makes the bacterium resistant to Chloramphenicol, a front-line antibiotic in the treatment of Salmonella. The vast majority of samples from people infected in the second wave of iNTS contained this gene.

The researchers say the gene was not present in the samples from the first wave of iNTS, which would imply that the bacterium acquired it early in the evolution of the second wave.

Lead author Gordon Dougan, a professor with the Sanger Institute, says:

"Because it acquired resistance to chloramphenicol, this pathogen has much greater opportunity to survive and spread across the region."

"There has been some evidence that this disease can be passed from human to human. Now the race is on to discover how NTS is actually transmitted in sub-Saharan Africa so that effective intervention strategies can be implemented," he adds.

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