A new study out today (Sunday 30 September) reveals that the emergence and spread of a rapidly evolving invasive intestinal disease, that has a significant mortality rate (up to 45%) in infected people in sub-Saharan Africa, seems to have been potentiated by the HIV epidemic in Africa.
The team found that invasive non-Typhoidal Salmonella (iNTS) disease is caused by a new form of the bacteria Salmonella Typhimurium that has spread from two different focal hubs in Southern and Central Africa beginning 52 and 35 years ago, respectively. They also found that one of the major contributing factors for the successful spread of iNTS was the acquisition of genes that afford resistance to several front line drugs used to treat blood-borne infection such as iNTS.
iNTS is a blood-borne infection that kills approximately one of four people in sub-Saharan Africa who catch it. Yet, in the rest of the world, NTS is a leading cause of acute inflammatory diarrhoea that is self-limiting and tends to be fatal in less than 1 per cent of people infected. The disease is more severe in sub-Saharan Africa than the rest of the world because of factors such as malnutrition, co-infection with malaria or HIV and potentially the novel genotype of the Salmonella bacteria.
"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," says Chinyere Okoro, joint first author from the Wellcome Trust Sanger Institute. "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"
From sequenced samples, the team created a phylogenetic or 'family tree', depicting the pathogen's evolution, dating when each sample first emerged and overlaying this with geographical information about where these samples came from. They found that this invasive disease comprises of two very closely related waves; the first wave originated from a possible south-eastern hub, about 52 years ago and the second originated about 35 years ago, possibly from the Congo Basin.