A new deadlier strain of Salmonella is spreading across Africa by "taking advantage" of the ongoing HIV epidemic, according to a new study that provides the first evidence that the human immunodeficiency virus might be allowing new pathogens to evolve in HIV patients.

The latest findings, published in the journal Nature Genetics, show that the weakened immune systems caused by HIV allowed the new opportunistic Salmonella strain to spread. People usually contract salmonella from eating contaminated meat, which typically results in an unpleasant but brief period of symptoms that include diarrhea, fever and abdominal cramps that develop 12 to 72 hours after infection that lasts between four to seven days.

Researchers say that the bacterial infection is self-limiting, and tends to be fatal in less than 1 percent of those infected, but the new strain, called invasive, non-typhoidal salmonella (iNTS), is fatal in up to 45 percent of cases. So far, one in four people in Africa infected with the new strain has died.

While there are hundreds of different versions of Salmonella, the epidemic of the newly evolved Salmonella strain is thought to be the first time a bug has spread so widely in the emergence of HIV.

This strain of Salmonella, which causes symptoms like fever, headaches, breathing problems and sometimes death, has been recognized in Africa for more than a decade but it wasn't until now that scientists have realized that they were all part of a single epidemic causing deaths across the continent.

A group of international researchers sequenced the DNA of the 179 samples of the new Salmonella strain, and traced the spread of the disease by analyzing how closely related bacteria strains in different countries were. They found that nearly all of the strains analyzed belonged to one of two closely related lineages.

The first lineage had emerged in Malawi in 1960 and swept across eastern and central Africa in the 1980s and the second, more recent lineage arose in central Africa, in the Congo Basin, in the late 1970s and spread in distinct waves across sub-Saharan Africa.

Researchers found that the newer strain of bacteria seems to have replaced the first because it is resistant against chloramphenicol, the frontline antibiotic widely used to treat the older, first lineage of Salmonella.

More importantly, researchers said that the timing and the locations of the spread of the bacteria closely coincide with the spread of HIV across the continent.  

"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," researcher Dr. Robert Kingsley, of Wellcome Trust Sanger Institute, said in a statement.

Scientists explained that before HIV became rampant, iNTS mostly affected children, who rarely travelled. However, HIV created adult iNTS carriers who travel more, and now the newer salmonella strain dominates these infections across Africa.

HIV attacks the immune system and leaves its victims more vulnerable to other infections, leading researchers to believe that the strain took advantage of this weakness and spread. "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," researcher Chinyere Okoro, of the Wellcome Trust Sanger Institute, said in a statement.

The current example of the second wave of Salmonella represents more than a case of one set of bacteria taking over from another. 

The latest findings suggest that the salmonella strain associated with HIV may also be adapting to people. Researchers said that another version of Salmonella, which causes typhoid fever, appears to have completely adapted to humans, to the extent that people are getting infected from other people rather than catching if from the environment.

Researchers say that the African strain of iNTS seems to be evolving like the typhoid bacteria, and may also be spreading between people.

"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," lead author Professor Gordon Dougan concluded.