Contagious pathogens spread and evolve in those with suppressed immune systems.
Nguyen Huy Kham/Reuters
PROBLEM: Salmonella, which we typically contract from tainted food and experience as unpleasant but not extraordinarily threatening, poses a much greater risk to people with HIV. Their suppressed immune systems allow a specific strain, Salmonella typhimurium, to infect their blood and cause invasive, non-typhoidal salmonella (iNTS), a far more serious disease with a death risk of up to 45 percent. In sub-Saharan Africa, a highly invasive form of iNTS has developed; researchers are attempting to understand how and why the epidemic is spreading.
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METHODOLOGY: Researchers in the UK sequenced the DNA of Salmonella typhimurium from populations with iNTS in Africa, and compared this information to global populations, using "family trees" to trace the bacteria's evolution.
RESULTS: Almost every sample of the bacteria from all over Africa was found to belong to one of two closely related lineages. One form appears to have emerged 52 years ago. The other, which replaced it, is 35 years old. Chronologically and location-wise, both are associated with the HIV's spread across the continent. The second likely emerged after becoming resistant to the drugs used to treat the first.
CONCLUSION: The iNTS epidemic is caused by a strain of salmonella that has been undergoing genetic changes that coincide with the HIV epidemic, and that appears to be adapting to its humans carriers and to our efforts to treat it.
IMPLICATIONS: Ultimately, Salmonella typhimurium, like the form of salmonella that causes typhoid fever, may become directly transmittable between humans -- if it hasn't already -- making iNTS a contagious disease. This wasn't able to happen before because iNTS mainly affected young children, but HIV, by comprimising the immune systems of more mobile adults, allowed the bacteria to spread across Africa. Further research is needed to understand whether the suppressed immune systems of people with HIV or resistance to antibiotics -- or a combination of both -- is the cause of this evolution. It may be that all types of infectious diseases have this ability to become more potent as they spread through large populations of people with compromised immune systems, giving us yet another reason to do everything possible to stop HIV's spread.
The full study, "Intracontinental spread of human invasive Salmonella typhimurium pathovariants in sub-Saharan Africa," is published in the journal Nature Genetics.