Non-typhoidal Salmonella (NTS) are a common cause of bacteremia in Africa, affecting mainly infants and HIV infected adults. Host risk factors such as malaria, malnutrition, sickle-cell disease and HIV, which are prevalent in Africa, are associated with increased susceptibility to invasive disease. Effective measures are needed to control this disease that kills 1 in every 5 individuals who are admitted to hospital with it. Improved water and sanitation practices, and vaccination are proposed as possible control mechanisms. However, three factors make it difficult to determine the best control strategy and the most effective implementation methods: (i) a lack of understanding of NTS transmission (routes and reservoirs); (ii) a lack of understanding of the transmission intensity or force of infection; and (iii) the focus of the disease in high-risk groups.
Interestingly, the burden of invasive NTS (iNTS) disease has been noted to decrease in parallel with decreasing malaria prevalence. The mechanisms underlying this relationship are not well understood. It is not known whether this represents a reduction in transmission of the disease in the community in association with declining malaria, or a reduction in invasion due to improved host-susceptibility as malaria infection declines. This information is important as the two hypotheses point towards different control strategies.
We propose a retrospective analysis of the trends in transmission of NTS in two settings in Kilifi: one where malaria prevalence has decreased over time and one where malaria prevalence has remained stable (and high) over time. We will test serum samples, collected in annual cross-sectional surveys in these two locations between 1998 and 2016, for anti-NTS antibodies. The results will be analysed for changes in transmission intensity of NTS over time and between the two locations.
Our preliminary findings indicate that in a setting with decreasing incidence of invasive Non-typhoidal Salmonella (NTS) disease, the seroprevalence of anti- NTS antibodies against S. Enteritidis has decreased over time while the seroprevalence of anti-NTS antibodies against S. Typhimurium has remained stable over the same time period. The age at first infection with S. Enteritidis has also increased. This points to an overall decrease in the transmission of Salmonella Enteritidis infections in the community. Whether this decline is related to the decline in malaria transmission that has been observed in the same setting is yet to be determined.
Dr Esther Muthumbi
PhD Student, Research Medical Officer
Professor Anthony Scott, London School of Hygiene and Tropical Medicine (UK)
Dr Sean Elias, University of Oxford (UK)
Professor Cal MacLennan, University of Oxford (UK)