Abstract
Invasive Salmonella diseases, both typhoid and invasive non-typhoidal Salmonella (iNTS), are seasonal bloodstream infections causing important morbidity and mortality globally in Africa. The reservoirs and transmission of both are not fully understood. We hypothesised that differences in the time-lagged relationships of rainfall or temperature with typhoid and iNTS incidence might infer differences in epidemiology. We assessed the dynamics of invasive Salmonella incidence over a 16-year period of surveillance, quantifying incidence peaks, seasonal variations, and nonlinear effects of rainfall and temperature exposures on the relative risks of typhoid and iNTS, using monthly lags. An increased relative risk of iNTS incidence was short-lasting but immediate after the onset of the rains, whereas that of typhoid was long-lasting but with a two months delayed start, implying a possible difference in transmission. The relative-risk function of temperature for typhoid was bimodal, with higher risk at both lower (with a 1 month lag) and higher (with a ≥4 months lag) temperatures, possibly reflecting the known patterns of short and long cycle typhoid transmission. In contrast, the relative-risk of iNTS was only increased at lower temperatures, suggesting distinct transmission mechanisms. Environmental and sanitation control strategies may be different for iNTS compared to typhoid disease.
Highlights
Invasive Salmonella diseases, both typhoid and invasive non-typhoidal Salmonella, are seasonal bloodstream infections causing important morbidity and mortality globally in Africa
Invasive Salmonella diseases, both typhoid and invasive non-typhoidal Salmonella disease, are serious bloodstream infections co-existing in Africa, and are leading causes of morbidity and mortality worldwide
We empirically report on 15-year seasonal dynamics of climate and invasive Salmonella diseases, and described distinct and different relationships with rainfall and temperature for both forms of invasive Salmonella disease, by monthly lags
Summary
Invasive Salmonella diseases, both typhoid and invasive non-typhoidal Salmonella (iNTS), are seasonal bloodstream infections causing important morbidity and mortality globally in Africa. We assessed the dynamics of invasive Salmonella incidence over a 16-year period of surveillance, quantifying incidence peaks, seasonal variations, and nonlinear effects of rainfall and temperature exposures on the relative risks of typhoid and iNTS, using monthly lags. INTS disease carries a case-fatality of 10–20% in both adults and children, 10-fold higher than typhoid[9,10], and leads to an estimated 2.1–6.5 million cases and 415,164–1,301,520 deaths globally per year[11,12]. Some studies have reported differences in the effects of climate on the risk of Salmonella, suggesting setting-specific infection dynamics[13,29–34]. Much is unknown about delayed structures of climate dynamics and invasive Salmonella disease, and how they relate to reservoirs and transmission routes, in poor resource settings. A known high incidence of both typhoid and iNTS disease, and uninterrupted and consistent longitudinal blood culture surveillance of typhoid and iNTS for 20 years, allowing the current comprehensive analysis
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