The Burden of Typhoid Fever in Sub-Saharan Africa: A Perspective

Typhoid conjugate vaccines for preventing typhoid fever (enteric fever)

RATIONALE

Typhoid fever is a major cause of enteric disease-related morbidity and mortality. Vaccination reduces disease burden and prevents outbreaks, but policies and programmes should be informed by the most recent evidence as newer vaccines become available.

OBJECTIVES

To assess the benefits and harms of typhoid conjugate vaccines (TCVs) compared to no vaccine, placebo, typhoid-inactive agents (vaccines for another disease) or other typhoid vaccines for preventing morbidity and mortality associated with typhoid fever in adults and children.

SEARCH METHODS

In April 2024, we searched the Cochrane Central Register of Controlled Trials, MEDLINE, Embase, CINAHL, Global Index Medicus, United States Advisory Committee on Immunization Practices and the World Health Organization vaccine repository for randomised controlled trials (RCTs), with no restrictions. We also searched clinical trial registries for ongoing trials (www.

CLINICALTRIALS

gov and the WHO International Clinical Trials Registry Platform), grey literature, bibliographic citations of reviews and key articles for additional studies. We contacted study authors for information about ongoing studies.

ELIGIBILITY CRITERIA

We included RCTs and cluster-RCTs of children and adults living in typhoid-endemic areas or travelling to typhoid-endemic areas. We included studies comparing TCVs to controls (i.e. no vaccine, placebo or vaccines for another disease), non-conjugated typhoid vaccines or other TCVs.

OUTCOMES

Outcomes included acute typhoid fever, defined by laboratory-confirmed isolation of Salmonella typhi, all-cause mortality, adverse events (AEs) and serious adverse events (SAEs).

RISK OF BIAS

Review authors independently assessed risk of bias for all outcomes, using the Cochrane RoB 2 tools. We resolved disagreements through discussion or adjudication. We assessed the intention-to-treat effect and used the overall RoB judgement to assess the certainty of evidence for each outcome.

SYNTHESIS METHODS

Three review authors independently screened titles and abstracts for eligible studies, followed by full-text assessment. Disagreements were resolved through discussion or adjudication by a fourth author. Four authors independently extracted characteristics of included studies and outcome data using a piloted, standardised data extraction form. We synthesised results for each outcome where possible, using the Mantel-Haenszel statistical method and random-effects analysis model. Where meta-analysis was not possible due to the nature of the data, we planned to synthesise results based on direction of effect. We used GRADE to assess the certainty of evidence for each outcome, assessing risk of bias, inconsistency, indirectness, imprecision and other bias.

INCLUDED STUDIES

We included 19 trials (17 RCTs and two cluster-RCTs). The 19 trials enrolled 395,650 participants, with ages ranging from six weeks to 60 years. Vaccines were delivered as a single dose in 14 studies; two doses, ranging from four to 24 weeks apart, in six studies; and three doses, four weeks apart, in one study. Comparators included: no vaccine, placebo and other vaccines. Seven studies compared TCV with non-conjugated typhoid vaccines. Six studies compared one TCV to another TCV.

SYNTHESIS OF RESULTS

TCV compared to control may result in a large reduction in acute typhoid fever (risk ratio (RR) 0.20, 95% confidence interval (CI) 0.12 to 0.32; I2 = 70%; 6 studies, 101,896 participants; low-certainty evidence) and probably results in little to no difference in all-cause mortality (RR 0.80, 95% CI 0.35 to 1.85; I2 = 52%; 4 studies, 100,337 participants; moderate-certainty evidence). TCV results in little to no difference in AEs when compared to control (RR 0.91, 95% CI 0.76 to 1.09; I2 = 0%; 3 studies, 29,465 participants; high-certainty evidence) and a slight reduction in SAEs compared to control (RR 0.82, 95% CI 0.71 to 0.95; I2 = 0%; 6 studies, 89,625 participants; high-certainty evidence). TCV compared to non-conjugated typhoid vaccines may result in little to no difference in acute typhoid fever (RR 0.90, 95% CI 0.48 to 1.69; 1 study, 78 participants; low-certainty evidence). There were no deaths in the included studies. When compared to non-conjugated typhoid vaccines, TCV likely results in little to no difference in AEs (RR 1.00, 95% CI 0.77 to 1.31; I2 = 0%; 3 studies, 244 participants; moderate-certainty evidence) and likely results in a slight reduction in SAEs (RR 0.30, 95% CI 0.05 to 1.88; I2 = 0%; 2 studies, 732 participants; moderate-certainty evidence). For TCV compared to another TCV, none of the studies reported on acute typhoid fever. Vi tetanus toxoid vaccine (Vi-TT) may result in little to no difference in all-cause mortality compared to a different TCV (RR 5.19, 95% CI 0.54 to 49.80; I2 = 0%; 2 studies, 2422 participants; low-certainty evidence). Vi-TT likely results in little to no difference in AEs compared to another TCV (RR 1.18, 95% CI 0.92 to 1.51; I2 = 39%; 4 studies, 2916 participants; moderate-certainty evidence) and may result in little to no difference in SAEs (RR 2.48, 95% CI 0.74 to 8.36; I2 = 0%; 3 studies, 2866 participants; low-certainty evidence). The certainty of evidence was consistently reduced due to imprecision, indirectness and bias.

AUTHORS' CONCLUSIONS

This review highlights that TCVs, compared to controls, are effective in preventing typhoid fever, and may confer protection for up to four years. TCVs compared to non-conjugated typhoid vaccines may result in little to no difference in acute typhoid fever and AEs, and likely result in a slight reduction in SAEs. Vi-TT compared to another TCV may result in little to no difference in all-cause mortality or SAEs, and likely results in little to no difference in AEs.

FUNDING

NG, TL and TK were partly supported by, and the Cochrane Infectious Diseases Group (CIDG) editorial base is funded by, the Research, Evidence and Development Initiative (READ-It), funded by UK aid for the benefit of low- and middle-income countries (project number 300342-104). The views expressed in this review do not necessarily reflect the official policies of the UK government.

REGISTRATION

Protocol available via doi.org/10.1002/14651858.CD015746.

Exogenous cystine increases susceptibility of drug-resistant Salmonella to gentamicin by promoting oxidation of glutathione metabolism and imbalance of intracellular redox levels

Introduction

Antibiotic overuse has caused the development of bacterial resistance, which is a major threat to public health. Intracellular metabolic processes are essential for maintaining the normal physiological activities of bacteria, and an increasing body of research has demonstrated a significant association between metabolic alterations and the development of drug resistance. Numerous studies have demonstrated that the addition of adjuvants can counteract bacterial antibiotic resistance.

Method

Cystine treatment was verified in vitro to promote the lethal effect of gentamicin on Salmonella using in vitro bactericidal counting methods. The metabolic differences in Salmonella enterica Typhimurium standard strain ATCC 14028 with or without the addition of cystine were analyzed via untargeted metabolomics. The multifunctional electronic enzyme marker was used to determine intracellular reduced glutathione/oxidized glutathione (GSH/GSSG), ferrous iron on (Fe2+), and reactive oxygen species (ROS) levels. The expression of glutathione and stress genes was determined using real-time quantitative PCR.

Result

We confirmed that exogenous cystine increased the lethal effect of gentamicin against strain S. enterica Typhimurium (ATCC 14028) and other clinically resistant Salmonella serotypes. Exogenous cystine stimulated the metabolism of the cell and activated the glutathione pathway while altering the GSH/GSSG ratio, which placed bacteria in a state of redox imbalance with increased Fe2+ and ROS levels. Our results suggest that when bacterial redox levels are reprogrammed, bacterial susceptibility to antibiotics can also change.

Discussion

This study confirms that cystine enhances the antimicrobial efficacy of gentamicin against drug-resistant Salmonella. Through the application of metabolomics, the underlying metabolic mechanisms by which cystine exerts its effects on Salmonella have been elucidated, offering a novel perspective in the domain of metabolic reprogramming aimed at counteracting drug resistance. Furthermore, these findings reinforce the potential role of small-molecule metabolites as effective adjuvants to enhance antibiotic action.

Seroprevalence of Typhoid Fever and Its Associated Risk Factors Among Clinically Diagnosed Febrile Patients Visiting the Outpatient Department at Debark Hospital and Drug Susceptibility Patterns of Isolates

Typhoid fever is caused by the bacterium Salmonella Typhi, which poses major health problems in developing countries, including Ethiopia. However, there is limited information regarding typhoid fever, contributing factors with it, and its drug susceptibility pattern in the research area. The aim of this study was to determine the seroprevalence of typhoid fever and its associated risk factors among clinically diagnosed febrile patients at Debark Hospital and evaluate the drug susceptibility patterns of the isolates. A hospital-based cross-sectional study was conducted among 158 febrile patients from December 2022 to April 2023. Blood and stool samples were collected from each febrile patient. The Widal test was used to test Salmonella Typhi O and H antigens sera from blood, and bacteria were cultured from the stool. Gram staining and biochemical analyses were carried out for each isolate. Antibiotic susceptibility testing was performed for the isolates using Kirby-Bauer disk diffusion techniques. Descriptive statistics and logistic regression were used for analysis. In this study, the seroprevalence of typhoid fever among febrile patients at Debark Hospital was 22.8%. Gender (adjusted odds ratio (AOR): 95% confidence interval (CI): 0.02, 0.31, p ≤ 0.001), marital status (AOR: 95% CI: 2.63, 4.66, p ≤ 0.001), family size (AOR: 95% CI: 0.01, 0.31, p ≤ 0.001), residence (AOR: 95% CI: 0.09, 0.83, p = 0.021), practice of using toilet (AOR: 95% CI: 0.08, 0.086, p = 0.027), washing fruits and vegetables before eating (AOR: 95% CI: 0.12, 0.87, p = 0.025), and awareness of typhoid fever transmission and prevention (AOR: 95% CI: 0.12, 0.91, p = 0.032) are the associated risk factors for typhoid fever. Fifty percent of the isolates showed multidrug resistance to two or more antibiotics. It was suggested that improving personal hygiene, providing safe drinking water, and careful use of antibiotics could considerably reduce the prevalence of typhoid fever in the study area.

Salmonella Typhi Haplotype 58 biofilm formation and genetic variation in isolates from typhoid fever patients with gallstones in an endemic setting in Kenya

Although typhoid fever has largely been eliminated in high-income countries, it remains a major global public health concern especially among low- and middle-income countries. The causative agent, Salmonella enterica serovar Typhi (S. Typhi), is a human restricted pathogen with a limited capacity to replicate outside the human host. Human carriers, 90% of whom have gallstones in their gallbladder, continue to shed the pathogen for an ill-defined period of time after treatment. The genetic mechanisms involved in establishing the carrier state are poorly understood, but S. Typhi is thought to undergo specific genetic changes within the gallbladder as an adaptive mechanism. In the current study, we aimed to identify the genetic differences in longitudinal clinical S. Typhi isolates from asymptomatic carriers with gallstones in a typhoid endemic setting in Nairobi, Kenya. Whole-genome sequences were analyzed from 22 S. Typhi isolates, 20 from stool samples, and 2 from blood samples, all genotype 4.3.1 (H58). Out of this, 19 strains were from four patients also diagnosed with gallstones, of whom three had typhoid symptoms and continued to shed S. Typhi after treatment. All isolates had point mutations in the quinolone resistance-determining region (QRDR), and only sub-lineage 4.3.1.2.EA3 encoded multidrug resistance genes. There was no variation in antimicrobial resistance patterns among strains from the same patient/household. Non-multidrug resistant (MDR) isolates formed significantly stronger biofilms in vitro than the MDR isolates, p<0.001. A point mutation within the treB gene (treB A383T) was observed in strains isolated after clinical resolution from patients living in 75% of the households. For missense mutations in Vi capsular polysaccharide genes, tviE P263S was also observed in 18% of the isolates. This study provides insights into the role of typhoid carriage, biofilm formation, AMR genes, and genetic variations in S. Typhi during asymptomatic carriage.

The Salmonella typhi Cell Division Activator Protein StCAP Impacts Pathogenesis by Influencing Critical Molecular Events

Conserved molecular signatures in multidrug-resistant Salmonella typhi can serve as novel therapeutic targets for mitigation of infection. In this regard, we present the S. typhi cell division activator protein (StCAP) as a conserved target across S. typhi variants. From in silico and fluorimetric assessments, we found that StCAP is a DNA-binding protein. Replacement of the identified DNA-interacting residue Arg34 of StCAP with Ala34 showed a dramatic (15-fold) increase in Kd value compared to the wild type (Kd 546 nm) as well as a decrease in thermal stability (10 °C shift). Out of the two screened molecules against the DNA-binding pocket of StCAP, eltrombopag, and nilotinib, the former displayed better binding. Eltrombopag inhibited the stand-alone S. typhi culture with an IC50 of 38 μM. The effect was much more pronounced on THP-1-derived macrophages (T1Mac) infected with S. typhi where colony formation was severely hindered with IC50 reduced further to 10 μM. Apoptotic protease activating factor1 (Apaf1), a key molecule for intrinsic apoptosis, was identified as an StCAP-interacting partner by pull-down assay against T1Mac. Further, StCAP-transfected T1Mac showed a significant increase in LC3 II (autophagy marker) expression and downregulation of caspase 3 protein. From these experiments, we conclude that StCAP provides a crucial survival advantage to S. typhi during infection, thereby making it a potent alternative therapeutic target.

Salmonella Typhi Haplotype 58 (H58) Biofilm Formation and Genetic Variation in Typhoid Fever Patients with Gallstones in an Endemic Setting in Kenya

The causative agent of typhoid fever, Salmonella enterica serovar Typhi, is a human restricted pathogen. Human carriers, 90% of whom have gallstones in their gallbladder, continue to shed the pathogen after treatment. The genetic mechanisms involved in establishing the carrier state are poorly understood, but S. Typhi is thought to undergo specific genetic changes within the gallbladder as an adaptive mechanism. In the current study, we aimed to identify biofilm forming ability and the genetic differences in longitudinal clinical S. Typhi isolates from asymptomatic carriers with gallstones in Nairobi, Kenya. Whole genome sequences were analyzed from 22 S. Typhi isolates, 20 from stool and 2 from blood samples, all genotype 4.3.1 (H58). Nineteen strains were from four patients also diagnosed with gallstones, of whom, three had typhoid symptoms and continued to shed S. Typhi after treatment. All isolates had point mutations in the quinolone resistance determining region (QRDR) and only sub-lineage 4.3.1.2EA3 encoded multidrug resistance genes. There was no variation in antimicrobial resistance patterns among strains from the same patient/household. Non-multidrug resistant (MDR), isolates formed significantly stronger biofilms in vitro than the MDR isolates, p<0.001. A point mutation within the treB gene (treB A383T) was observed in strains isolated after clinical resolution from patients living in 75% of the households. Missense mutations in Vi capsular polysaccharide genes, tviE P263S was also observed in 18% of the isolates. This study provides insights into the role of typhoid carriage, biofilm formation, AMR genes and genetic variations in S. Typhi from asymptomatic carriers.

Fecal Shedding, Antimicrobial Resistance and In Vitro Biofilm formation on Simulated Gallstones by Salmonella Typhi Isolated from Typhoid Cases and Asymptomatic Carriers in Nairobi, Kenya

Typhoid fever, caused by the human restricted pathogen Salmonella Typhi, remains a major global public health concern. Even after successful treatment, approximately 3-5% of patients with typhoid fail to clear the bacteria within one year and become chronic carriers. Most typhoid carriers have gallstones in their gallbladder, and biofilm formation on gallstones is highly correlated with chronic carriage. This study's goal was to identify asymptomatic typhoid carriers in an endemic setting in Kenya, and to compare acute versus chronic isolates. A cohort of typhoid fever patients identified through blood and/or stool culture, and their household contacts, were followed up after treatment to detect longitudinal S. Typhi stool shedding. An abdominal ultrasound scan was used to identify individuals with gallstones. A total of 32 index patients and 32 household contacts were successfully followed-up. Gallstones were detected in 4 cases and 1 household contact. The duration of S. Typhi shedding was significantly longer in individuals with gallstones compared to those without, P<0.001. Eighty-three (83) S. Typhi strains were tested for susceptibility to commonly used antimicrobials and examined by in vitro biofilm formation assays. Out of 37 infected individuals, 32.4% had infections caused by multidrug resistant (MDR) S. Typhi strains and only 18.9% were infected by susceptible strains. Non-MDR strains formed significantly better biofilms in vitro than the MDR strains (P<0.001). This study provides data on S. Typhi chronic carriage that will influence public health approaches aimed at reducing typhoid transmission and the burden of infection.

Genomic Characterization of Multidrug-Resistant Pathogenic Enteric Bacteria from Healthy Children in Osun State, Nigeria

Antimicrobial resistance (AMR) is responsible for the spread and persistence of bacterial infections. Surveillance of AMR in healthy individuals is usually not considered, though these individuals serve as reservoirs for continuous disease transmission. Therefore, it is essential to conduct epidemiological surveillance of AMR in healthy individuals to fully understand the dynamics of AMR transmission in Nigeria. Thirteen multidrug-resistant Citrobacter spp., Enterobacter spp., Klebsiella pneumoniae, and Escherichia coli isolated from stool samples of healthy children were subjected to whole genome sequencing (WGS) using Illumina and Oxford nanopore sequencing platforms. A bioinformatics analysis revealed antimicrobial resistance genes such as the pmrB_Y358N gene responsible for colistin resistance detected in E. coli ST219, virulence genes such as senB, and ybtP&Q, and plasmids in the isolates sequenced. All isolates harbored more than three plasmid replicons of either the Col and/or Inc type. Plasmid reconstruction revealed an integrated tetA gene, a toxin production caa gene in two E. coli isolates, and a cusC gene in K. quasivariicola ST3879, which induces neonatal meningitis. The global spread of AMR pathogenic enteric bacteria is of concern, and surveillance should be extended to healthy individuals, especially children. WGS for epidemiological surveillance will improve the detection of AMR pathogens for management and control.

Enteric fever cluster identification in South Africa using genomic surveillance of Salmonella enterica serovar Typhi

The National Institute for Communicable Diseases in South Africa participates in national laboratory-based surveillance for human isolates of Salmonella species. Laboratory analysis includes whole-genome sequencing (WGS) of isolates. We report on WGS-based surveillance of Salmonella enterica serovar Typhi (Salmonella Typhi) in South Africa from 2020 through 2021. We describe how WGS analysis identified clusters of enteric fever in the Western Cape Province of South Africa and describe the epidemiological investigations associated with these clusters. A total of 206 Salmonella Typhi isolates were received for analysis. Genomic DNA was isolated from bacteria and WGS was performed using Illumina NextSeq technology. WGS data were investigated using multiple bioinformatics tools, including those available at the Centre for Genomic Epidemiology, EnteroBase and Pathogenwatch. Core-genome multilocus sequence typing was used to investigate the phylogeny of isolates and identify clusters. Three major clusters of enteric fever were identified in the Western Cape Province; cluster one (n=11 isolates), cluster two (n=13 isolates), and cluster three (n=14 isolates). To date, no likely source has been identified for any of the clusters. All isolates associated with the clusters, showed the same genotype (4.3.1.1.EA1) and resistome (antimicrobial resistance genes: bla _TEM-1B, catA1, sul1, sul2, dfrA7). The implementation of genomic surveillance of Salmonella Typhi in South Africa has enabled rapid detection of clusters indicative of possible outbreaks. Cluster identification allows for targeted epidemiological investigations and a timely, coordinated public health response.