The Severe Typhoid Fever in Africa Program: Study Design and Methodology to Assess Disease Severity, Host Immunity, and Carriage Associated With Invasive Salmonellosis

BACKGROUND

Invasive salmonellosis is a common community-acquired bacteremia in persons residing in sub-Saharan Africa. However, there is a paucity of data on severe typhoid fever and its associated acute and chronic host immune response and carriage. The Severe Typhoid Fever in Africa (SETA) program, a multicountry surveillance study, aimed to address these research gaps and contribute to the control and prevention of invasive salmonellosis.

METHODS

A prospective healthcare facility-based surveillance with active screening of enteric fever and clinically suspected severe typhoid fever with complications was performed using a standardized protocol across the study sites in Burkina Faso, the Democratic Republic of Congo (DRC), Ethiopia, Ghana, Madagascar, and Nigeria. Defined inclusion criteria were used for screening of eligible patients for enrollment into the study. Enrolled patients with confirmed invasive salmonellosis by blood culture or patients with clinically suspected severe typhoid fever with perforation were eligible for clinical follow-up. Asymptomatic neighborhood controls and immediate household contacts of each case were enrolled as a comparison group to assess the level of Salmonella-specific antibodies and shedding patterns. Healthcare utilization surveys were performed to permit adjustment of incidence estimations. Postmortem questionnaires were conducted in medically underserved areas to assess death attributed to invasive Salmonella infections in selected sites.

RESULTS

Research data generated through SETA aimed to address scientific knowledge gaps concerning the severe typhoid fever and mortality, long-term host immune responses, and bacterial shedding and carriage associated with natural infection by invasive salmonellae.

CONCLUSIONS

SETA supports public health policy on typhoid immunization strategy in Africa.

Structural analysis of O-polysaccharide chains extracted from different Salmonella Typhimurium strains

Salmonella Typhimurium is the major cause of invasive nontyphoidal Salmonella disease in Africa, with high mortality among children and HIV-infected individuals. Currently, no vaccine is available for use in humans. Antibodies directed against the O-polysaccharide of the lipopolysaccharide molecule of Salmonella mediate bacterial killing and are protective, and conjugation of the O-polysaccharide to a carrier protein represents a possible strategy for vaccine development. Here we have purified the O-polysaccharide from six different strains of S. Typhimurium and fully characterized them using analytical methods including HPLC-SEC, HPAEC-PAD, GC, GC-MS, 1D and 2D NMR spectroscopy. All the O-polysaccharide samples showed a similar bimodal molecular mass distribution, but differed with respect to the amount and position of O-acetylation and glucosylation. For some strains, O-acetyl groups were found not only on C-2 of abequose (factor 5 specificity), but also on C-2 and C-3 of rhamnose; glucose was found to be linked 1→4 or 1→6 to galactose in different amounts according to the strain of origin. This structural variability could have an impact on the immunogenicity of corresponding glycoconjugate vaccines and different strains need to be evaluated in order to identify the appropriate source of O-polysaccharide to use for the development of a candidate conjugate vaccine with broad coverage against S. Typhimurium.

Preferential expression of AChR epsilon-subunit in thymomas from patients with myasthenia gravis

The role of antigen expression by thymomas in myasthenia gravis (MG) is not clear. Previous reports of acetylcholine receptor (AChR) mRNA expression by the highly sensitive reverse transcription-polymerase chain reactions (RT-PCR) produced varying results. To try to clarify this issue, we first used RT-PCR but then turned to the more accurate and quantitative RNase protection assays (RPA) to assess AChR subunit mRNA expression in thymomas from 25 patients (22 with MG). By RT-PCR, all five AChR subunits could be detected in many thymomas. However, by RPA, the mRNA for the adult-specific AChR epsilon-subunit was found in 13/25 (52%) thymomas, but not mRNA for the other subunits. AChR epsilon-subunit was more frequently detected in thymomas of A or AB histology (WHO classification) than those with B1-B3 histology. Overall, 6/6 with thymomas of A or AB histology were positive compared with only 8/19 with B histology (p=0.02). Autoantibodies in the two patients with the highest levels of epsilon-subunit mRNA bound better to adult (alpha(2)betadeltaepsilon) AChR than to fetal (alpha(2)betadeltagamma) AChR, whereas the other sera bound better to fetal AChR. The greater abundance of mRNA for AChR epsilon-subunit than for other subunits suggests that the AChR epsilon-subunit may play a distinctive role in autosensitization in MG-associated thymomas, particularly those of type A or AB.

Affordable pediatric CD4 counting by flow cytometry in Malawi

Background

Rapid expansion of antiretroviral therapy in Malawi has occurred in the relative absence of suitable pediatric CD4 counting facilities. We have recently validated in adults a simplified affordable flow cytometric CD4 counting method, the Blantyre count. There is a need for this technology to transfer to government laboratories run by local staff, and to be validated in children, where %CD4/lymphocyte values are required.

Methods

We assessed agreement of %CD4/lymphocyte values determined by the Blantyre count and Panleucogate methods on an EPICS XL-MCL flow cytometer on 113 venous blood samples from HIV-seropositive children in Blantyre, Malawi. All assays were performed by two Malawian laboratory technicians.

Results

Overall bias between the two methods was −0.13% (95% CI −0.37 to 0.11) and limits of agreement were −2.69 to 2.43% (95% CI −3.11 to −2.27 and 2.01 to 2.85). Limits of agreement were within −3.00 and 3.00 for each laboratory technician. Coefficient of variation for the Blantyre count assay was 2.0% and samples showed good stability over 5 days.

Conclusions

The Blantyre count method can accurately determine %CD4/lymphocyte values in blood of HIV-seropositive children on an EPIC XL-MCL flow cytometer at a reagent cost of US $0.21 per test or less. The assay can be competently carried out by local laboratory technicians.