Causative agents of bacterial sepsis have changed in The Gambia following introduction of the pneumococcal vaccine

Bacterial sepsis is a leading cause of disease and death in sub-Saharan Africa (SSA). Many of the causative agents can be prevented with vaccines such as Haemophilus influenzae type b (Hib) conjugate vaccine and pneumococcal conjugate vaccines (PCVs). The impact of these vaccines on infection needs to be monitored through surveillance. However, microbiological facilities and expertise are not readily available in many SSA settings.

In The Gambia, the introduction of routine infant immunization with Hib in 1997 reduced Hib disease to negligible levels. Subsequently, S. pneumoniae, S. aureus, E. coli and NTS became the leading causes of sepsis in the country. In August 2009, a 7-valent PCV was introduced as part of the Expanded Programme on Immunisation (EPI), which was replaced by the 13-valent vaccine in 2011.

New research, published in Oxford University Press, and conducted by scientists across the various research themes of the Medical Research Council Unit The Gambia at the London School of Hygiene and Tropical Medicine, is the 1st facility-based surveillance that provides over a decade of information on the main bacterial causes of sepsis and meningitis; and the pattern of antimicrobial resistance in The Gambia. This is important for monitoring vaccine priorities and management of patients. The MRCG at LSHTM is the only health facility in The Gambia where microbiological cultures are routinely carried out on patients with suspected sepsis, and where such data are analysed and reported as part of an ongoing surveillance.

The researchers found that there was a 16% reduction in S. pneumoniae causing sepsis in the country and this profound in the children under 2 years and adults. Likewise, S. aureus became the most common bacteria isolated from patients with sepsis.  The increases in S. aureus sepsis was among children aged 2 – 23 months. Nevertheless, 6 years after the introduction of PCV13, S. pneumoniae remains an important cause of sepsis across all age groups.

The researchers also found that Escherichia coli (E. coli) and non-typhoidal Salmonellae (NTS) remain important causes of bacterial sepsis in The Gambia. While the prevalence of E. coli remained stable, that of NTS significantly declined post-PCV with the overall number of cases being relatively small.

Saffiatou Darboe, Supervisor of the Clinical Microbiology Lab at the MRCG at LSHTM, co-first and corresponding author of the study said, “we have demonstrated, for the first time, how the major bacterial causes of sepsis have changed in relation to the introduction of the pneumococcal vaccine. Antimicrobial resistance was generally low for the Gram-positive organisms including very few methicillin-resistant S. aureus isolates compared to some sub-Saharan African countries. However, monitoring resistance for emergence of multi-drug resistance in Gram-negatives including quinolone resistance is warranted.

Senior author for the study, Dr. Anna Roca commented, “Generating data of etiological causes of sepsis is key for the Gambia and the region in terms of highlighting prevention priorities and re-defining treatment guidelines. The Unit can generate these data thanks to the strong investment in laboratory facilities and human capacity. This is an example of the synergies and close collaboration between the Unit platforms and the research themes”.

Overall, the data shows that S. aureus have emerged as the leading cause of bacterial sepsis, and therefore new vaccines targeting this bacterium should be a priority for the region. Antimicrobial resistance surveillance remains vigilant

This work is supported by the MRCG at the LSHTM. This study was part of a Clinical Infectious Diseases supplement looking at the impact of pneumococcal vaccine in Africa.