Epidemiological characteristics of serotype 3 in Tunisia

The epidemiology of serotype 3 remain important globally. It has been questioned in several studies from different region in the world. Despite this, few studies have been particularly focused on the emergency of serotype 3 in developing countries.  In our study, we aim to investigate the epidemiological characteristics of serotype 3 in Sfax-Tunisia collected over 7 year period from Habib Bourguiba Hospital. In a previous study between 2012-2016, serotype distribution of  S. pneumoniae study in that region were reported by Ktari et al (Ktari et al., 2016). In this report, serotype 3 were classified at the third rank order of frequent serotype after the most prevalent serotype 19F and 14.

Furthermore, The incidence of serotype 3 in all Tunisia were different from a city to another. For example, a study report done by midouni et al in a Tunisian pneumology hospital, revealed that serotype 3 were in decreasing order of frequency unlike In Sfax. Using MLST, sequence type were identified as ST180, which is the most detected sequence type aside of the Double LocusVariant (DLV) ST505 and ST1220 as a singleton (Midouni et al., 2019).  A study conducted in Farhat Hached Hospital of Sousse depicated that the frequency of S.pneumoniae serotype 3 were also not high (60 years and young children. Whereas, the first detection of ST505 were in 2014. This is were mostly obtained from adult and infant age bettween < 2 years. Those ST were presented almost during the period of study with the exception of ST260 which were found only in 2016, indeed, the first appearnce of a newly ST1253 in 2018 isolated from patient age 24 years.

Streptococcus penumoniae serotype 3 have a great virulence and an importante rate of mortality (Weinberger et al., 2010). While, most of serotype is WZY dependent, serotype 3 led to the expectation that the capsular polysaccharide (CPS) is synthesize through a synthase-depdent pathway (Geno et al., 2015). Indeed,  the large  and mucoid polysaccharide capsule of serotype 3 isolates were capable to interfer with the immunity system by inhibiting phagocytosis. Thus, all this characteristic may explain the pathogenicity of serotype 3and being  a great evader from the antigen contained in the conjugate PCV-13 vaccin. However, countries where the pneumococcal conjugate PCV13 is licensured. Their mainly deduction that there was no overall reduction in PCV-13 serotype 3 suggesting a lower efficacy of PCV-13. (Echániz-Aviles et al., 2019; Moore et al., 2015).

In Tunisia, the etiology and epidemiology of serotype 3 is poorly understood. It’s been as a unvaccinated serotype. Our study’s goal was to prospectively investigate the molecular epidemiology of pneumococcal of serotype 3 in the region of Sfax.  From a public health perspective, routine surveillance of pneumococcal diseases (PD) is critical for development of effective infection control policies and strategies for preventive and treatment measures. In this concern, the local surveillance of serotype 3 is important by detecting such occurrences in clonal dissemination and changes in serotype prevalence throughout the coming year. Furthermore, it is worth noting that, what really matters is to provide a boradly database of genetic types which may greatly facilitate future studies.

In conclusion, the present results may reflect the current epidemiological situation of S. pneumoniae 3 in the region of Sfax-South Tunisia. Although, this was a pilot study conducted in a single local hospital. Large scale and nationwide surveillance using the molecular typing technique should be performed in Tunisia.