Senegal is a hyperendemic country for dengue, experiencing annual outbreaks in various regions since 2017. These outbreaks are marked by the co-circulation of DENV1-3 virus serotypes. The first case of dengue in Senegal was reported in 1970 in the Thiès region, and since then, there have been multiple outbreaks and sporadic cases.
2021 Outbreak in Rosso
On October 8, 2021, a dengue outbreak was reported at the Rosso health post, a sentinel site of the syndromic surveillance network located in the north of the country. This was reported to the WHO Collaborating Centre for Arboviruses and Hemorrhagic Fever Viruses at the Institut Pasteur de Dakar. Upon notification, a multidisciplinary team was dispatched to conduct epidemiological and virological investigations.
During the investigation, a rapid diagnostic test (RDT) was used for the combined detection of the dengue virus non-structural protein 1 (NS1) and IgM/IgG antibodies. Additionally, confirmatory testing was performed using real-time RT-PCR and IgM ELISA at the reference laboratory in Dakar. Positive samples by qRT-PCR were subjected to whole-genome sequencing using nanopore technology.
Epidemiological Results
- Positive Cases: Out of 173 patients recruited, 102 positive cases were identified (confirmed by RT-PCR, NS1 antigen detection, and/or IgM).
- Virus Serotypes: The serotyping of the virus showed that the outbreak was caused by serotype DENV-1, different from the DENV-2 serotype involved in the Rosso outbreak three years earlier, indicating a serotype replacement.
- Diagnostic Concordance: Almost all NS1-positive samples in the field were confirmed by qRT-PCR, with a concordance rate of 92.3%.
- Sequencing and Phylogenetic Analysis: Whole-genome sequencing and phylogenetic analysis of the strains suggested a reintroduction in Rosso of a DENV-1 strain different from the one responsible for the outbreak in the Louga area five years earlier.
Demographic Distribution
- Age and Gender: The median age of patients was 21 years, with a higher prevalence in individuals under 15 years old (37%). A significant difference in the positivity rate was found by gender, with men being more prone to dengue infection than women.
- Common Symptoms: The most commonly reported symptoms were fever, headache, arthralgia, and myalgia.
Epidemic Curve
The epidemic curve showed that the first case had symptom onset on September 26, 2021, with peaks on the following dates: October 1, October 7-10, and October 12-15.
The findings highlight the need to improve dengue virus surveillance in Senegal, with a broader deployment of DENV antigen tests, allowing easy on-site diagnosis of suspected cases and early outbreak detection. Continuous monitoring of circulating serotypes is also emphasized to better understand the viral epidemiology across the country.
