Seasonal patterns in malaria cases: An outbreak investigation in Dar es Salaam, Tanzania, 2023

Introduction: Malaria is endemic and causes considerable morbidity and mortality in Mainland Tanzania. Circulating rumours on social media about an unusual rise in malaria cases in the Dar es Salaam region from April to June 2023 activated this investigation. The aim of this investigation was to ascertain whether the reported increase in malaria cases was an outbreak. Methods: The retrospective cross-sectional study was conducted in all five Councils of the Dar es Salaam region. We reviewed outpatient department (OPD) and malaria testing results using data from the District Health Information System (DHIS2) from January to June 2021 to 2023 to identify patterns and trends. Furthermore, laboratory-confirmed malaria cases from the registers for the months of January to June 2023 were line-listed and analysed. The malaria positivity rates were calculated by dividing the number of positive mRDT tests by the total number of tests conducted. The cumulative sum (CUSUM) method and paired t-test were used to establish an outbreak. Results: A total of 2992 confirmed malaria cases from January to June 2023 were investigated. Majority 1884 (63%) were male, from the Dar es Salaam City Council, formerly named Ilala, 1441 (48%), aged 19-45 years, 1681 (57%). Data from DHIS2 revealed an increase of outpatients’ department (OPD) and inpatient department (IPD) confirmed malaria cases in May and June 2023. We found that there was a significant decrease in the mean of malaria cases by year 2021(1165, 95%CI:1161.92 – 1168.08) as compared to year 2022 (861,95%CI: 857.78, 864.22, p<0.05). The threshold was calculated by using the CUSUM method, which shows that from 1st January to 23rd June 2023, there were increase in malaria confirmed cases. A high peak was noted at month of May 2023, week 17. Conclusion: Findings suggest a seasonal increase of malaria cases in May-June 2023 rather than a true outbreak. Strengthened surveillance and continuous monitoring are recommended to ensure early detection and effective response to potential future outbreaks.