Abstract
Malaria epidemiology in Kenya is heterogeneous because of geographic and climatic differences. Semi-arid and arid zones are prone to seasonal increases in malaria cases above expected levels after rainy seasons, leading to malaria outbreaks. In November 2023, Marsabit County experienced a 155% increase in rainfall above its monthly average. The malaria surveillance system detected a 345% increase in cases, rising from 210 during Epiweek 1 to Epiweek 6 of the previous year to 934 in 2024. An outbreak investigation was conducted to guide prevention and control efforts. We reviewed routine surveillance data from December 2023 to February 2024, abstracted data from 12 facilities that surpassed action thresholds (5-year weekly median + third quantile), conducted data quality assessments, and conducted two community-focused group discussions. The data were analyzed as frequencies and proportions. Of the 757 malaria cases abstracted, the median age was 17 years (interquartile range: 10-28 years), with 227 cases (30.0%) in individuals aged 10-20 years. Males accounted for 424 cases (56.0%), 421 cases (55.6%) were tested using Plasmodium falciparum (P. falciparum)-specific rapid diagnostic tests, and 44.4% were tested using microscopy. Among the cases identified via microscopy tests, 94.9% involved P. falciparum, 2.7% involved Plasmodium vivax, and 2.4% involved Plasmodium ovale. There were 90 cases (11.9%) of severe malaria and three deaths. The reporting accuracy was 90% for a third of the facilities, and the completeness of monthly summaries was 39%. Potential exposures reported by the community included proximity to a national park, proximity to stagnant water, and sleeping outside during herding. The outbreak was predominantly due to P. falciparum malaria but also involved non-falciparum malaria, with most cases occurring in males aged 10-20 years. We recommend the continuous monitoring of malaria species to improve malaria surveillance.