Malaria Outbreak Facilitated by Increased Human Activities around Swamps: Kisoro District, Uganda, December 2017-March 2018

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Abstract Background: Malaria elimination is increasingly becoming a global priority. On 19 December 2018, a district in southwestern Uganda known to have achieved pre-elimination levels of malaria (<30 cases per month) reported a sudden increase in cases to the Ministry of Health. We investigated to determine the magnitude and scope of the outbreak and identify exposures associated with transmission. Methods:We reviewed medical records in all health facilities in the affected Kanaba and Murora sub-counties to find cases. We calculated attack rates (AR) by age, sex, and village using the projected 2016 population. In a case-control study, we compared potential exposures between case-patients, selected randomly from the line-list, and neighborhood- and age-matched asymptomatic controls. We conducted an entomological and environmental assessment of randomly-selected households and potential breeding sites of the affected sub-counties. Results: We found 3,130 malaria cases (compared with 879 cases during the same period the previous year). Persons in the age groups 10-19 (AR=14/1,000) and 20-29 (AR=12/1,000) years were the most affected. Villages closest to the swamp had the highest attack rate. In the case-control study, 89% (129/145) of case-patients and 73% (106/145) of controls reported frequent late-night mosquito bites (ORCLR=3.9; 95%CI: 1.8-8.4); 70% (102/145) of case-patients and 59% (86/145) of controls had a household size >5 (ORCLR=1.8; 95%CI: 1.04-3.1); 78% (113/145) of case-patients and 86% (125/145) of controls usually slept under a bednet (ORCLR=0.56; 95%CI: 0.29-1.1). Anopheles gambiae sensu lato was identified in breeding places around Sereri and Mpundu swamps. In total, 64% (23/36) of female adult Anopheles captured in case-patients’ households by pyrethrum spray catches were fed, suggesting low bed-net usage the previous night in the surveyed households. Conclusion: This malaria outbreak was likely propagated by favorable breeding conditions, including recent heavy rainfall, and exacerbated by new human activities around two swamps. To achieve malaria elimination, low-endemic areas should be particularly aware of activities that can lead to resurgences in malaria, such as night-time exposures, lack of bed-net usage, and human activities focused around breeding sites. We recommended increased awareness about bed-net usage and use of larvicides in the residential area and swamps to break the breeding cycle.

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