Abstract:
Background: Lassa fever (LF) is a zoonotic viral hemorrhagic disease endemic to West Africa, with Nigeria bearing the highest burden with seasonal outbreaks. Cases start rising in the dry season in the last quarter of the year, into the first quarter of the following year, with outbreaks occurring in multiple endemic states without one state bearing a disproportionate burden. In 2025, however, Ondo State accounted for 30% of the national caseload, with 85% of them concentrated in three of the state’s 18 Local Government Areas (LGA), indicating unusual clustering. With no vaccine currently available, preventive and control measures rely on understanding the transmission dynamics that drive peculiar transmissions. We aimed to identify key drivers of transmission in Ondo state, which accounted for one third of Nigeria’s confirmed LF cases, during the 2025 outbreak.
Methods: We conducted a 1:2 unmatched case-control study in the three most affected Local Government Areas (LGAs), which comprised 85% of the state’s LF burden between December 2024 and March 2025. Cases were laboratory confirmed LF patients, and controls were afebrile individuals from the same LGA within ±3 years of the corresponding case’s age. Data on socio demographic characteristics, housing, hygiene practices, rodent exposure, hunting, and travel history were collected using a semi-structured questionnaire and environmental assessments. Continuous variables were summarized using means and standard deviations, and proportions for categorical variables. Conditional logistic regression was used to identify risk factors for LF infection.
Results: Of 213 participants, 71 were cases (mean age: 36?±?13 years) and 142 controls (mean age: 35?±?13 years). Overall, 135 (63%) had a tertiary education, and 137 (64%) resided in urban areas. Most lived in brick houses (63.7%), while 48.8% and 20% reported recent travel and hunting activities, respectively. Occasional rodent presence was reported by 64.3%, and only 10.7% covered food during storage. Independent risk factors for LF infection included being a farmer (aOR: 5.79; 95% CI: 1.37–24.57), residing in mud houses (aOR: 4.46; 95% CI: 1.07–18.61), and contact with confirmed LF cases (aOR: 5.16; 95% CI: 1.49–17.87). Environmental assessments revealed poor food hygiene practices, inadequate infection prevention and control (IPC) in health facilities, and extensive rodent infestations in homesteads.
Conclusion: The findings highlight the multifactorial drivers of LF transmission, including poor housing, environmental exposures, and contact with infected individuals. Targeted hygiene education at community level, housing improvements, and enhanced IPC measures are critical to reducing transmission in endemic settings.
Keywords: Disease Outbreak; Lassa Virus Infection; Case Control Study, Risk Factors, Environment and Public Health
Biography:
Juliet Namugenyi graduated with a BSc in Biomedical Laboratory Technology from Makerere University Kampala, Uganda, and went on to complete a Masters of Science in Clinical Epidemiology and Biostatistics from the same University. She has over 10 years of experience in infectious diseases research in HIV and TB/HIV coinfections, and is a published author. She is currently in the final year of a field Epidemiology fellowship of the Africa CDC’s flagship Africa Epidemic Services Program, attached to the Nigeria Center for Disease Control and Prevention.
