Drivers of Lassa fever in an endemic area of southwestern Nigeria (2017-2021): An epidemiological study

Abstract

Background: Reporting two million human Lassa fever (LF) cases with around 10,000 associated annual mortality, the West African sub-region is endemic for Lassa fever virus (LASV). The true incidence of LF is difficult to determine because most LASV-infected individuals show no differentiating clinical signs and symptoms. We investigated the distribution of cases, post-hospitalization survival patterns, and evaluated factors contributing to infection and clinical course of the disease during an outbreak of LF in Ondo State, Nigeria, from 2017 to 2021.

Methods: We extracted LF data from the Integrated Disease Surveillance and Response weekly report of the Nigerian Centre for Disease Control for 2017-2021. Kaplan-Meier estimate was used to describe the probability of survival among the LF cases. Also, a univariable binary logistic regression was used to explore factors associated with mortality among the study participants. Key informant was interviewed and environmental assessments were also done.

Results: LASV infection was confirmed in 1,115 (24.5%) of 4,551 cases with clinical signs suggestive of LF (age 35.24 ± 20.77) and case fatality rate of 25.5%. Hospitalized patients who did not recover within 17 days had less than 50% chance of survival. Age is a strong predictor of survival; hospitalized patients >40 years were significantly more likely than younger ones to experience mortality (Odds ratio:2.46; 95% CI = 1.67-3.62; p < 0.01). Similarly, male patients were significantly less likely than the females to survive beyond 10 days of hospitalization. Open sun drying of food items and congested urban residential settings with history of frequent rat sightings are possible factors for the increase of LF cases in the study area.

Conclusion: Current case definition in Ondo State identified close to 25% of laboratory confirmed LASV infection. Human activities during the dry season (October-March) are associated with increased LF cases. We propose a One Health disease surveillance approach that synchronizes farming activities with educational campaigns as a mitigation strategy against LASV infection and mortality in Nigeria.

Fig 1. The distribution of suspected and confirmed cases and deaths by LGA based on Ondo State LF data, 2017-2021.

Fig 2. Map of Ondo State showing the 18 Local Government Areas.

The intensity of the colour indicates the number of LF cases. Source: Department of Geography, University of Ibadan; https://Grid3.org. The administrative boundary used in the thematic mapping of Lassa fever across LGAs in Ondo State was obtained from the Grid 3. The link to LGA boundaries in Nigeria is: https://data.grid3.org/datasets/GRID3::grid3-nga-operational-lga-boundaries/about.

Fig 3. Seasonal trend of suspected LF cases, Ondo State, 2017-2021.

Fig 4. A. Kaplan-Meier survival estimate of LF cases in Ondo State, 2021.

B: Kaplan-Meier survival estimate of LF cases (by gender) in Ondo State, 2021.

Fig 5. Socio-ecological factors sustaining and exacerbating the endemicity and outbreak of LF in Owo and adjoining towns in Ondo State.

This Figure was generated using keynote (the presentation App on Macbook). All the images and clipart within the figure were drawn by hand with the App.