Space-Time Trends in Lassa Fever in Sierra Leone by ELISA Serostatus, 2012-2019

Abstract

Lassa fever (LF) is a viral hemorrhagic disease found in Sub-Saharan Africa and is responsible for up to 300,000 cases and 5000 deaths annually. LF is highly endemic in Sierra Leone, particularly in its Eastern Province. Kenema Government Hospital (KGH) maintains one of only a few LF isolation facilities in the world with year-round diagnostic testing. Here we focus on space-time trends for LF occurring in Sierra Leone between 2012 and 2019 to provide a current account of LF in the wake of the 2014-2016 Ebola epidemic. Data were analyzed for 3277 suspected LF cases and classified as acute, recent, and non-LF or prior LF exposure using enzyme-linked immunosorbent assays (ELISAs). Presentation rates for acute, recent, and non-LF or prior LF exposure were 6.0% (195/3277), 25.6% (838/3277), and 68.4% (2244/3277), respectively. Among 2051 non-LF or prior LF exposures, 33.2% (682/2051) tested positive for convalescent LF exposure. The overall LF case-fatality rate (CFR) was 78.5% (106/135). Both clinical presentations and confirmed LF cases declined following the Ebola epidemic. These declines coincided with an increased duration between illness onset and clinical presentation, perhaps suggesting more severe disease or presentation at later stages of illness. Acute LF cases and their corresponding CFRs peaked during the dry season (November to April). Subjects with recent (but not acute) LF exposure were more likely to present during the rainy season (May to October) than the dry season (p < 0.001). The findings here suggest that LF remains endemic in Sierra Leone and that caseloads are likely to resume at levels observed prior to the Ebola epidemic. The results provide insight on the current epidemiological profile of LF in Sierra Leone to facilitate LF vaccine studies and accentuate the need for LF cohort studies and continued advancements in LF diagnostics.

Keywords: Ebola virus disease; Lassa fever; Lassa virus; Sierra Leone; case-fatality rate; enzyme-linked immunosorbent assay.

Figure 1

Lassa fever screenings at Kenema Government Hospital by admission status, 2012–2019. Note. Admitted refers to subjects admitted to KGH Lassa Fever Ward following confirmed Lassa fever diagnosis.

Figure 2

Annual fatality rates by Lassa fever serostatus. (a) Acute LF exposure; (b) Recent LF exposure (Ag−/IgM+); (c) Non-LF or prior LF exposure (Ag−/IgM−).

Figure 3

Receiver Operating Characteristic (ROC) analyses for subject age at clinical presentation for suspected Lassa fever cases. (a) ROC results for determining patient survival outcome on clinical presentation; (b) ROC result for determining acute Lassa fever seropositivity on clinical presentation. The leftmost numbers inside the plot are the age values, and the middle and rightmost values correspond to their respective sensitivity and 1-specificity values.

Figure 4

Suspected and confirmed Lassa fever cases screened at Kenema Government Hospital, 2012–2019. (a) Presentations between 1 January 2012 and 31 December 2013; (b) Presentations between 1 January 2014 and 31 December 2014; (c) Presentations between 1 January 2016 and 31 December 2017; (d) Presentations between 1 January 2018 and 31 December 2019. Following the 2014–2016 Ebola epidemic, suspected and confirmed cases were observed in closer proximity to Kenema District.

Figure 5

Monthly distribution of suspected LF cases by serostatus. Month of clinical presentation was determined according to the date the initial ELISA Ag and IgM tests were performed. (a) Acute Lassa fever exposure (Ag+), (b) Recent Lassa fever exposure (Ag−/IgM+); (c) Non-Lassa fever cases or prior Lassa fever exposure (Ag−/IgM−). All tests were for suspected LF cases defined according to its suspected case definition. The blue-shaded regions represent the seasonal rainfall period in Sierra Leone (May to October).