Identifying risk factors for Lassa fever infection in Sierra Leone, 2019-2021

Background Lassa fever (LF) virus (LASV) is endemic in Sierra Leone and poses a significant public health threat to the region; however, no risk factors for LASV infection have been reported in Sierra Leone. The objective of this study was to identify the risk factors for LASV infection in an endemic community in Sierra Leone. Methods We conducted a case-control study by enrolling 37 laboratory-confirmed LF cases identified through the national LF surveillance system in Sierra Leone, and 140 controls resided within a one-kilometre radius of the case household. We performed conditional multiple logistic regression analysis to identify the risk factors for LASV infection. Results Of the 37 cases enrolled, 23 died (62% case fatality rate). Cases were younger than controls (19.5 years vs 28.9 years, p<0.05) and more frequently female (64.8% vs. 52.8%). Compared to the controls, LF cases had contact with rodents (rats or mice) in their households more frequently in the preceding three weeks (83.8% vs.47.8%). Households with a cat reported a lower presence of rodents (73% vs 38%, p<0.01) and contributed to a lower rate of LF (48.6% vs 55.7%) although not statistically significant (p=0.56). The presence of rodents in the households (Matched Adjusted Odds Ratio [mAOR]: 11.1), and younger age (mAOR: 0.99) were independently associated with LASV infection. Conclusion Rodent access to households is likely a key risk factor for LASV infection in rural Sierra Leone and potentially in other countries within the West African region. Controlling rodent access to households might help reduce household-level LASV infection in Sierra Leone.