Boosting understanding of Lassa Fever virus epidemiology: Field testing a novel assay to identify past Lassa Fever virus infection in blood and oral fluids of survivors and unexposed controls in Sierra Leone

Background Despite identification 50 years ago, the true burden of Lassa Fever (LF) across Africa remains undefined for reasons including research focus on hospitalised patients, lack of validated field-feasible tools which reliably identify past infection, and the fact that all assays require blood samples making large-scale surveys difficult. Designated a priority pathogen of epidemic potential requiring urgent research by the World Health Organisation, a better understanding of LF sero-epidemiology is essential to developing and evaluating new interventions including vaccines. We describe the first field testing of a novel species-neutral Double Antigen Binding Assay (DABA) designed to detect antibodies to LF in plasma and oral fluid. Methodology/Principal findings Paired plasma and oral fluid were collected in Sierra Leone from survivors discharged from Kenema Government Hospital Lassa Fever Unit between 1980 and 2018, and from controls recruited in Freetown in 2019. Epidemiological sensitivity and specificity of the DABA measured against historical diagnosis in survivors and self-declared non-exposed controls was 81.7% (95% CI 70.7%– 89.9%) and 83.3% (72.7%- 91.1%) respectively in plasma, and 71.8% (60.0%– 81.9%) and 83.3% (72.7%– 91.1%) respectively in oral fluid. Antibodies were identified in people infected up to 15 years and, in one case, 40 years previously. Participants found oral fluid collection easy and painless with 80% happy to give an oral fluid sample regularly. Conclusions/Significance Given the difficulties of assay validation in a resource-limited setting, including unexpected exposures and diagnostics of varying accuracy, the new assay performed well in both plasma and oral fluid. Sensitivity and specificity are expected to be higher when case/control ascertainment is more definitive and further work is planned to investigate this. Even at the performance levels achieved, the species-neutral DABA has the potential to facilitate the large-scale seroprevalence surveys needed to underpin essential developments in LF control, as well as support zoonotic investigations.
Author summary Lassa Fever (LF) is a life-threatening rodent-borne viral haemorrhagic disease of epidemic potential particularly affecting some West African communities. Although it is estimated to cause 100,000–300,000 infections and 5,000 deaths annually, the true burden of the disease in the community is not well understood since research has largely focussed on hospitalised cases (thought to account for ~15% of infections) because of the difficulty of identifying mild or asymptomatic cases in the community. Gaining a better understanding of LF epidemiology, particularly the exposure and immunity status of populations in countries where the virus is endemic in the rodent hosts, is critical to developing control measures including vaccines which can protect against disease and/or reduce risk of epidemic transmission. A key step is to be able to identify who has been previously infected by virus, and therefore likely immune, and who remains susceptible by examining the prevalence of antibodies to the virus in the community. However, to date all tests require blood to be drawn which can be problematic for community participation as well as risky for collectors. This study reports on a project to develop and validate an antibody test that can be used with an easy-to-give oral fluid sample.