Co-Infection of Lassa Fever-Malaria (LF) in Endemic Settings: Influence on LF Case Detection in Health Facilities in Ondo State, Nigeria

In malaria-endemic settings, a positive malaria result may explain fever while Lassa fever (LF) remains clinically possible.This study assessed LF-malaria co-infection, probable missed LF suspicion, and diagnostic delays in selected health facilities in Ondo State, Nigeria.A convergent mixed-methods design combined retrospective review of 463 LF and malaria records from 2021 -2023 across 10 purposively selected facilities in five high-burden LGAs with nine key-informant interviews.Co-infection required positive LF and malaria laboratory results.Quantitative data were analysed using descriptive statistics, chi-square tests and logistic regression; qualitative findings were analysed thematically and integrated through a joint display.All 463 records were purposefully selected as confirmed LF cases.Of these, 297 (64.1%) had positive LF and malaria results.Malaria was suspected in 440 (95.0%) records, and 324 (70.0%) had prior malaria treatment.Probable missed LF suspicion occurred in 103 (22.2%) records.Malaria positivity was associated with missed suspicion (adjusted odds ratio [aOR] 5.30, 95% CI 2.13-13.19;p < 0.001).Interviews highlighted symptom overlap, malaria-first reasoning, treatment failure, restricted RT-PCR access, specimen-transport barriers, notification gaps and uneven refresher training.Overall, 386 (83.4%) patients recovered and 53 (11.4%) died; the case-fatality proportion among confirmed LF records was 11.4%.In this selected sample, malaria appeared to mask LF rather than establish an independent cause of mortality.Malaria confirmation should not close the LF detection pathway.Parallel assessment, timely LF specimen collection, reliable transport, effective surveillance communication and practical diagnostic algorithms may support earlier detection.