Lassa fever (LF) is endemic and poses public health threats in Edo State. Identification of primary clusters will help prioritize public health interventions in the state. We investigated retrospective cases of LF to identify primary cluster of the disease for household exposure management. Method: We reviewed retrospective data (n = 1400) of LF case-patients at a referral hospital in Edo State from 2008 to 2014 based on World Health Organization case definition for LF. We determined primary

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... ster of confirmed cases on Bernoulli model and evaluated environmental factors in the primary cluster: presence of rodent deterrents, proximity of households at 2 km radius to rice farm, rice post-harvest storage facility, refuse dump, forest, hospital and main road using Multi-criteria analysis at p < 0.05. Results: Of the reviewed cases, 171 (12.2%) were confirmed case-patients. The median age of confirmed case-patients was 30 years (Q1:16.0 and Q3: 43.5). Of the confirmed case-patients, 101 (59.1%) were male. A primary spatial cluster (4.45 km radius; geographic centre at 6.717900oN, 6.243500oE) was identified in Esan West LGA. Associated environmental factors included presence of rodent deterrents (p < 0.001), proximities of households to refuse dump (p < 0.001), rice post-harvest storage facility (p = 0.01) and rice farm (p = 0.03). Conclusion: Esan West LGA was identified as a primary cluster for Lassa fever. Presence of rodent deterrents, household proximity to refuse dump sites, rice storage facilities and rice farms were associated environmental factors. We recommend improved rice post-harvest storage and use of rodent deterrents in Edo State, Nigeria.