Fifty years of imported Lassa fever: a systematic review of primary and secondary cases

Rationale for Systematic Review: Lassa fever is the most common cause of imported haemorrhagic fevers cases in non-endemic countries. As a disease with a high case fatality rate that has regularly caused clusters of nosocomial transmission in endemic areas, prompt diagnosis is vital. We conducted a systematic review of imported cases of the last 50 years with the aim of defining the clinical and epidemiological characteristics that will enhance early diagnosis, prompt initiation of treatment and an appropriate public health response to Lassa fever cases. Methods: We performed a retrospective, systematic review of 36 primary and two secondary cases of Lassa fever in non-endemic countries outside West Africa by searching the PubMed database. This yielded 56 relevant publications that were included in our analysis. Results: The case fatality rate of 35.1% for imported cases was higher than that reported for endemic countries. The majority of patients showed clinical features consistent with Lassa fever and had a typical exposure. There was a considerable delay in diagnosis in imported cases with high associated numbers of contacts. Ribavirin was rarely used for post-exposure prophylaxis. Only two secondary transmissions occurred. Thirty-one percent of patients received Lassa fever-specific treatment and five required intensive care. Conclusions: Although importation of Lassa fever to non-endemic countries is a rare event, it has repeatedly happened over five decades. Suspicion of Lassa fever should be based on careful consideration of clinical features and exposure history in order to assist early diagnosis in returning travellers from West Africa.