Originally discovered in 1969, Lassa fever (LF) is a zoonotic viral hemorrhagic fever endemic to West Africa and caused by Lassa Virus (LASV). Diagnosis of LF in patients returning from West Africa may be challenging for providers unfamiliar with the spectrum of its clinical presentation. Additionally, although LASV is not transmitted via casual contact, contact-tracing investigations of returning cases have often been large in scale. To quantify the frequency of case patients presenting with distinctive clinical features, time from patient presentation to clinician suspicion of a LF diagnosis, and the risk for secondary LF transmission, a retrospective review of all 33 LF cases imported to non-endemic countries between 1969 and 2016 was performed.
METHODS:
PubMed was searched from August 1 through December 11, 2017 for publications using the terms “Lassa” and “Lassa Fever”. Additional publications were identified by reviewing references in retrieved reports, and official correspondence by epidemiologists involved in these cases. Seventy-four publications discussing clinical or epidemiological aspects of the 33 imported LF cases were selected for review. Information was collected pertaining to case demographics, distinctive clinical features suggestive of LF, time from patient presentation to clinical suspicion, number of total and high-risk contacts traced. Distinctive clinical features were defined as fever and at least one of the following: pharyngitis, sore throat, tonsillitis, conjunctivitis, oropharyngeal ulcers, or proteinuria. High-risk contacts were defined as having unprotected contact with patients or their body fluids.
RESULTS:
Of the 28 cases for whom clinical information was available, 15 (54%) had distinctive clinical features and 13 (46%) did not. Time from patient presentation to clinician suspicion of LF ranged from 1 to 22 days (median 4.5 days). In 7 cases, diagnosis of LF was made retrospectively following patient discharge or death. Contact tracing investigation details were reported for 17/33 cases (52%) and ranged from 3 to 552 contacts per investigation. In total, 3,162 contacts were traced, of which 107 were classified as high-risk exposures. Only 2 cases of secondary transmission were reported, both in high-risk contacts in Germany.
CONCLUSIONS:
Approximately half of the imported cases of LF had distinctive clinical features, and delays in clinical suspicion of this diagnosis were common. Additionally, there was no secondary transmission of LF to contacts with low-risk exposures, and infection of high-risk contacts was rare. Future public health investigations of returning travelers with LF should focus exclusively on identification and tracing of contacts with high-risk exposures.