A hospital epidemic of Lassa fever in Zorzor, Liberia, March-April 1972

In March 1972 cases of Lassa fever were recognized among patients and staff at an American missionary hospital in Zorzor, Liberia. Eleven cases were diagnosed clinically, by virus isolation, and by serological tests. Clinically and epidemiologically this nosocomial epidemic resembled a previous one in 1970 in Jos, Nigeria. The index case, apparently sporadically infected in her home village, was hospitalized on an open Obstetrical (OB) Ward. Subsequently, 3 OB patients and 7 members of the hospital staff became ill, all within 1 week of the date of discharge of the index case. The case-fatality rate was 36%. Of 93 other contacts of the index and secondary cases, 8 had CF antibodies indicating recent infection but none had a history of illness. None of 40 hospital staff members without exposure had antibodies. Length and intimacy of exposure to the index case was associated with a high risk of clinical infection. Neither the exact mode(s) of spread within the hospital nor the source of infection of the index case were elucidated. This epidemic provided evidence for 1) the occurrence of Lassa virus in widely separated foci in West Africa, 2) the frequent occurrence of very mild or subclinical Lassa infections, and 3) the importance of close personal contact for infection resulting in clinical illness.