Lassa Fever in the Eastern Province of Sierra Leone, 1970–1972

Twelve patients hospitalized with a confirmed diagnosis of Lassa fever were studied during an epidemic of this disease in Sierra Leone. Clinical observations confirmed and extended those made in previous outbreaks in Liberia and Nigeria. Two patients were treated with plasma containing antibodies to Lassa virus; both had a favorable response. Lassa virus was isolated from serum, throat swabs, or urine from all patients sampled during the first 15 days of illness. Virus was recovered from the pharynx of patients with circulating complement-fixing (CF) antibodies. CF antibodies were detectable by the 3rd week of illness. A strain of Lassa virus from one of the patients was compared with strains recovered in Liberia and Nigeria by CF test. Serum from convalescent patients infected with Nigerian virus did not fix complement in the presence of strains from Liberia or Sierra Leone; this result may indicate antigenic differences between Lassa virus strains which must be confirmed by neutralization test.