The
nucleoprotein of Lassa virus, strain Josiah, was expressed in Escherichia coli as an N-terminally truncated,
histidine-tagged
recombinant protein. Following affinity purification the
protein was completely denatured and spotted onto
nitrocellulose membrane. A total of 1 microgram of
protein was applied for detection of Lassa virus
antibodies (LVA) in a simple immunoblot assay. Specific anti-Lassa
immunoglobulin M (
IgM)
antibodies could be detected by increasing the amount of
protein to 5 microgram. A panel of 913 serum specimens from regions in which Lassa virus was endemic and from regions in which Lassa virus was not endemic was used for evaluating the sensitivity and specificity of the LVA immunoblot in comparison to those of an indirect immunofluorescence (IIF) assay. The sera originated from field studies conducted in the Republic of Guinea (570 serum samples) and Liberia (99 serum samples), from inpatients of the clinical department of the Bernhard-Nocht-Institute, Hamburg, Germany (94 serum samples), and from healthy German blood donors (150 serum samples). In comparison to the IIF assay the LVA immunoblot assay had a specificity of 90.0 to 99.3%, depending on the origin of the specimens. The sensitivity was found to be highest for the Guinean samples (90.7%) and was lower for the Liberian samples (75%). Acute
Lassa fever was diagnosed by PCR in 12 of 59 (20.3%) patients with
fever of unknown origin (FUO) from the Republic of Guinea. On admission to the hospital, nine
Lassa fever patients (75%) were reactive by the
IgM immunoblot assay. One of the patients was infected with a new Lassa variant, which showed 10.4% variation on the
amino acid level in comparison to the prototype strain of Lassa virus, Josiah. Seven PCR-negative patients were reactive by immunoblotting. The positive and negative predictive values of a single
IgM immunoblot result for acute, PCR-confirmed
Lassa fever were therefore 53.6 and 93.0%, respectively. Because of its high negative predictive value, a single
IgM immunoblot result will be valuable for excluding acute
Lassa fever for cases of FUO in areas where
Lassa fever is endemic.