The Sensitivity and Specificity of Lassa Virus IgM by ELISA as Screening Tool at Early Phase of Lassa Fever Infection

"Background: Early diagnosis; prompt treatment; and disease containment are vital measures in the management of Lassa fever (LF); a lethal and contagious arenaviral hemorrhagic disease prevalent in West Africa. Lassa Virus (LAV)-specific Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) test; the gold standard for diagnosis; is unavailable in most centers. Serologic detection of LAV IgM is a more accessible tool and this work was to investigate its adequacy as an early marker for LF. Patients and Methods: A prospective case-control study conducted July 2007-March 2011 in a tertiary referral health center in Nigeria. Blood samples for test and control were evaluated for Lassa specific antigens and IgM using RT-PCR (primers S36+ and LVS 339) and indirect ELISA (Lassa Nucleo-protein (NP)-Antigen) respectively. RT-PCR outcome was used as standard to test for the sensitivity and specificity of IgM. Results: Of the 37 confirmed cases of LF infection by RT-PCR; 21 (57) were IgM positive. Amongst the 35 confirmed negative cases (control group); eight were IgM positive. The diagnostic sensitivity and specificity of the IgM assay were 57 and 77 respectively. The negative and positive predictive values of the IgM serological assay were 63 and 72 ; respectively; while the efficiency of the test was 67. Conclusion: The specificity and sensitivity of IgM as a screening tool for early detection of LF appear weak and; hence; the need for a reliable LF ""rapid screening kit"" since RT-PCR is unavailable in most centers. In the interim; ""high clinical index of suspicion;"" irrespective of IgM status; requires urgent referral to confirmatory centers."

Lassa Fever: another infectious menace

Nigeria is presently suffering from another Lassa fever epidemic. This was confirmed in the statement of the Minister of Health of the Federation in which he said; ""There has been an upsurge in the reported cases of Lassa fever since the beginning of this year; especially in the Federal Capital Territory and its environs. Within two weeks; 12 cases with five deaths due to the disease were recorded. 25 contacts are confirmed by laboratory investigations to have been infected; including 4 health staff working in the National Hospital; Abuja.""1 Lassa fever is an acute viral haemorrhagic fever first described in 1969 in the town of Lassa in Borno state; Nigeria.2 It is endemic in West African countries; and causes 300;000 cases annually with 5000 deaths.3 Lassa fever epidemics occur in Nigeria; Liberia; Sierra Leone; Guinea and the Central African Republic.4 Lassa virus; the agent of the disease is a member of the Arenaviridae family. The virus is pleomorphic with single-stranded and bisegmented RNA genome.3 Its primary host is Natal Multimammate Mouse (Mastomys natalensis). Transmission to man occurs via exposure to the rat excrement through respiratory or gastrointestinal tracts5; exposure of broken skin or mucus membrane to infected material; direct contact; sexually and transplacentally. The prevalence of antibodies to the virus is 8-229 in Sierra Leone; 4-55in Guinea;12 and 21in Nigeria.13 The disease is mild or asymptomatic in 80of infected people; but 20have a severe multisystemic disease. Clinical features are difficult to differentiate from that of other viral haemorrhagic fevers and common febrile illness such as Malaria; Typhoid fever and so on. Definitive diagnosis is by viral isolation; Antigen and Antibody detection and Reverse Transcriptase PCR. Treatment is with Ribavirin; an antiviral agent. No vaccine is currently available. Prevention is by keeping rats away from homes