ANTIGENIC ANALYSIS OF NIGERIAN STRAINS OF WEST NILE VIRUS

Abstract

Sero-immunological methods were employed to investigate the antigenic relationships among West Nile virus isolates originating in Nigeria. The techniques included neutralization test in new born mice, complement fixation, haemagglutination inhibition and agar-gel precipitation tests. These were further supplemented by plaque formation, plaque inhibition test in tissue culture and chemical elution on calcium phosphate columns. They were further classified according to the zoogeographic classification established for WN virus strains from different parts of tho world. Eight Nigerian strains of WN virus were originally examined. In the course, of preliminary investigation, one of these strains was found to be a strain of Usutu virus. It did not belong to the WN virus group because of its deviation from the generally accepted serological relationships existing among related virus strains. The remaining seven strains were differentiated into two serological intratypic groups based on their cross reactions with two other strains which served as proto-types for each sub-group. Five related strains which constitute one antigenic sub-type were found to be related to the strain of WN virus originating from general palaearctic zoogeographic zone (European and Middle East). Theoretical considerations of the origin and the spread of these virus strains from the general palaearctic zone were examined. Among other things, it appeared that these strains must have been brought to Nigeria through domestic animals, especially camels and further spread by birds and susceptible arthropod vectors. Application of these findings include epidemiological studies, screening of sera and serodiagnosis. It was found that strain 7019 which had a broad spectrum of reactivity with the other strains was a most suitable strain for detecting and evaluating antibodies to WN virus. One new antigenic sub-group comprising strains 4029 and 38045 were discovered not to belong to either the Palaearctic, Oriental or Ethiopian group, the three zoogeographic areas of the world to which WN virus strains have been classified. These new strains appeared unique so far to Nigeria and possibly to West Africa in general.