Genetic analysis of an influenza B virus isolated from a patient with encephalopathy in Japan

Genetic diversity of influenza B virus: the frequent reassortment and cocirculation of the genetically distinct reassortant viruses in a community

To characterize the genetic diversity of influenza B viruses isolated during one influenza season, the antigenic and genetic relationships among 20 strains of influenza B virus isolated in February and March 2001 at one pediatric clinic in Yamagata City, Japan, were investigated. The HA gene and seven other gene segments were phylogenetically divided into three distinct sublineages (Harbin/7/94-, Tokyo/6/98-, and Shiga/T30/98-related lineage) of the Yamagata/16/88-like lineage. The NS genes of the viruses belonging to the Harbin/7/94-related lineage have additional three nucleotides at positions 439-447, and were phylogenetically distinguishable from those of the currently circulating Yamagata/16/88- and Victoria/2/87-like lineages, but were closely related to that of the Yamagata/16/88-like lineage isolated before 1994. Moreover, four strains of influenza B virus isolated in the same community between 2002 and 2003 were further examined. Phylogenetic analysis revealed that a virus of Victoria/2/87-like lineage isolated in 2003 had acquired the NA, NS, M, and PA gene segments from a Shiga/T30/98-like virus, and two strains of Harbin/7/94-related lineage had acquired the various gene segments from Shiga/T30/98-like virus through a reassortment event. These results indicate that genetically distinct multiple viruses can combine to cause an influenza B epidemic in a community and that the frequent reassortment among these viruses plays a role in generating the genetic diversity of influenza B viruses.

Multiple genotypes of influenza B virus circulated between 1979 and 2003

The segmented genome of influenza B virus allows exchange of gene segments between cocirculating strains. Through this process of reassortment, diversity is generated by the mixing of genes between viruses that differ in one or more gene segments. Phylogenetic and evolutionary analyses of all 11 genes of 31 influenza B viruses isolated from 1979 to 2003 were used to study the evolution of whole genomes. All 11 genes diverged into two new lineages prior to 1987. All genes except the NS1 gene were undergoing linear evolution, although the rate of evolution and the degree to which nucleotide changes translated into amino acid changes varied between lineages and by gene. Frequent reassortment generated 14 different genotypes distinct from the gene constellation of viruses circulating prior to 1979. Multiple genotypes cocirculated in some locations, and a sequence of reassortment events over time could not be established. The surprising diversity of the viruses, unrestricted mixing of lineages, and lack of evidence for coevolution of gene segments do not support the hypothesis that the reassortment process is driven by selection for functional differences.

Encephalitis associated with influenza B virus infection in 2 children and a review of the literature

Two children with influenza B-associated encephalitis (IBAE) presented to our hospital during the winter of 2000-2001, both of whom had cases notable for mutism in association with encephalitis. A review of the literature identified 13 additional reports consistent with IBAE that contained sufficient data for analysis. Eleven of 15 reported cases occurred in children aged <or=18 years; of these, more than one-half occurred in children <11 years of age. Neurologic symptoms appeared within the first 4 days of illness in 13 cases. Speech abnormalities were observed in 4 patients and consisted of mutism in 3. Although the majority of patients recovered fully, 3 were left with neurologic sequelae, and 1 died. These cases reveal the spectrum of IBAE and its potential for long-term sequelae. Clinicians caring for children should remain vigilant for this rare complication of influenza B virus infection.