Bats harbor many viruses, which are periodically transmitted to humans resulting in outbreaks of disease (e.g., Ebola, SARS-CoV). Recently, influenza virus-like sequences were identified in bats; however, the viruses could not be cultured. This discovery aroused great interest in understanding the evolutionary history and pandemic potential of bat-influenza. Using synthetic genomics, we were unable to rescue the wild type bat virus, but could rescue a modified bat-influenza virus that had the HA and NA coding regions replaced with those of A/PR/8/1934 (H1N1). This modified bat-influenza virus replicated efficiently in vitro and in mice, resulting in severe disease. Additional studies using a bat-influenza virus that had the HA and NA of A/swine/Texas/4199-2/1998 (H3N2) showed that the PR8 HA and NA contributed to the pathogenicity in mice. Unlike other influenza viruses, engineering truncations hypothesized to reduce interferon antagonism into the NS1 protein didn't attenuate bat-influenza. In contrast, substitution of a putative virulence mutation from the bat-influenza PB2 significantly attenuated the virus in mice and introduction of a putative virulence mutation increased its pathogenicity. Mini-genome replication studies and virus reassortment experiments demonstrated that bat-influenza has very limited genetic and protein compatibility with Type A or Type B influenza viruses, yet it readily reassorts with another divergent bat-influenza virus, suggesting that the bat-influenza lineage may represent a new Genus/Species within the Orthomyxoviridae family. Collectively, our data indicate that the bat-influenza viruses recently identified are authentic viruses that pose little, if any, pandemic threat to humans; however, they provide new insights into the evolution and basic biology of influenza viruses.

Influenza Influenza viruses are classified as genera A, B, and C, in accordance with the antigenic differences in their nucleoproteins (NP) and matrix 1 (M1) proteins (28). Influenza A (IAV) and B (IBV) viruses can result in severe upper respiratory disease in humans, while influenza C viruses (ICV) cause relatively mild disease (9, 23). Among influenza viruses, IAV and IBV are very similar in terms of genome structure and organization. IBV, along with influenza A(H3N2) and A(H1N1) viruses [including A(H1N1)pdm09 virus], cause seasonal influenza epidemics annually (9, 23). In the United States alone during 1976 to 2007, approximately 3,000 to 49,000 deaths each year have been attributed to these epidemics (42). Some reports indicate that in older children and healthy adults, influenza A(H3N2) virus is responsible for the most severe cases, followed by IBV, while influenza A(H1N1) virus infections tend to manifest as the mildest cases of illness (1,5,23,25). In some seasons, however, IBV may be the predominate strain responsible for influenza activities. This was best exemplified by the 1979-1980 season, in which IBV was the predominant strain circulating in the United States; therefore, it was responsible for influenza outbreaks and excess pneumonia and influenza deaths nationwide (39). Furthermore, IBV has been reported to be associated with central nervous system complications, such as Reye's syndrome and encephalitis in children (1).IBVs continue to circulate worldwide alongside IAVs. Actively circulating IBVs are divided into two genetically and antigenically

H9N2 avian influenza virus (AIV) has an extended host range, but the molecular basis underlying H9N2 AIV transmission to mammals remains unclear. We isolated more than 900 H9N2 AIVs in our 3-year surveillance in live bird markets in China from 2009 to 2012. Thirty-seven representative isolates were selected for further detailed characterization. These isolates were categorized into 8 genotypes (B64 to B71) and formed a distinct antigenic subgroup. Three isolates belonging to genotype B69, which is a predominant genotype circulating in China, replicated efficiently in mice, while the viruses tested in parallel in other genotypes replicated poorly, although they, like the three B69 isolates, have a leucine at position 226 in the hemagglutinin (HA) receptor binding site, which is critical for binding human type sialic acid receptors. Further molecular and single mutation analysis revealed that a valine (V) residue at position 190 in HA is responsible for efficient replication of these H9N2 viruses in mice. The 190V in HA does not affect virus receptor binding specificity but enhances binding affinity to human cells and lung tissues from mouse and humans. All these data indicate that the 190V in HA is one of the important determinants for H9N2 AIVs to cross the species barrier to infect mammals despite multiple genes conferring adaptation and replication of H9N2 viruses in mammals. Our findings provide novel insights on understanding host range expansion of H9N2 AIVs. IMPORTANCE Influenza virus hemagglutinin (HA) is responsible for binding to host cell receptors and therefore influences the viral host rangeand pathogenicity in different species. We showed that the H9N2 avian influenza viruses harboring 190V in the HA exhibit enhanced virus replication in mice. Further studies demonstrate that 190V in the HA does not change virus receptor binding specificity but enhances virus binding affinity of the H9N2 virus to human cells and attachment to lung tissues from humans and mouse. Our findings suggest that more attention should be given to the H9N2 AIVs with HA-190V during surveillance due to their potential threat to mammals, including humans. Since the first isolate of an avian influenza virus (AIV) H9N2 subtype was reported in the United States in 1966, three distinct lineages of H9N2 viruses that caused outbreaks in domestic poultry in Asia have been identified (1, 2). H9N2 is a predominant subtype of AIVs circulating in poultry farms in Asia and the Middle East and has caused substantial economic losses over the past decade (3-9). Although a large amount of H9N2 vaccines, including inactivated and vectored vaccines, have been used in areas of endemicity, outbreaks caused by H9N2 AIVs are still not efficiently controlled. Importantly, H9N2 AIV has been reported to infect mammals, including humans, pigs, and dogs (10-13), indicating that it has an extended host range. It should be noted that several human infections with an H9N2 AIV have been recorded (14,15). Numerous human infections have also been confirm...

Rift Valley fever (RVF) is a zoonotic disease that causes severe epizootics in ruminants, characterized by mass abortion and high mortality rates in younger animals. The development of a reliable challenge model is an important prerequisite for evaluation of existing and novel vaccines. A study aimed at comparing the pathogenesis of RVF virus infection in US sheep using two genetically different wild type strains of the virus (SA01-1322 and Kenya-128B-15) was performed. A group of sheep was inoculated with both strains and all infected sheep manifested early-onset viremia accompanied by a transient increase in temperatures. The Kenya-128B-15 strain manifested higher virulence compared to SA01-1322 by inducing more severe liver damage, and longer and higher viremia. Genome sequence analysis revealed sequence variations between the two isolates, which potentially could account for the observed phenotypic differences. We conclude that Kenya-128B-15 sheep infection represents a good and virulent challenge model for RVF.