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In February 2013, a patient hospitalized with pneumonia and acute respiratory distress syndrome (ARDS) was confirmed to be infected with a rare subtype of H7N9 avian influenza A virus (A/Shanghai/1/2013) [1]. Since then, a total of 132 laboratory-confirmed human H7N9 infections have been reported in eastern China and Taiwan, resulting in a 28% mortality rate (as of 30 May 2013) [2, 3]. No human H7N9 cases have been reported in other countries. The virus did not spread efficiently among humans, but limited, nonsustained human-to-human transmission could not be excluded in a few family clusters [4]. Viruses of the H7N9 subtype had not been previously isolated from humans, and infections with low-pathogenic avian influenza viruses have not been associated with severe infection or death. This is important because current seasonal influenza vaccines contain only H1N1 and H3N2 influenza A antigens, and all population groups are immunologically naive to both H7 and N9 surface antigens. The presence of H7 surface antigen is also troubling as highly pathogenic viruses of this HA subtype were shown to infect humans and 1 fatal case was reported [5]. Although vaccines are the most effective option for controlling influenza infections, none are available for prevention of H7N9 infections [6]. Human influenza H7N9 viruses are resistant to adamantanes as they harbor an S31N amino acid substitution in the M2; thus, neuraminidase (NA) inhibitors (NAIs) are the only option for control of H7N9 influenza infections [1]. The NAI oseltamivir is recommended for treatment of human H7N9 infections [4, 7]. Challenges in H7N9 antiviral treatment include the development of ARDS as a common complication in patients hospitalized with pneumonia [1, 8] as well as limited clinical experience and knowledge about the effectiveness of oseltamivir on human H7N9 infections. Moreover, human H7N9 viruses with NAI resistance-associated mutations (R152K and R292K, N2 numbering) were reported during treatment, emphasizing that the emergence of drug-resistant viruses should be monitored [1, 9]. Mouse models are well established for evaluating viral pathogenesis and the efficacy of antiviral drugs, as mice and humans develop similar changes in the respiratory tract, with the predominant involvement of the lower airway in mice [10]. Whereas pathological changes caused by human H7N9 viruses in mice have been reported recently [11–13], ARDS was not described, and therapeutic strategies have not been fully explored. Here we evaluate the susceptibility of a panel of N9 influenza viruses to NAIs in vitro, establish the pathogenicity of A/Anhui/1/2013 (H7N9) in BALB/c mice, and examine the efficacy of oseltamivir treatment against lethal H7N9 virus challenge.

A novel avian-origin influenza A/H7N9 virus infecting humans was first identified in March 2013 and, as of 30 May 2013, has caused 132 human infections leading to 33 deaths. Phylogenetic studies suggest that this virus is a reassortant, with the surface hemagglutinin (HA) and neuraminidase (NA) genes being derived from duck and wild-bird viruses, respectively, while the six “internal gene segments” were derived from poultry H9N2 viruses. Here we determine the pathogenicity of a human A/Shanghai/2/2013 (Sh2/H7N9) virus in healthy adult mice in comparison with that of A/chicken/Hong Kong/HH8/2010 (ck/H9N2) virus, highly pathogenic avian influenza (HPAI) A/Hong Kong/483/1997 (483/H5N1) virus, and a duck influenza A H7N9 virus of different genetic derivation, A/duck/Jiangxi/3286/2009 (dk/H7N9). Intranasal infection of mice with Sh2/H7N9 virus doses of 103, 104, and 105 PFU led to significant weight loss without fatality. This virus was more pathogenic than dk/H7N9 and ck/H9N2 virus, which has six internal gene segments that are genetically similar to Sh2/H7N9. Sh2/H7N9 replicated well in the nasal cavity and lung, but there was no evidence of virus dissemination beyond the respiratory tract. Mice infected with Sh2/H7N9 produced higher levels of proinflammatory cytokines in the lung and serum than did ck/H9N2 and dk/H7N9 but lower levels than 483/H5N1. Cytokine induction was positively correlated with virus load in the lung at early stages of infection. Our results suggest that Sh2/H7N9 virus is able to replicate and cause disease in mice without prior adaptation but is less pathogenic than 483/H5N1 virus., IMPORTANCE An H7N9 virus isolate causing fatal human disease was found to be more pathogenic for mice than other avian H9N2 or H7N9 viruses but less pathogenic than the highly pathogenic avian influenza virus (HPAI) H5N1. Similarly, the ability of Sh2/H7N9 to elicit proinflammatory cytokines in the lung and serum of mice was intermediate to ck/H9N2 and dk/H7N9 on the one hand and HPAI H5N1 on the other. These findings accord with the observed epidemiology in humans, in whom, as with seasonal influenza viruses, H7N9 viruses cause severe disease predominantly in older persons while HPAI H5N1 can cause severe respiratory disease and death in children and young adults.

Influenza A (H5N1) virus with an amino acid substitution in neuraminidase conferring high-level resistance to oseltamivir was isolated from two of eight Vietnamese patients during oseltamivir treatment. Both patients died of influenza A (H5N1) virus infection, despite early initiation of treatment in one patient. Surviving patients had rapid declines in the viral load to undetectable levels during treatment. These observations suggest that resistance can emerge during the currently recommended regimen of oseltamivir therapy and may be associated with clinical deterioration and that the strategy for the treatment of influenza A (H5N1) virus infection should include additional antiviral agents.