What's New with Pandemic Flu

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Abstract

The term “bird flu” refers to avian influenza viruses. The recently emerged avian H7N9 influenza viruses replicate efficiently in birds and in mammals. Although H7N9 viruses rarely infect humans, they were the cause of sporadic human infections in China in 2014. In this review, we focus on the avian H7N9 influenza virus, summarize the characteristic biological features, and assess its pandemic potential. We also review information about highly pathogenic avian influenza viruses as possible sources of pandemic outbreaks.

Introduction

The first influenza pandemic of the 21st century arrived in late March 2009 and was eventually named the 2009 influenza A H1N1 pandemic (H1N1pdm09) virus. One of its predecessors, the pandemic of 1918 to 1920, caused an estimated 50 to 100 million human deaths [1]. The 1957 (H2N2) and 1968 (H3N2) pandemics caused an estimated 2 million and 1 million deaths respectively.

The 2009 pandemic was also very deadly. The first positive human samples were identified from patients in California and Texas [2, 3]; however, the virus originated in Mexico prior to detection in the United States. A postoutbreak report from the World Health Organization in 2010 noted 19,000 deaths worldwide, but a recent analysis [4] estimated 10 times that number of deaths.

The influenza virus genome is composed of 8 negative-sense single-stranded RNA segments, each encoding at least one protein and totaling at least 11 proteins in all.

The A/California/04/2009 (H1N1) pandemic strain was the first pandemic strain for which a complete genome became publically available through GenBank [2, 3]. Among the newest threats for the next predicted worldwide pandemic are a variety of “avian influenza viruses.” Because humans lack protective antibodies against these viruses, H5N1 or H7N9 viruses that can be transmitted to humans could spread worldwide, resulting in an influenza pandemic.

Most humans infected with H7N9 influenza virus exhibit general influenza-like symptoms, including fever and cough. More than half of the infections typically progress to severe pneumonia, acute respiratory distress syndrome, and multi-organ failure [5, 6, 7, 8, 9]. Most H7N9 virus-infected patients possess one or more underlying medical conditions, such as chronic obstructive pulmonary disease, diabetes, hypertension, obesity, and/or chronic lung and heart disease [5, 6, 9, 10], suggesting that these comorbidities may increase the risk of severe H7N9 virus infection.

Section snippets

Zoonoses

Influenza A viruses are primarily sustained in poultry, wild waterfowl, pigs, horses, and humans; however, reports of influenza virus have also occurred for marine mammals, dogs, and other mammalian species, such as bats [10]. These viruses possess mammalian-adapting amino acid changes that likely contribute to their ability to infect mammals. Influenza A viruses are composed of two viral surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), by which viral subtypes are characterized.

Epidemiology

Epidemiological studies report that H7N9 virus infections have affected mainly middle-aged or older individuals [5, 6, 10]. Two-thirds of the infected individuals have been male [5, 6, 8, 10]. The high number of cases among elderly men may be associated with the fact that elderly men have frequent work-related or non-job-related contact with poultry. The following evidence suggests that contact with live poultry is the source of human H7N9 virus infections [10]: (i) active surveillance in live

Origins of H7N9 Influenza Viruses

Phylogenetic analyses revealed that the novel H7N9 viruses were likely to have emerged via reassortment of at least four avian influenza A virus strains [10]. The H7N9 NA gene is closely related to those of the avian H2N9 and/or H11N9 influenza viruses found in wild migratory birds along the East Asian flyway. The H7N9 HA gene belongs to the Eurasian lineage of avian influenza viruses and is closely related to those of avian H7N3 viruses isolated from ducks in eastern China in 2010–2011 [10]. The

Host Range

How does avian influenza virus overcome the normal host restriction barriers to infect humans? Two viral proteins play a major role in the host range of influenza viruses: the surface glycoprotein, HA, which determines host-specific receptor binding, and PB2, which is the polymerase subunit that determines the replicative ability. Influenza virus infections are initiated when the HA glycoprotein binds to receptors on host cells. Avian influenza viruses preferentially bind to the major

Risk Assessment of Human-to-Human Transmission

Influenza viruses transmit from human to human through direct and indirect contact via aerosols, respiratory droplets, and fomites; therefore, the primary concern with H7N9 viruses is that they may gain efficient human-to-human transmissibility and cause a pandemic. Several research groups have evaluated the transmissibility of H7N9 viruses in two animal models, namely, ferrets and guinea pigs although the efficiency of transmission is lower than that of human influenza viruses [10, 12].

The

Sequence Analysis

Elucidating evolutionary trajectories of individual viral segments enables researchers to determine how frequently segments jump between different host species. In addition, sequence analysis can identify key reassortment events and thereby track down the geographic origin of pandemic viruses. As scientists continue to better understand how influenza viruses evolve and reassort, it may be possible to design better novel vaccines and antiviral drugs and to predict the efficiency of existing

HPAI

Most of the poultry outbreaks in the United States have been caused by highly pathogenic avian influenza (HPAI) (H5N2) viruses. HPAI H5 viruses infect the avian gastrointestinal and respiratory tracts and can spread rapidly, causing high mortality in infected poultry. Many birds have died, and millions of chickens and turkeys in several states have been destroyed in an attempt to control the outbreaks. Signs of HPAI in poultry can include sudden death; lack of energy, appetite, and

Specimen Collection for HPAI H5 and H7N9 Viruses

Specimen collection and processing for patients who may be infected with novel HPAI influenza A viruses are detailed below. Clinicians and public health personnel should consider the following recommendations for surveillance and testing of HPAI H5 viruses [17].

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    Consider the possibility of infection with novel influenza A viruses in patients with medically attended influenza-like infection and acute respiratory infection who have had recent contact (<10 days prior to illness onset) with

Summary

To date, the novel H7N9 and HPAI H5 influenza viruses have not caused a pandemic in humans due to their inability to support sustained human-to-human transmission. The viruses exhibit efficient replication but limited transmissibility in mammals. H7N9 has acquired amino acid changes that enable adaptation to mammals, and it may reassort with circulating human viruses. The viruses can also easily acquire resistance to the NA inhibitor oseltamivir. Perhaps most importantly for its potential to

References (17)

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Cited by (1)

  • Risk perceptions of public health and food safety hazards in poultry husbandry by citizens, poultry farmers and poultry veterinarians

    2018, Poultry Science
    Citation Excerpt :

    In high risk periods, avian influenza transmission from wild bird to chickens may be prevented by keeping outdoor poultry temporarily indoors. Avian influenza transmission from birds to humans is rare (Wildoner, 2016) and if a flock is infected, the flock is culled as soon as possible, so the infection risk for the general public is small. Although the “real” risks of Campylobacter, dioxins and avian influenza for the public health are not clear, it seems that professionals perceive these risks in different husbandry systems better in accordance with literature than do citizens.

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