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Highly Pathogenic Avian Influenza
Wild aquatic birds are thought to be the natural reservoir of all known subtypes of influenza A virus. Based on their virulence in chickens, avian influenza A/H5 and A/H7 subtypes are further classified into low pathogenic or highly pathogenic strains. Since the late 1990s, the avian A/H5N1 influenza virus with highly pathogenic phenotype has caused the deaths of hundreds of millions of domestic birds in outbreaks spreading from Asia to Europe, Africa, and the Middle East. Highly pathogenic A/H5N1 is currently thought to be enzootic in poultry in six countries (Bangladesh, China, Egypt, India, Indonesia, and Vietnam), with sporadic outbreaks occurring elsewhere.
The first cases of zoonotic human infection with A/H5N1 were reported from Hong Kong in 1997. Since the re-emergence of A/H5N1 infections in 2003, there have been 610 laboratory-confirmed human cases from 15 countries reported to the World Health Organization (WHO) by mid-December 2012, and 360 (59%) of these patients died. The last confirmed human infection reported to WHO occurred in July 2012.
Although the pathogenicity of influenza virus is thought to be multigenic, depending on the specific collection of genes of a particular strain infecting a particular species, the severe primary viral pneumonia and diffuse lung injury that has been seen in humans infected with highly pathogenic A/H5N1 may be partly due to its hemagglutinin glycoprotein (HA). HA is responsible for the binding and fusogenic functions of the influenza A virus, and the A/H5N1 HA currently binds to receptors terminating in alpha-2,3-linked sialic acid moieties, which in birds are found throughout the respiratory and enteric tracts. In humans, however, these receptors are located in the lower respiratory tract. By contrast, the receptors for human influenza viruses are located in the upper respiratory tract.
Avian Flu Pandemic: Evaluating the Possibility*
The different patterns of A/H5N1 receptor expression in humans and birds are also believed to account for the host restriction of avian influenza viruses. To date, most humans becoming infected with the avian A/H5N1 influenza virus were those in close contact with infected birds or contaminated environments, and there has been no sustained human-to-human transmission. There is concern that the virus might acquire the ability to be transmitted between mammals, including humans, via respiratory droplets or aerosols. Human-to-human respiratory transmission of A/H5N1 influenza virus would probably lead to a pandemic, because humans lack immunity to influenza viruses possessing an H5 HA.
The possibility that A/H5N1 could acquire the ability to be transmitted from person to person has been investigated. Researchers artificially induced mutations in specific strains of virus that then underwent serial passages in ferrets, animals that are susceptible to human and avian influenza viruses and develop disease similar to that seen in humans. They found that the A/Indonesia/5/2005 strain might require as few as five mutations and the A/Vietnam/1203/2004 strain four mutations and a reassortment event to become transmissible in ferrets by respiratory droplets. Numerous viruses from surveillance studies are three and four nucleotide mutations away from acquiring the mutations of the experimental strains. Mathematical modeling of surveillance data has indicated that the remaining mutations could occur naturally within a single mammalian host.
Publication of the research using ferrets was initially delayed*, because some critics feared that data from the papers could be used to create a biological superweapon, and there has always been concern about the accidental release of laboratory pathogens. However, the techniques used in the research are well known; the papers had been so widely discussed that experts knew every detail anyway; and it was finally decided that the benefits outweighed the risks. Furthermore, the controversy surrounding the work resulted in the development of new guidelines for dangerous research.
Avian Flu Testing: Is There a Rapid Flu Test?
As of December 2012, human infections with avian influenza viruses have rarely been identified in North America, and no highly pathogenic H5N1 viruses have ever been identified in humans, poultry, wild birds, or other animals in the United States. However, enhanced surveillance efforts by health departments, hospitals, and physicians to identify patients at increased risk are still recommended by the Centers for Disease Control and Prevention (CDC). Current CDC guidelines** focus on the importance of obtaining a history of travel in patients with respiratory illness. For patients with a clinical syndrome consistent with highly pathogenic H5N1 infection plus possible exposure to the virus during the seven days before the onset of illness, the CDC recommends that physicians consider testing and consult their state health department. The preferred method for laboratory diagnosis is the H5N1-specific reverse-transcription polymerase chain reaction (RT-PCR) test conducted under biosafety level 2 conditions. This test was developed by the CDC and approved by the FDA.
The FDA has approved one rapid test for avian influenza, the AVantage™ A/H5N1 Flu Test from Arbor Vita (Fremont, CA). This test is different from the other approved rapid flu tests. Those tests are immunoassays that detect the core nucleoproteins of human influenza viruses A and/or B. They do not provide information on subtypes and have limited sensitivity.
Arbor Vita’s flu test is a qualitative immunoassay that detects influenza A/H5N1 virus directly from nasal and nasopharyngeal swabs. The company has developed a novel approach to diagnostics that employs PDZ domain proteins. PDZ domains are protein-protein interaction modules that interact with specific short amino acid motifs (PDZ-ligands [PL]) at the carboxy termini of target proteins to assemble protein complexes in multicellular organisms. Pathogenic viruses, including influenza A, use short linear peptide motifs that target PDZ domains to perturb host signaling networks. The company’s lateral-flow immunochromatographic assay uses recombinant proteins containing PDZ domains that selectively capture the nonstructural protein 1 (NS1) of H5N1 via the PL, and gold-conjugated panreactive monoclonal anti-influenza A antibodies as detection reagents. The test does not require equipment or cold storage, and results are available within 45 minutes. Although the test has not been evaluated in prospective clinical studies in enzootic countries, the limits of detection for two different H5N1 clade 2.2 cultured strains were 36 TCID50/mL and 134 TCID50/mL. The test demonstrated 100% specificity (95% CI, 99.6%–100%) in a prospective surveillance study conducted in the United States. As with other rapid influenza tests, a negative result does not rule out avian influenza virus infection, and should not be used by itself to make decisions on treatment.
Conclusion
The highly pathogenic A/H5N1 avian influenza virus continues to cause serious problems in domestic birds and has caused human zoonotic infections primarily in those individuals who are in close contact with infected birds. As of December 2012, there have not been any A/H5N1 viruses identified in humans, birds, or other animals in the United States. In addition, sustained human-to-human transmission has not yet been observed. However, there is concern that the virus will naturally acquire the ability to be transmitted in humans via respiratory droplets or aerosols, and the CDC continues to recommend enhanced surveillance.
*For additional reading, please see the Special Section in the June 22, 2012 issue of Science; http://www.sciencemag.org/content/336/6088/1521.
**More information can be found at http://www.cdc.gov/flu/avianflu/avian-flu-summary.htm and http://www.cdc.gov/flu/avianflu/guidance-labtesting.htm.
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