Email updates

Keep up to date with the latest news and content from Critical Care and BioMed Central.

This article is part of the series Disaster management.

Highly Accessed Review

Clinical review: Update of avian influenza A infections in humans

Christian Sandrock1* and Terra Kelly2

Author Affiliations

1 School of Medicine, University of California, Davis, Sacramento, CA 95817, USA

2 School of Veterinary Medicine, University of California, Davis, Sacramento, CA 95817, USA

For all author emails, please log on.

Critical Care 2007, 11:209  doi:10.1186/cc5675

The electronic version of this article is the complete one and can be found online at: http://ccforum.com/content/11/2/209


Published:22 March 2007

© 2007 BioMed Central Ltd

Abstract

Influenza A viruses have a wide host range for infection, from wild waterfowl to poultry to humans. Recently, the cross-species transmission of avian influenza A, particularly subtype H5N1, has highlighted the importance of the non-human subtypes and their incidence in the human population has increased over the past decade. During cross-species transmission, human disease can range from the asymptomatic to mild conjunctivitis to fulminant pneumonia and death. With these cases, however, the risk for genetic change and development of a novel virus increases, heightening the need for public health and hospital measures. This review discusses the epidemiology, host range, human disease, outcome, treatment, and prevention of cross-transmission of avian influenza A into humans.

Introduction

Human influenza pandemics over the last 100 years have been caused by H1, H2, and H3 subtypes of influenza A viruses. More recently, avian influenza virus subtypes (that is, H5, H7) have been found to directly infect humans from their avian hosts. The recent emergence, host expansion, and spread of a highly pathogenic avian influenza (HPAI) H5N1 subtype in Asia have heightened concerns globally, both in regards to mortality from HPAI H5N1 infection in humans and the potential of a new pandemic. This paper will review the current human infections with avian influenza and their public health and medical implications.

Influenza A viruses

Influenza A, B and C are the most important genera of the Orthomyxoviridae family, casusing both pandemic and seasonal disease in humans. Influenza A viruses are enveloped, single-stranded RNA viruses with a segmented genome (Table 1) [1]. They are classified into subtypes on the basis of the antigenic properties of the hemagglutinin (HA) and neuraminidase (NA) glycoproteins expressed on the surface of the virus [1,2]. Influenza A viruses are characterized by their pathogenicity, with highly pathogenic avian influenza (HPAI) causing severe disease or death in domestic poultry [3]. Molecular changes in the RNA genome occur through two main mechanisms: point mutation (antigenic drift) and RNA segment reassortment (antigenic shift) [4,5]. Point mutations cause minor changes in the antigenic character of viruses and are the primary reason a vaccination for influenza A is given yearly. Reassortment occurs when a host cell is infected with two or more influenza A viruses, leading to the creation of a novel subtype. The influenza subtypes of the 1957 (H2N2) and 1968 (H3N2) pandemics occurred through reassortment, while the origins of the 1918 (H1N1) pandemic are unclear.

Table 1. Characteristics of influenza viruses

The HA glycoprotein mediates attachment and entry of the virus by binding to sialic acid receptors on the cell surface. The binding affinity of the HA to the host sialic acid allows for the host specificity of influenza A [6,7]. Avian influenza subtypes prefer to bind to sialic acid linked to galactose by α-2,3 linkages, which are found in avian intestinal and respiratory epithelium (Table 2) [8]. Human virus subtypes bind to α-2,6 linkages found in human respiratory epithelium [8,9]. Swine contain both α-2,3 and α-2,6 linkages in their respiratory epithelium, allowing for easy co-infection with both human and avian subtypes (thus acting as a 'mixing vessel' for new strains) [10]. Humans have been found to contain both α-2,3 and α-2,6 linkages in their lower respiratory tract and conjunctivae, which allows for human infections by avian subtypes [9,11,12]. The HA glycoprotein is the main target for immunity by neutralizing antibodies.

Table 2. Characteristics and pathogenicity of influenza A viruses

The NA glycoprotein allows the spread of the virus by cleaving the glycosidic linkages to sialic acid on host cells and the surface of the virus. The virus is then spread in secretions or other bodily fluids. The NA glycoprotein is not the major target site for neutralization of the virus by antibodies.

Host range of influenza A viruses

Influenza A viruses infect a wide range of hosts, including many avian species, and various mammalian species, such as swine, ferrets, felids, mink, whales, horses, seals, dogs, civets, and humans [13-31]. Wild birds (ducks, geese, swans, and shorebirds) are important natural reservoirs of these viruses, and all of the known 16 HA and 9 NA subtypes have been found in these birds [32-35]. In most cases, these subtypes are found within the gastrointestinal tract of the birds, are shed in their feces, and rarely cause disease [32]. Since 2002, however, HPAI H5N1 viruses originating in Asia have been reported from approximately 960 wild bird species, causing disease in some instances and asymptomatic shedding in others [36-48]. The virus has now spread across Asia, Europe, the Middle East, and some African countries. Additional species, such as tigers, leopards, cats, stone martens, and humans have also become infected with HPAI H5N1 [49]. This spread of H5N1 into a wide range of animal and avian species may enhance the spread of the virus into the human population as it interacts with animals in a number of ways (increased land use, markets, consumption) [44]. Thus, the potential contact, transmission, and mutability of HPAI H5N1 worldwide will increase as the number of species and their interactions increase, complicating prevention, surveillance and treatment possibilities.

Epidemiology and pathogenicity of avian influenza infections in humans

The incidence of avian influenza infections in humans has increased over the past decade (Table 3). Initially, cases of avian influenza (H7N7) in humans occurred in association with poultry outbreaks, manifesting as self-limiting conjunctivitis [30,50-53]. Then, in 1997, a large scale HPAI H5N1 outbreak occurred among poultry in Hong Kong, with 18 documented human cases [29,31,54,55]. Two subsequent poultry outbreaks in Hong Kong in 1999 and 2003 with HPAI H5N1 occurred without human cases until 2003 when two members of a family in Hong Kong contracted HPAI H5N1 [56]. In December of 2003, HPAI H5N1 surfaced in poultry in Korea and China, and from 2003 to 2006 the outbreak stretched worldwide in the largest outbreak in poultry history. Human cases of HPAI H5N1 followed the poultry outbreak, with a total of 256 cases and 151 fatalities thus far [57]. Other limited outbreaks have occurred, causing variable human disease (Table 3) [52,58]. However, HPAI H5N1 remains the largest and most significant poultry and human avian influenza outbreak.

Table 3. Avian influenza A outbreaks reported in humans

Epidemiological investigations of human cases of avian influenza show that the virus was acquired by direct contact with infected birds [29-31,50-56]. Influenza A is transmitted through the fecal-oral and respiratory routes among wild birds and poultry [32]. Human interaction with these infected secretions and birds was the major mode of transmission, with contact including consumption of undercooked or raw poultry products, handling of sick or dead birds without protection, or food processing at bird cleaning sites. All birds were domesticated (chicken, duck, goose) and no transmission from birds in the wild (migrating) or contaminated waterways has been documented. In a few cases, limited human to human transmission has been reported among health care workers and family members (Table 4) [59-63]. In each of these cases, no personal protective equipment was used, which is the major factor in transmission between humans [60].

Table 4. Person to person transmission of avian influenza

Clinical manifestations of avian influenza in humans

The clinical manifestations of avian influenza in humans has ranged from mild conjunctivitis to severe pneumonia with multi-organ system failure (Table 5) [50,51]. The median age of patients was 17.2 years in the 1997 HPAI H5N1 outbreak and 16 years in the 2003 to 2006 Southeast Asian cases (range 2 months to 90 years) [17,55,65-68]. The incubation period ranged from two to eight days from contact with sick or dead birds to symptom onset. The predominant clinical findings appear to vary with each influenza A subtype; for example, in 2003 during the Netherlands outbreak (H7N7) 92% (82 of 89) of patients presented with conjunctivitis and a minority with respiratory symptoms [53]. However, with HPAI in Hong Kong in 1997 and in Southeast Asia currently, pneumonia progressing to multiorgan failure, acute respiratory distress syndrome (ARDS), and death are the predominant findings [17,55,65-68]. Rye syndrome, pulmonary hemorrhage, and predominant nausea, vomiting, and diarrhea complicate these cases [68]. Laboratory findings include both thrombocytopenia and lymphopenia [65,66]. Chest radiographic findings include interstitial infiltrates, lobar consolidation, and air bronchograms. The clinical course of patients with HPAI H5N1 is rapid, with 68% percent of patients developing ARDS and multiorgan failure within 6 days of disease onset [69]. The case fatality rate ranges form 67% to 80%, depending on the case series [17,55,65,66]. Once the patients reached the critical care unit, however, the mortality rate was 90% [69]. The average time of death from disease onset was nine to ten days.

Table 5. Clinical, laboratory, and radiographic findings of avian influenza in humans

Avian influenza A infections in humans differ from seasonal influenza in several ways. The presence of conjunctivitis is more common with avian influenza A infections than with seasonal influenza. Gastrointestinal symptoms, as seen with HPAI H5N1, and reports of primary influenza pneumonia and development of ARDS are also more common with avian influenza A infections [65,67,69]. Finally, the rapid progression to multi-organ failure and eventually death occurs at a much higher rate with avian influenza A infections [69].

Post-mortem studies have illustrated findings consistent with an overwhelming systemic inflammatory response syndrome, including diffuse alveolar damage, acute tubular necrosis and atrophy, disseminated intravascular coagulation, and multi-organ damage [70,71]. Interestingly, the virus has been isolated from the lungs, intestine, spleen, and brain, suggesting viremia, but active replication of the virus has been limited to the lungs [71]. This overwhelming inflammatory response, with acute lung injury and ARDS as the predominant features, coincides with the findings of preferential binding of the avian influenza A viruses to α-2,3 linkages in type II pneumocytes of the lower respiratory tract of humans and a vigorous cytokine response, including increased interleukin-6, interleukin-10, and interferon beta release [11,12,70,71].

Diagnosis

The clinical diagnosis of avian influenza infection in humans is difficult and relies on the epidemiological link to endemic areas, contact with sick or dead poultry, or contact with a confirmed case of avian influenza (Table 6). Since many infectious diseases present with similar symptoms, the only feature significant to the clinician may be contact in an endemic area, through travel or infected poultry, and the clinician should always elicit a detailed patient history.

Table 6. Case definition of avian influenza

The definitive diagnosis is made from isolation of the virus in culture from clinical specimens. This method not only provides the definitive diagnosis, but the viral isolate is now available for further testing, including pathogenicity, antiviral resistance, and DNA sequencing and analysis. Alternatively, antibody testing can be performed, with a standard four-fold titer increase to the specific subtype of avian influenza virus. Neutralizing antibody titer assays for H5, H7 and H9 are performed by the micorneutralization technique [72]. Western blot analysis with recombinant H5 is the confirmatory test for any positive microneutralization assay [59,60,72]. More recently, rapid diagnosis can be performed with reverse transcription-PCR on clinical samples with primers specific for the viral subtype [73-75]. This test should be performed only on patients meeting the case definition of possible avian influenza A infection.

Any suspected case of avian influenza in a human should be investigated by the public health officials in the province or country of origin [39,76]. Additionally, governmental labs are often equipped with the appropriate biolevel safety 3 laboratories, primer libraries, and associated expertise to confirm the diagnosis quickly and efficiently. Any clinical specimens should be submitted with the assistance of the public health experts.

Treatment

Treatment of avian influenza infections in humans includes antiviral therapy and supportive care. Controlled clinical trials on the efficacy of antivirals (NA inhibitors), supportive therapy, or adjuvant care have never been performed, so current recommendations stem from the experiences of past avian influenza outbreaks and animal models.

The adamantanes (rimantadine and amantadine) and NA inhibitors (oseltamivir and zanamivir) are the antivirals used for treatment and prophylaxis of influenza infections in humans. In avian influenza virus infections, adamantanes have no role due to widespread resistance through a M2 protein alteration. In addition, over 90% of isolates of H1 and H3 human subtypes during seasonal influenza have had resistance to the adamantanes [77]. Their role has now been limited to prophylaxis in the community when the circulation strain is know to be susceptible to the adamantanes [78-80].

NA inhibitors (oseltamivir and zanamivir) have been studied for both treatment and prophylaxis with the human influenza A subtypes H1, H2, and H3 as well as influenza B (Table 7) [80-82]. In animal models with HPAI H5N1, their efficacy has been well documented, with improved survival rates seen after infection [83-85]. Oseltamivir has been used in avian influenza outbreaks involving H7N7 and HPAI H5N1, and therapy with oseltamivir has been shown to decrease the viral load in nasal secretions in patients infected with HPAI H5N1 [11,86,87]. Resistance to oseltamivir has been documented in a HPAI H5N1 subtype in a Vietnamese girl treated with 75 mg daily for 4 days as post-exposure prophylaxis [68]. The NA glycoprotein had a histidine to tyrosine substitution at position 274, conveying a markedly higher IC50 for oseltamivir [68,88]. In one study, the viral count of HPAI H5N1 in nasal secretions did not decrease with the administration of oseltamivir when the H5N1 isolate carried this resistance mutation [68]. However, resistance produced by this change may be overcome with higher doses of oseltamivir in vitro, and this change has not been documented to confer resistance to zanamivir [88].

Table 7. Neuroaminidase inhibitors

The timing of treatment with NA inhibitors is paramount, as early therapy is directly related to improved survival [66,83-85]. The greatest level of protection was seen if the NA inhibitors were started within 48 hours of infection, and protection rapidly dropped after 60 hours [78,79]. These initial studies, however, were performed with seasonal human influenza A and B, where the period of viral shedding is approximately 48 to 72 hours. In HPAI H5N1 cases from Southeast Asia, survival appeared to be improved in patients who received oseltamavir earlier (4.5 days versus 9 days after onset of symptoms) [66]. Both of these time periods are much longer than documented in animal models, so the window of optimal therapy is still unknown, particularly if viral shedding exceeds the average 48 to 72 hour period seen in seasonal influenza A and B infections.

Combination therapy with influenza A viruses has not been studied [84]. Ribaviron by inhalation has been evaluated in vitro with some avian influenza A subtypes and has been found to reduce mortality from influenza B in a mouse model [89]. Further animal model studies are indicated to determine if there is a role for ribaviron or combination therapy with avian influenza A viruses.

Supportive care with intravenous rehydration, mechanical ventilation, vasopressor therapy, and renal replacement therapy are required if multiorgan failure and ARDS are a feature of disease [69,90]. Due to the progression of pneumonia to ARDS, non-invasive ventilation is not recommended, and early intubation may be beneficial before overt respiratory failure ensues. Corticosteroids have been used in some patients with HPAI H5N1, but no definitive role for steroids has been determined. Other immunomodulatory therapy has not been reported [91].

Vaccination

Human vaccination for avian influenza viruses has not been widely used, although multiple vaccination trials are underway. Prior avian vaccines in humans have been poorly immunogenic and thus have limited use. An inactivated H5N3 has been tested and was tolerated but with limited immunogenicity [91,92]. Other H5 vaccines have resulted in the development of neutralizing antibodies, but to a limited degree [93,94]. Recently, a large randomized trial looked at an H5N1 attenuated vaccine from the Vietnam strain [95]. Only a modest immune response was seen, with micro-neutralization antibodies being developed at 12 times the dose used in the seasonal influenza vaccine. The side effects were minimal. A number of other industry trials with adjuvant vaccines are currently ongoing. Although promising, human vaccination against avian influenza viruses is still under development. Underscoring this development is the uncertainty of a pandemic strain, which may have vastly different antigenic properties from any developed H5 vaccine.

Infection control

Health care infection control is a crucial component in the management of avian influenza infection or a new pandemic strain. Experience from the severe ARDS outbreak in 2002 has illustrated that appropriate infection control measures are paramount to reduce spread to health care workers and, possibly, the community [96-98]. Therefore, the World Health Organization (WHO) and Centers for Disease Control and Prevention (CDC) recommend contact and airborne precautions for any initial suspected case of avian influenza in a human [99]. In late October 2006, the CDC released updated interim guidance on the use of masks and respirators in the health care setting (Table 8) [99]. In certain high risk procedures, additional protection may be considered given the likelihood of generating aerosol particles that may enhance transmission (Table 9) [99]. Respiratory protection should be worn along with an impermeable gown, face shield, and gloves. Initial cases should be placed in a negative pressure isolation room with 6 to 12 air changes per hour. Hand hygiene with antibacterial soap or alcohol based washless gel should be standard, with appropriate basins at each patient room. Seasonal vaccination of all health care workers should be preformed and further emphasized in order to reduce the likelihood of co-infection with two stains of influenza. Visitors and family members should be strictly monitored and their access to the patient limited to reduce the likelihood of spread. Finally, antiviral chemoprophylaxis should be available to any health care workers exposed to an infected individual. Any symptomatic worker should be taken off duty and workplace surveillance should occur. With these aggressive measures, risk to health care workers, patients, and family members will be reduced.

Table 8. Masks and respirators for health care workers

Table 9. High risk aerosol procedures in avian influenza

Conclusion

Avian influenza viruses have occurred with increased incidence within the human population, reflecting the delicate and tangled interaction between wildlife, domesticated animals, and humans. Disease in humans can be limited to conjunctivitis or an influenza-like illness, but HPAI H5N1 causes mainly severe pneumonia, respiratory failure, and death. Most cases have occurred through direct transmission from infected poultry or waterfowl, with only a few limited cases of human to human transmission. Treatment has been successful with the NA inhibitors if started early, and vaccine development is underway with a more immunogenic attenuated H5N1 virus preparation. Infection control measures are the mainstay for prevention and disease reduction. Avian influenza viruses may constitute part of the next pandemic, so appropriate knowledge, prevention, and treatment will reduce the likelihood of this occurrence.

Abbreviations

ARDS = acute respiratory distress syndrome; CDC = Centers for Disease Control and Prevention; HA = hemagglutinin; HPAI = highly pathogenic avian influenza; NA = neuroaminidase; WHO = World Health Organization.

Competing interests

The authors declare that they have no competing interests.

Note

This article is part of a thematic series on Disaster management edited by J Christopher Farmer.

Other articles in this series can be found online at http://ccforum.com/articles/theme-series.asp?series=CC_Disaster webcite

References

  1. World Health Organization Expert Committee: A revision of the system of nomenclature for influenza viruses: a WHO Memorandum.

    Bull WHO 1980, 58:585-591. PubMed Abstract OpenURL

  2. Fouchier RAM, Munster V, Wallensten A, Bestebroer TM, Herfst S, Smith D, Rimmelzwaan GF, Olsen B, Osterhaus AD: Characterization of a novel influenza A virus hemagglutinin subtype (H16) obtained from black-headed gulls.

    J Virol 2005, 79:2814-2822. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  3. Highly Pathogenic Avian Influenza: Manual of Diagnostic Tests and Vaccines for Terrestrial Animals [http://www.oie.int/eng/normes/mmanual/A_00037.htm] webcite

  4. Kawaoka Y, Webster RG: Molecular mechanism of acquisition of virulence in influenza virus in nature.

    Microb Pathog 1988, 5:311-318. PubMed Abstract | Publisher Full Text OpenURL

  5. Kendal AP: Epidemiologic implications of changes in the influenza virus genome.

    Am J Med 1987, 82:4-14. PubMed Abstract | Publisher Full Text OpenURL

  6. Gambaryan AS, Tuzikov AB, Bovin NV, Yamnikova SS, Lvov DK, Webster RG, Matrosovich MN: Differences between influenza virus receptors on target cells of duck and chicken and receptor specificity of the 1997 H5N1 chicken and human influenza viruses from Hong Kong.

    Avian Dis 2003, 47:1154-1160. PubMed Abstract OpenURL

  7. Ito T, Couceiro JNSS, Kelm S, Baum LG, Krauss S, Castrucci MR, Donatelli I, Kida H, Paulson JC, Webster RG, Kawaoka Y: Molecular basis for the generation in pigs of influenza A viruses with pandemic potential.

    J Virol 1998, 72:7367-7373. PubMed Abstract | PubMed Central Full Text OpenURL

  8. Couceiro JN, Paulson JC, Baum LG: Influenza virus strains selectively recognize sialyloligosaccharides on human respiratory epithelium: the role of the host cell in selection of hemagglutinin receptor specificity.

    Virus Res 1993, 29:155-165. PubMed Abstract | Publisher Full Text OpenURL

  9. Matrosovich MN, Matrosovich TY, Gray T, Roberts NA, Klenk HD: Human and avian influenza (AI) viruses target different cell types in cultures of human airway epithelium.

    Proc Natl Acad Sci USA 2004, 101:4620-4624. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  10. Matrosovich M, Zhou N, Kawaoka Y, Webster R: The surface glycoproteins of H5 influenza viruses isolated from humans, chickens, and wild aquatic birds have distinguishable properties.

    J Virol 1999, 73:1146-1155. PubMed Abstract | PubMed Central Full Text OpenURL

  11. Shinya K, Ebina M, Yamada S, Ono M, Kasai N, Kawaoka Y: Avian flu: influenza virus receptors in the human airway.

    Nature 2006, 440:435-436. PubMed Abstract | Publisher Full Text OpenURL

  12. van Riel D, Munster VJ, de Wit E, Rimmelzwaan GF, Fouchier RA, Osterhaus AD, Kuiken T: H5N1 virus attachment to lower respiratory tract.

    Science 2006, 312:399. PubMed Abstract | Publisher Full Text OpenURL

  13. Pensaert M, Ottis K, Vandeputte J, Kaplan MM, Bachmann PA: Evidence for the natural transmission of influenza A virus from wild ducks to swine and its potential importance for man.

    Bull WHO 1981, 59:75-78. PubMed Abstract OpenURL

  14. Scholtissek C, Burger H, Bachmann PA, Hannoun C: Genetic relatedness of hemagglutinins of the H1 subtype of influenza A viruses isolated from swine and birds.

    Virology 1983, 129:521-523. PubMed Abstract | Publisher Full Text OpenURL

  15. Hinshaw VS, Webster RG, Easterday BC, Bean WJ Jr: Replication of avian influenza A viruses in mammals.

    Infect Immun 1981, 34:354-361. PubMed Abstract | PubMed Central Full Text OpenURL

  16. Rimmelzwaan GF, van Riel D, Baars M, Bestebroer TM, van Amerongen G, Fouchier RA, Osterhaus AD, Kuiken T: Influenza A virus (H5N1) infection in cats causes systemic disease with potential novel routes of virus spread within and between hosts.

    Am J Pathol 2006, 168:176-183. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  17. Keawcharoen J, Oraveerakul K, Kuiken T, Fouchier RA, Amonsin A, Payungporn S, Noppornpanth S, Wattanodorn S, Theambooniers A, Tantilertcharoen R, et al.: Avian influenza H5N1 in tigers and leopards.

    Emerg Infect Dis 2004, 10:2189-2191. PubMed Abstract OpenURL

  18. Englund L, Klingeborn B, Mejerland T: Avian influenza A virus causing an outbreak of contagious interstitial pneumonia in mink.

    Acta Vet Scand 1986, 27:497-504. PubMed Abstract OpenURL

  19. Klingeborn B, Englund L, Rott R, Juntti N, Rockborn G: An avian influenza A virus killing a mammalian species – the mink. Brief report.

    Arch Virol 1985, 86:347-351. PubMed Abstract | Publisher Full Text OpenURL

  20. Hinshaw VS, Bean WJ, Geraci J, Fiorelli P, Early G, Webster RG: Characterization of two influenza A viruses from a pilot whale.

    J Virol 1986, 58:655-656. PubMed Abstract | PubMed Central Full Text OpenURL

  21. Guo Y, Wang M, Zheng GS, Li WK, Kawaoka Y, Webster RG: Seroepidemiological and molecular evidence for the presence of two H3N8 equine influenza viruses in China in 1993–94.

    J Gen Virol 1995, 76:2009-2014. PubMed Abstract OpenURL

  22. Guo Y, Wang M, Kawaoka Y, Gorman O, Ito T, Saito T, Webster RG: Characterization of a new avian-like influenza A virus from horses in China.

    Virology 1992, 188:245-255. PubMed Abstract | Publisher Full Text OpenURL

  23. Callan RJ, Early G, Kida H, Hinshaw VS: The appearance of H3 influenza viruses in seals.

    J Gen Virol 1995, 76:199-203. PubMed Abstract OpenURL

  24. Hinshaw VS, Bean WJ, Webster RG, Rehg JE, Fiorelli P, Early G, Geraci JR, St Aubin DJ: Are seals frequently infected with avian influenza viruses?

    J Virol 1984, 51:863-865. PubMed Abstract | PubMed Central Full Text OpenURL

  25. Webster RG, Hinshaw VS, Bean WJ, Van Wyke KL, Geraci JR, St Aubin DJ, Petursson G: Characterization of an influenza A virus from seals.

    Virology 1981, 113:712-724. PubMed Abstract | Publisher Full Text OpenURL

  26. Crawford PC, Dubovi EJ, Castleman WL, Stephenson I, Gibbs EP, Chen L, Smith C, Hill RC, Ferro P, Pompey J, et al.: Transmission of equine influenza virus to dogs.

    Science 2005, 310:482-485. PubMed Abstract | Publisher Full Text OpenURL

  27. Roberton SI, Bell DJ, Smith GJ, Nicholls JM, Chan KH, Nguyen DT, Tran PQ, Streicher U, Poon LL, Chen H, et al.: Avian influenza H5N1 in viverrids: implications for wildlife health and conservation.

    Proc R Soc B 2006, 273:1729-1732. PubMed Abstract | Publisher Full Text OpenURL

  28. Banks J, Speidel E, Alexander DJ: Characterization of an avian influenza A virus isolated from a human – is an intermediate host necessary for the emergence of pandemic influenza viruses?

    Arch Virol 1998, 143:781-787. PubMed Abstract | Publisher Full Text OpenURL

  29. Claas EC, Osterhaus AD, van Beek R, De Jong JC, Rimmelzwaan GF, Senne DA, Krauss S, Shortridge KF, Webster RG: Human influenza A H5N1 virus related to a highly pathogenic avian influenza virus.

    Lancet 1998, 351:472-477. PubMed Abstract | Publisher Full Text OpenURL

  30. Fouchier RA, Schneeberger PM, Rozendaal FW, Broekman JM, Kemink SA, Munster V, Kuiken T, Rimmelzwaan GF, Schutten M, Van Doornum GJ, et al.: Avian influenza A virus (H7N7) associated with human conjunctivitis and a fatal case of acute respiratory distress syndrome.

    Proc Natl Acad Sci USA 2004, 101:1356-1361. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  31. Subbarao K, Klimov A, Katz J, Regnery H, Lim W, Hall H, Perdue M, Swayne D, Bender C, Huang J, et al.: Characterization of an avian influenza A (H5N1) virus isolated from a child with a fatal respiratory illness.

    Science 1998, 279:393-396. PubMed Abstract | Publisher Full Text OpenURL

  32. Webster RG, Bean WJ, Gorman OT, Chambers TM, Kawaoka Y: Evolution and ecology of influenza A viruses.

    Microbiol Rev 1992, 56:152-179. PubMed Abstract | PubMed Central Full Text OpenURL

  33. Hinshaw VS, Air GM, Gibbs AJ, Graves L, Prescott B, Karunakaran D: Antigenic and genetic characterization of a novel hemagglutinin subtype of influenza A viruses from gulls.

    J Virol 1982, 42:865-872. PubMed Abstract | PubMed Central Full Text OpenURL

  34. Kawaoka Y, Yamnikova S, Chambers TM, Lvov DK, Webster RG: Molecular characterization of a new hemagglutinin, subtype H14, of influenza A virus.

    Virology 1990, 179:759-767. PubMed Abstract | Publisher Full Text OpenURL

  35. Rohm C, Zhou N, Suss J, Mackenzie J, Webster RG: Characterization of a novel influenza hemagglutinin, H15: criteria for determination of influenza A subtypes.

    Virology 1996, 217:508-516. PubMed Abstract | Publisher Full Text OpenURL

  36. Ellis TM, Bousfield RB, Bissett LA, Dyrting KC, Luk GS, Tsim ST, Sturm-Ramirez K, Webster RG, Guan Y, Malik Peiris JS: Investigation of outbreaks of highly pathogenic H5N1 avian influenza in waterfowl and wild birds in Hong Kong in late 2002.

    Avian Pathol 2004, 33:492-505. PubMed Abstract | Publisher Full Text OpenURL

  37. Liu J, Xiao H, Lei F, Zhu Q, Qin K, Zhang XW, Zhang XL, Zhao D, Wang G, Feng Y, et al.: Highly pathogenic H5N1 influenza virus infection in migratory birds.

    Science 2005, 309:1206. PubMed Abstract | Publisher Full Text OpenURL

  38. Sturm-Ramirez KM, Ellis T, Bousfield B, Bissett L, Dyrting K, Rehg JE, Poon L, Guan Y, Peiris M, Webster RG: Reemerging H5N1 viruses in Hong Kong in 2002 are highly pathogenic to ducks.

    J Virol 2004, 78:4892-4901. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  39. Li KS, Guan Y, Wang J, Smith GJ, Xu KM, Duan L, Rahardjo AP, Puthavathana P, Buranathai C, Nguyen TD, et al.: Genesis of a highly pathogenic and potentially pandemic H5N1 virus in eastern Asia.

    Nature 2004, 430:209-213. PubMed Abstract | Publisher Full Text OpenURL

  40. Kou Z, Lei FM, Yu J, Fan ZJ, Yin ZH, Jia CX, Xiong KJ, Sun YH, Zhang XW, Wu XM, et al.: New genotype of avian influenza H5N1 viruses isolated from tree sparrows in China.

    J Virol 2005, 79:15460-15466. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  41. Mase M, Tsukamoto K, Imada T, Imai K, Tanimura N, Nakamura K, Yamamoto Y, Hitomi T, Kira T, Nakai T, et al.: Characterization of H5N1 influenza A viruses isolated during the 2003–2004 influenza outbreaks in Japan.

    Virology 2005, 332:167-176. PubMed Abstract | Publisher Full Text OpenURL

  42. Van Borm S, Thomas I, Hanquet G, Lambrecht B, Boschmans M, Dupont G, Decaestecker M, Snacken R, van den Berg T: Highly pathogenic H5N1 influenza virus in smuggled Thai Eagles, Belgium.

    Emerg Infect Dis 2005, 11:702-705. PubMed Abstract OpenURL

  43. Chen H, Smith GJ, Li KS, Wang J, Fan XH, Rayner JM, Vijaykrishna D, Zhang JX, Zhang LJ, Guo CT, et al.: Establishment of multiple sublineages of H5N1 influenza virus in Asia: implications for pandemic control.

    Proc Natl Acad Sci USA 2006, 103:2845-2850. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  44. Hulse-Post DJ, Sturm-Ramirez KM, Humberd J, Seiler P, Govorkova EA, Krauss S, Scholtissek C, Puthavathana P, Buranathai C, Nguyen TD, et al.: Role of domestic ducks in the propagation and biological evolution of highly pathogenic H5N1 influenza viruses in Asia.

    Proc Natl Acad Sci USA 2005, 102:10682-10687. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  45. Webster RG, Yakhno M, Hinshaw VS, Bean WJ, Murti KG: Intestinal influenza: replication and characterization of influenza viruses in ducks.

    Virology 1978, 84:268-278. PubMed Abstract | Publisher Full Text OpenURL

  46. Ito T, Okazaki K, Kawaoka Y, Takada A, Webster RG, Kida H: Perpetuation of influenza A viruses in Alaskan waterfowl reservoirs.

    Arch Virol 1995, 140:1163-1172. PubMed Abstract | Publisher Full Text OpenURL

  47. Morse SS: Factors in the emergence of infectious diseases.

    Emerg Infect Dis 1995, 1:7-15. PubMed Abstract OpenURL

  48. Munster VJ, Wallensten A, Baas C, Rimmelzwaan GF, Schutten M, Olsen B, Osterhaus AD, Fouchier RA: Mallards and highly pathogenic avian influenza ancestral viruses, northern Europe.

    Emerg Infect Dis 2005, 11:1545-1551. PubMed Abstract OpenURL

  49. Woolhouse ME, Gowtage-Sequeria S: Host range and emerging and reemerging pathogens.

    Emerg Infect Dis 2005, 11:1842-1847. PubMed Abstract OpenURL

  50. Webster RG, Geraci J, Petursson G, Skirnisson K: Conjunctivitis in human beings caused by influenza A virus of seals.

    N Engl J Med 1981, 304:911. PubMed Abstract OpenURL

  51. Kurtz J, Manvell RJ, Banks J: Avian influenza virus isolated from a woman with conjunctivitis.

    Lancet 1996, 348:901-902. PubMed Abstract | Publisher Full Text OpenURL

  52. Peiris M, Yuen KY, Leung CW, Chan KH, Ip PL, Lai RW, Orr WK, Shortridge KF: Human infection with influenza H9N2.

    Lancet 1999, 354:916-917. PubMed Abstract | Publisher Full Text OpenURL

  53. Koopmans M, Wilbrink B, Conyn M, Natrop G, van der Nat H, Vennema H, Meijer A, van Steenbergen J, Fouchier R, Osterhaus A, Bosman A: Transmission of H7N7 avian influenza A virus to human beings during a large outbreak in commercial poultry farms in the Netherlands.

    Lancet 2004, 363:587-593. PubMed Abstract | Publisher Full Text OpenURL

  54. Yuen KY, Chan PK, Peiris M, Tsang DN, Que TL, Shortridge KF, Cheung PT, To WK, Ho ET, Sung R, Cheng AF: Clinical features and rapid viral diagnosis of human disease associated with avian influenza A H5N1 virus.

    Lancet 1998, 351:467-471. PubMed Abstract | Publisher Full Text OpenURL

  55. Chan PKS: Outbreak of avian influenza A (H5N1) virus infection in Hong Kong in 1997.

    Clin Infect Dis 2002, 34(Suppl 2):S58-S64. PubMed Abstract | Publisher Full Text OpenURL

  56. Peiris JS, Yu WC, Leung CW, Cheung CY, Ng WF, Nicholls JM, Ng TK, Chan KH, Lai ST, Lim WL, et al.: Re-emergence of fatal human influenza A subtype H5N1 disease.

    Lancet 2004, 363:617-619. PubMed Abstract | Publisher Full Text OpenURL

  57. World Health Organization: Cumulative Number of Confirmed Human Cases of Avian Influenza A/(H5N1) Reported to WHO [http:/ / www.who.int/ csr/ disease/ avian_influenza/ country/ cases_table_2006_10_16/ en/ index.html] webcite

  58. Tweed SA, Skowronski DM, David ST, Larder A, Petric M, Lees W, Li Y, Katz J, Krajden M, Tellier R, et al.: Human illness from avian influenza H7N3, British Columbia.

    Emerg Infect Dis 2004, 10:2196-2199. PubMed Abstract OpenURL

  59. Katz JM, Lim W, Bridges CB, Rowe T, Hu-Primmer J, Lu X, Abernathy RA, Clarke M, Conn L, Kwong H, et al.: Antibody response in individuals infected with avian influenza A (H5N1) viruses and detection of anti-H5 antibody among household and social contacts.

    J Infect Dis 1999, 180:1763-1770. PubMed Abstract | Publisher Full Text OpenURL

  60. Buxton Bridges C, Katz JM, Seto WH, Chan PK, Tsang D, Ho W, Mak KH, Lim W, Tam JS, Clarke M, et al.: Risk of influenza A (H5N1) infection among health care workers exposed to patients with influenza A (H5N1), Hong Kong.

    J Infect Dis 2000, 181:344-348. PubMed Abstract | Publisher Full Text OpenURL

  61. Ungchusak K, Auewarakul P, Dowell SF, Kitphati R, Auwanit W, Puthavathana P, Uiprasertkul M, Boonnak K, Pittayawonganon C, Cox NJ, et al.: Probable person-to-person transmission of avian influenza A (H5N1).

    N Engl J Med 2005, 352:333-340. PubMed Abstract | Publisher Full Text OpenURL

  62. Liem NT, Lim W, World Health Organization International Avian Influenza Investigation Team, Vietnam: Lack of H5N1 avian influenza transmission to hospital employees, Hanoi, 2004.

    Emerg Infect Dis 2005, 11:210-215. PubMed Abstract OpenURL

  63. Schultsz C, Dong VC, Chau NV, Le NT, Lim W, Thanh TT, Dolecek C, de Jong MD, Hien TT, Farrar J: Avian influenza H5N1 and healthcare workers.

    Emerg Infect Dis 2005, 11:1158-1159. PubMed Abstract OpenURL

  64. Avian Influenza, human (09): Indonesia, Archive number 20070111.0133 [http://www.promedmail.org/pls/promed] webcite

  65. Tran TH, Nguyen TL, Nguyen TD, Luong TS, Pham PM, Nguyen VC, Pham TS, Vo CD, Le TQ, Ngo TT, et al.: Avian influenza A (H5N1) in 10 patients in Vietnam.

    N Engl J Med 2004, 350:1179-1188. PubMed Abstract | Publisher Full Text OpenURL

  66. Chotpitayasunondh T, Ungchusak K, Hanshaoworakul W, Chunsuthiwat S, Sawanpanyalert P, Kijphati R, Lochindarat S, Srisan P, Suwan P, Osotthanakorn Y, et al.: Human disease from influenza A (H5N1), Thailand, 2004.

    Emerg Infect Dis 2005, 11:201-209. PubMed Abstract OpenURL

  67. The Writing Committee of the World Health Organization (WHO) Consultation on human influenza A/H5: Avian influenza A (H5N1) infection in humans.

    N Engl J Med 2005, 353:1374-1385. PubMed Abstract | Publisher Full Text OpenURL

  68. de Jong MD, Bach VC, Phan TQ, Vo MH, Tran TT, Nguyen BH, Beld M, Le TP, Truong HK, Nguyen VV, et al.: Fatal avian influenza A (H5N1) in a child presenting with diarrhea followed by coma.

    N Engl J Med 2005, 352:686-691. PubMed Abstract | Publisher Full Text OpenURL

  69. Gruber PC, Gomersall CD, Joynt GM: Avian influenza (H5N1): implications for intensive care.

    Intensive Care Med 2006, 32:823-829. PubMed Abstract | Publisher Full Text OpenURL

  70. Cheung CY, Poon LL, Lau AS, Luk W, Lau YL, Shortridge KF, Gordon S, Guan Y, Peiris JS: Induction of proinflammatory cytokines in human macrophages by influenza A (H5N1) viruses: a mechanism for the unusual severity of human disease?

    Lancet 2002, 360:1831-1837. PubMed Abstract | Publisher Full Text OpenURL

  71. To KF, Chan PK, Chan KF, Lee WK, Lam WY, Wong KF, Tang NL, Tsang DN, Sung RY, Buckley TA, et al.: Pathology of fatal human infection associated with avian influenza A H5N1 virus.

    J Med Virol 2001, 63:242-246. PubMed Abstract | Publisher Full Text OpenURL

  72. Bridges CB, Lim W, Hu-Primmer J, Sims L, Fukuda K, Mak KH, Rowe T, Thompson WW, Conn L, Lu X, et al.: Risk of influenza A (H5N1) infection among poultry workers, Hong Kong, 1997–1998.

    J Infect Dis 2002, 185:1005-1010. PubMed Abstract | Publisher Full Text OpenURL

  73. Spackman E, Senne DA, Myers TJ, Bulaga LL, Garber LP, Perdue ML, Lohman K, Daum LT, Suarez DL: Development of a real-time reverse transcriptase PCR assay for type A influenza virus and the avian H5 and H7 hemagglutinin subtypes.

    J Clin Microbiol 2002, 40:3256-3260. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  74. Lee CW, Suarez DL: Application of real-time RT-PCR for the quantitation and competitive replication study of H5 and H7 subtype avian influenza virus.

    J Virol Methods 2004, 119:151-158. PubMed Abstract | Publisher Full Text OpenURL

  75. Payungporn S, Phakdeewirot P, Chutinimitkul S, Theamboonlers A, Keawcharoen J, Oraveerakul K, Amonsin A, Poovorawan Y: Single-step multiplex reverse transcription-polymerase chain reaction (RT-PCR) for influenza A virus subtype H5N1 detection.

    Viral Immunol 2004, 17:588-593. PubMed Abstract | Publisher Full Text OpenURL

  76. The World Health Organization Global Influenza Program Surveillance Network: Evolution of H5N1 avian influenza viruses in Asia.

    Emerg Infect Dis 2005, 11:1515-1521. PubMed Abstract OpenURL

  77. Regoes RR, Bonhoeffer S: Emergence of drug-resistant influenza virus: population dynamical considerations.

    Science 2006, 312:389-391. PubMed Abstract | Publisher Full Text OpenURL

  78. Hammer SM, Inouye RT: Antiviral agents. In Clinical Virology. 2nd edition. Edited by Richman DD, Whitley RJ, Hayden FG. Washington DC: ASM Press; 2002:171-242. OpenURL

  79. De Clercq E: Antiviral agents active against influenza A viruses.

    Nat Rev Drug Discov 2006, 5:1015-1025. PubMed Abstract | Publisher Full Text OpenURL

  80. Jefferson TO, Demicheli V, Di Pietrantonj C, Jones M, Rivetti D: Neuraminidase inhibitors for preventing and treating influenza in healthy adults.

    Cochrane Databse Syst Rev 2006, 3:CD001265. OpenURL

  81. Cass LM, Efthymiopoulos C, Bye A: Pharmacokinetics of zanamivir after intravenous, oral, inhaled or intranasal administration to healthy volunteers.

    Clin Pharmacokinet 1999, 36(Suppl 1):1-11. PubMed Abstract | Publisher Full Text OpenURL

  82. Massarella JW, He GZ, Dorr A, Nieforth K, Ward P, Brown A: The pharmacokinetics and tolerability of the oral neuraminidase inhibitor oseltamivir (Ro 64-0796/GS4104) in healthy adult and elderly volunteers.

    J Clin Pharmacol 2000, 40:836-843. PubMed Abstract | Publisher Full Text OpenURL

  83. Leneva IA, Roberts N, Govorkova EA, Goloubeva OG, Webster RG: The neuraminidase inhibitor GS4104 (oseltamivir phosphate) is efficacious against A/Hong Kong/156/97 (H5N1) and A/Hong Kong/1074/99 (H9N2) influenza viruses.

    Antiviral Res 2000, 48:101-115. PubMed Abstract | Publisher Full Text OpenURL

  84. Govorkova EA, Fang HB, Tan M, Webster RG: Neuraminidase inhibitorrimantadine combinations exert additive and synergistic anti-influenza virus effects in MDCK cells.

    Antimicrob Agents Chemother 2004, 48:4855-4863. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  85. Leneva IA, Goloubeva O, Fenton RJ, Tisdale M, Webster RG: Efficacy of zanamivir against avian influenza A viruses that possess genes encoding H5N1 internal proteins and are pathogenic in mammals.

    Antimicrob Agents Chemother 2001, 45:1216-1224. PubMed Abstract | Publisher Full Text | PubMed Central Full Text OpenURL

  86. Kiso M, Mitamura K, Sakai-Tagawa Y, Shiraishi K, Kawakami C, Kimura K, Hayden FG, Sugaya N, Kawaoka Y: Resistant influenza A viruses in children treated with oseltamivir: descriptive study.

    Lancet 2004, 364:759-765. PubMed Abstract | Publisher Full Text OpenURL

  87. de Jong MD, Tran TT, Truong HK, Vo MH, Smith GJ, Nguyen VC, Bach VC, Phan TQ, Do QH, Guan Y, et al.: Oseltamivir resistance during treatment of influenza A (H5N1) infection.

    New Engl J Med 2005, 353:2667. PubMed Abstract | Publisher Full Text OpenURL

  88. Le QM, Kiso M, Someya K, Sakai YT, Nguyen TH, Nguyen KH, Pham ND, Ngyen HH, Yamada S, Muramoto Y, et al.: Avian flu: isolation of drug-resistant H5N1 virus.

    Nature 2005, 437:1108. PubMed Abstract | Publisher Full Text OpenURL

  89. Sidwell RW, Bailey KW, Wong MH, Barnard DL, Smee DF: In vitro and in vivo influenza virus inhibitory effects of viramidine.

    Antiviral Res 2005, 68:10-17. PubMed Abstract | Publisher Full Text OpenURL

  90. Cheng VC, Tang BS, Wu AK, Chu CM, Yuen KY: Medical treatment of viral pneumonia including SARS in immunocompetent adult.

    J Infect 2004, 49:262-273. PubMed Abstract | Publisher Full Text OpenURL

  91. Nicholson KG, Colegate AE, Podda A, Stephenson I, Wood J, Ypma E, Zambon MC: Safety and antigenicity of non-adjuvanted and F59-adjuvanted influenza A/Duck/Singapore/97 (h5N3) vaccine: a randomized trial of two potential vaccines against H5N1 influenza.

    Lancet 2001, 357:1937-1943. PubMed Abstract | Publisher Full Text OpenURL

  92. Stephenson I, Bugarini R, Nicholson KG, Podda A, Wood JM, Zambon MC, Katz JM: Cross-reactivity to highly pathogenic avian influenza H5N1 viruses after vaccination with nonadjuvanted and MF59-adjuvanted influenza A/Duck/Singapore/97 (H5N3) vaccine: a potential priming strategy.

    J Infect Dis 2005, 191:1210-1215. PubMed Abstract | Publisher Full Text OpenURL

  93. Treanor JJ, Wilkinson BE, Masseoud F, Hu-Primmer J, Battaglia R, O'Brien D, Wolff M, Rabinovich G, Blackwelder W, Katz JM: Safety and immunogenicity of a recombinant hemagglutinin vaccine for H5 influenza in humans.

    Vaccine 2001, 19:1732-1737. PubMed Abstract | Publisher Full Text OpenURL

  94. Lipatov AS, Webby RJ, Govorkova EA, Krauss S, Webster RG: Efficacy of H5 influenza vaccines produced by reverse genetics in a lethal mouse model.

    J Infect Dis 2005, 191:1216-1220. PubMed Abstract | Publisher Full Text OpenURL

  95. Treanor JJ, Campbell JD, Zangwill KM, Rowe T, Wolff M: Saftey and immunogenicity of an inactivated subviron influenza A (H5N1) vaccine.

    New Engl J Med 2006, 354:1343-1351. PubMed Abstract | Publisher Full Text OpenURL

  96. World Health Organization: Summary of Probable SARS Cases with Onset of Illness from 1 November 2002 to 31 July 2003 [http://www.who.int/csr/sars/country/table2004_04_21/en/index.html] webcite

  97. Yu IT, Li Y, Wong TW, Tam W, Chan AT, Lee JH, Leung DY, Ho T: Evidence of airborne transmission of the severe acute respiratory syndrome virus.

    N Engl J Med 2004, 350:1731-1739. PubMed Abstract | Publisher Full Text OpenURL

  98. Fowler RA, Lapinsky SE, Hallett D, Detsky AS, Sibbald WJ, Slutsky AS, Stewart TE, Toronto SARS Critical Care Group: Critically ill patients with severe acute respiratory distress syndrome.

    JAMA 2003, 290:367-373. PubMed Abstract | Publisher Full Text OpenURL

  99. PandemicFlu.Gov: Interim Guidance On The Planning For The Use Of Surgical Masks And Respirators In Health Care Settings During An Influenza Pandemic [http://www.pandemicflu.gov/plan/healthcare/maskguidancehc.html] webcite