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EPIDEMIOLOGY: ON THE 1918 SPANISH INFLUENZA VIRUS GENOME

The following points are made by T.M. Tumpey et al (Science 2005 310:77):

1) The influenza pandemic of 1918 was exceptional, resulting in the deaths of up to 50 million people worldwide, including an estimated 675,000 deaths in the United States[1,2]. The pandemic's most striking feature was the unusually high death rate among healthy adults aged 15 to 34 years, which consequently lowered the average life expectancy in the United States by more than 10 years[3]. A similarly high death rate has not occurred in this age group in either prior or subsequent influenza A pandemics or epidemics[4].

2) Genomic RNA of the 1918 virus was recovered from archived formalin-fixed lung autopsy materials and from frozen, unfixed lung tissues from an Alaskan influenza victim who was buried in permafrost in November of 1918[5]. The complete coding sequences of all eight viral RNA segments have now been determined, and analysis of these sequences has provided insights into the nature and origin of this pathogen[5]. Plasmid-based reverse genetics has allowed for the generation of recombinant viruses containing 1918 hemagglutinin (HA) with or without the 1918 neuraminidase (NA) rescued in the genetic background of contemporary human H1N1 or H3N2 influenza viruses. The resulting strains were demonstrated to cause mortality in mice only at high infection doses; however, the virulence of the complete 1918 virus has not been evaluated.

3) In the present study, the authors generated a virus containing the complete coding sequences of the eight viral gene segments from the 1918 virus in an effort to understand the molecular basis of virulence of this pandemic virus. Genes encoding the 1918 influenza virus were reconstructed from deoxyoligonucleotides and corresponded to the reported coding sequences of the 1918 virus as previously described[5]. Because the 1918 5' and 3' noncoding regions have not been sequenced, the genes were constructed such that they had the noncoding regions corresponding to the closely related influenza A/WSN/33 (H1N1) virus. The 1918 virus and recombinant H1N1 influenza viruses were generated using the previously described reverse genetics system. All viruses containing one or more gene segments from the 1918 influenza virus were generated and handled under high-containment [biosafety level 3 enhanced (BSL3)] laboratory conditions in accordance with guidelines of the National Institutes of Health and the Centers for Disease Control and Prevention.

4) In summary: The pandemic influenza virus of 1918-1919 killed an estimated 20 to 50 million people worldwide. With the recent availability of the complete 1918 influenza virus coding sequence, the authors used reverse genetics to generate an influenza virus bearing all eight gene segments of the pandemic virus to study the properties associated with its extraordinary virulence. In stark contrast to contemporary human influenza H1N1 viruses, the 1918 pandemic virus had the ability to replicate in the absence of trypsin, caused death in mice and embryonated chicken eggs, and displayed a high-growth phenotype in human bronchial epithelial cells. Moreover, the coordinated expression of the 1918 virus genes most certainly confers the unique high-virulence phenotype observed with this pandemic virus.

References (abridged):

1. A. Crosby, America's Forgotten Pandemic (Cambridge Univ. Press, Cambridge, 1989)

2. N. P. Johnson, J. Mueller, Bull. Hist. Med. 76, 105 (2002)

3. W. P. Glezen, Epidemiol. Rev. 18, 64 (1996)

4. E. D. Kilbourne, in The Influenza Viruses and Influenza, E. D. Kilbourne, Ed. (Academic Press, New York, 1975), pp. 483-538

5. J. K. Taubenberger et al., Science 275, 1793 (1997)

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VIROLOGY: ON THE 1918 INFLUENZA EPIDEMIC

The following points are made by Edward C. Holmes (Science 2004 303:1787):

1) The global influenza pandemic of 1918, known as the "Spanish flu", is infamous for its ferocity. The first wave of the epidemic that began in the spring of that year was not characterized by exceptional mortality. However, the second wave, which peaked in September, October, and November of 1918, infected over one-third of the US population and globally killed between 20 and 40 million people, more than three times the number that died during World War I (1). Usually, the most serious consequences of influenza infection are reserved for the elderly, but in the case of the Spanish flu those worst affected were in the 15 to 45 age group.

2) Why the influenza pandemic of 1918 was so devastating, particularly in younger people, has intrigued virologists for decades. A variety of factors have been invoked to explain the high mortality of the 1918 influenza epidemic. Poor living conditions in the army camps where many of the victims died, as well as a lack of drugs to treat secondary bacterial pneumonia, may have greatly increased mortality rates. The high proportion of young people who fell victim to the epidemic has also been taken to imply that elderly people had acquired protective immunity from an earlier influenza outbreak (1).

3) However, the most popular theory is that the 1918 influenza A virus had unique pathogenic properties, most likely encoded within the hemagglutinin protein, a viral membrane glycoprotein that is critical for the virus to bind to host cell receptors and initiate membrane fusion, the first steps of viral infection. To date, 15 different subtypes of hemagglutinin have been identified in avian and mammalian influenza A viruses. All of these subtypes are found in wild waterfowl, which act as the reservoir host species for influenza A viruses. Only viruses carrying one of three hemagglutinin subtypes (H1, H2, H3) have crossed species barriers and successfully established themselves in humans, in each case causing a major pandemic: H1 in 1918, H2 in 1957, and H3 in 1968.

4) A unique glimpse into the 1918 epidemic is provided by the analysis of viral RNA sequences isolated from a few unlucky victims. To date, four such archival RNA samples have been recovered from lung biopsy tissues that had been taken from infected soldiers, embedded in paraffin, and fixed in formalin; another RNA sample was acquired from an Alaskan Inuit who died of the flu and was buried in the permafrost (4,5). However, despite the initial enthusiasm stimulated by the isolation of these ancient RNA molecules, intensive analysis of their primary sequences, particularly the sequence encoding hemagglutinin, has failed to find any obvious pointers as to why the 1918 influenza virus was so virulent. Particularly puzzling is the fact that although phylogenetic analysis places the 1918 virus strains at the base of the human branch of the H1 virus evolutionary tree and some distance away from avian strains, these viruses retain avian-like receptor binding sites (5).

5) Determination of the crystal structure of hemagglutinin from H1 viruses, including those from 1918, seems to provide a solution to the apparent paradox of how an avian virus can spread so effectively in humans. Stevens et al (3) have demonstrated that despite its phylogenetic position, the hemagglutinin protein of the 1918 virus is distinctly avian in structure, particularly within the receptor binding site. Gamblin et al (2) have gone one step further and have determined the structure not only of the 1918 virus hemagglutinin but also of hemagglutinins from related H1 viruses isolated in 1930 and 1934. In the case of the 1930 and 1934 viruses, they resolved the structure of the hemagglutinin bound to its host cell receptor, enabling them to assess the binding efficiency of these proteins.

References (abridged):

1. J. K. Taubenberger, A. H. Reid, T. A. Janczewski, T. G. Fanning, Philos. Trans. R. Soc. London B. 356, 1829 (2001)

2. S. J. Gamblin et al., Science 303, 1838 (2004)

3. J. Stevens et al., Science 303, 1866 (2004)

4. A. H. Reid et al., Emerg. Infect. Dis. 9, 1249 (2003)

5. J. K. Taubenberger, A. H. Reid, A. E. Frafft, K. E. Bijwaard, T. G. Fanning, Science 275, 1793 (1997)

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MEDICAL BIOLOGY: DISEASE AND THE SPECIES BARRIER

The following points are made by M.S. Klempner and D.S. Shapiro (New Engl. J. Med. 2004 350:1171):

1) Many infectious diseases cross the species barrier. Generally, this crossing occurs either because humans come into contact with a microorganism that is already capable of causing human infection or because an alteration occurs in the spectrum of species for which the organism is pathogenic -- the so-called "host range". The majority of these infectious agents are "zoonotic", meaning that their usual host is a nonhuman vertebrate. In some cases, the assignment of a "usual" host is rather arbitrary, since the pathogen may be found in many different species. The bacterium that causes Lyme disease, Borrelia burgdorferi, and rabies virus are examples of zoonotic pathogens that have a broad host range. These pathogens, along with many others, intermittently enter the human population as a result of contact between humans and an already competent pathogen.

2) In the case of Lyme disease, human contact occurs through the bite of an insect vector, the tick carrying the bacteria, that usually resides harmlessly in white-footed mice or deer. In the case of rabies, direct contact between humans and an infected vertebrate animal is the usual mode of transmission. Similarly, the recently described cases of monkeypox and tanapox infections in humans resulted from direct contact with infected prairie dogs and chimpanzees, respectively. In each of these cases, molecular evidence suggests that the genome of the microorganism is virtually the same whether the organism is isolated from the animal or the infected human.

3) Epidemiologic links to direct contact with infected poultry have been implicated in almost all the cases of human infection with avian influenza A, which expresses the H5 type hemagglutinin and the N1 type neuraminidase. And detailed sequence analysis of the isolates of avian influenza A (H5N1) virus from patients in Vietnam reveals that it is unchanged from the isolates from infected poultry.

4) The outbreak of SARS coronavirus (SARS-CoV) infections in humans represents something partway between a small step to man and the feared giant leap to mankind. There is strong evidence that this coronavirus was present in a population of animals in China, including the Himalayan palm civet (Paguma larvata). Through molecular changes, most likely in its spike glycoprotein, SARS-CoV broadened its host range in such a way that it became capable of attaching to human cells and infecting humans who were directly exposed to these animals. Seroprevalence studies involving persons who sold these animals for consumption in China demonstrated that antibodies to the new SARS-CoV developed in more than 70 percent of the animal handlers. Thus, under the right circumstances, the transmission from animals to humans was relatively efficient.

5) But SARS-CoV had an additional feature: it could be spread not only from animals to humans, but also from one human to another -- albeit inefficiently. It is important to emphasize the efficiency of human-to-human transmission. Although there are rare instances of human-to-human transmission of infectious diseases whose transmission usually requires direct contact with animals -- for example, rabies, which has been transmitted from human to human under extraordinary circumstances, such as corneal transplantation from an infected tissue donor -- SARS-CoV crossed the species barrier to humans and then was transmitted from clinically ill persons to household contacts, health care workers, and even passengers who were seated near the ill persons on airplane flights. However, from recent seroprevalence studies, we have learned that very few people who had direct contact with patients with confirmed SARS-CoV infection actually became infected.

6) The epidemiologic significance of the transmission of infectious agents from animals to humans depends on such factors as the density of the infected animal hosts (e.g., poultry), the frequency with which susceptible humans come into contact with these animals, and the biology of the pathogen, including its mode of transmission and the efficiency of its spread from human to human. Thus, in Asia, where there remain large numbers of poultry that are infected with the influenza A (H5N1) virus and are in contact with humans, even if interspecies transmission is rare, we can anticipate additional cases in humans.

New Engl. J. Med. http://www.nejm.org

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