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In both 1957 and 1968, a new influenza virus emerged because of reassortment events involving two influenza viruses. The segmented genome allows each influenza A virus to exchange genetic material with other influenza A viruses. In 1957, dual infection of an individual animal — probably a human, but possibly another species, such as a pig — with an avian H2N2 influenza and a human H1N1 influenza resulted in the emergence of a new influenza virus containing the hemagglutinin, the neuraminidase, and the gene for one of the polymerase proteins (PB1) from the avian virus, along with the remaining five genetic segments from the human H1N1 influenza virus.
The new reassortant virus circulated in humans until 1968, when it was replaced by another reassortant virus, the H3N2 Hong Kong virus — created by the replacement of the hemagglutinin (H2) and polymerase (PB1) genes of the H2N2 virus with two new avian genes, H3 and a new PB1. Today, the descendants of this virus continue to cause the majority of influenza infections in humans (see diagram). Five of the genes of today's H3N2 influenza virus have their origin in the 1918 pandemic.
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The concern at present relates to the widespread epidemic of avian H5N1 influenza in domestic fowl, as well as wild birds, with sporadic transmission to humans. In the past decade, numerous instances of bird-to-human transmission have been recognized (see table); although we have only recently become aware of them, these events are surely not new. What is new is the broadening of the range of avian and nonavian species that have become infected with the current H5N1 virus.3
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In addition, the 1918 virus is 100 times as lethal in mice as any other human influenza virus; the median lethal dose (LD50, or 103.5 to 3.75 median egg infectious doses [EID50]) is low, and the virus replicates rapidly so that high titers (>107 EID50 per milliliter) are found in the lungs of infected mice. High virus inocula result in the death of mice as early as three days after they have been infected. The 1918 virus was susceptible to the adamantine compounds (amantadine and rimantadine) and neuraminidase inhibitors (oseltamivir and zanamivir), and the availability of the recovered virus will facilitate studies of other therapeutics. For example, the vigorous release of cytokines in mice infected with the 1918 influenza virus is associated with rapid onset of pulmonary disease and death. Compounds that block the action of specific cytokines can now be evaluated as therapeutics that might help to reduce the mortality associated with pandemic influenza.
It is not possible to know whether the current H5N1 is capable of adapting to humans so that it can spread with high efficiency through low-titer aerosol transmission to initiate an influenza pandemic. However, Taubenberger et al. provide some guidance on the genetic changes that might be required for such an event. The role of PB1 must be critical, since in both 1957 and 1968, this polymerase gene was transferred along with the hemagglutinin during reassortment. By comparing the consensus sequence of the three avian influenza polymerase genes PA, PB1, and PB2 with the 1918 sequence, as well as with more contemporary influenza viruses, Taubenberger et al. have identified four amino acids of PA, one of PB1, and five of PB2 that are found in human influenza viruses (including the 1918 virus) but generally not in avian influenza viruses. In two instances, these amino acids are found in nuclear localization signaling regions, suggesting that some or all of these amino acid differences are critical for the virus to adapt to humans.
The genetic sequences of the 1997 Hong Kong H5N1 virus and the 2004 Vietnam H5N1 virus reveal that several human isolates of these viruses contain one of the five amino acid changes in PB2 that have been identified as important to the ability of the 1918 virus to infect humans. This finding suggests that several additional genetic changes must occur before these viruses will begin to spread efficiently from person to person. The genetic sequences of avian viruses may provide a window through which to monitor these sporadic transmissions for the potential of the viruses to adapt to humans. The occurrence of additional genetic changes in the avian H5N1 virus circulating in birds that match the consensus sequence for PA, PB1, or PB2 in human influenza would be cause for heightened concern.
On the basis of the rates of replacement of amino acids, Taubenberger et al. estimated that avian influenza polymerase genes had been circulating in humans as early as 1900. If this estimate is correct, then monitoring of the sequences of viruses isolated in instances of bird-to-human transmission for genetic changes in key regions may enable us to track viruses years before they develop the capacity to replicate with high efficiency in humans. Knowledge of the genetic sequences of influenza viruses that predate the 1918 pandemic would be extremely helpful in determining the events that may lead to the adaptation of avian viruses to humans before the occurrence of pandemic influenza. We could then conduct worldwide surveillance for similar events involving contemporary avian viruses.
Source Information
Dr. Belshe is a professor of medicine, pediatrics, and molecular microbiology in the Division of Infectious Diseases and Immunology, Department of Internal Medicine, Saint Louis University, St. Louis.
An interview with Dr. Belshe can be heard at www.nejm.org.
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