Humans spared

Published : Feb 15, 2008 00:00 IST

Human infections of bird flu have been entirely avian in origin and reflect strains circulating locally among poultry and wild birds.

DURING every bird flu outbreak since the disease first arrived in India in 2006, there have been false alarms about humans getting infected by the virus. It is only fortunate that there has been no incidence of humans contracting the H5N1 virus until now. But, given the scale of the present outbreak in West Bengal, this is only waiting to happen.

Worldwide, there have been a number of confirmed human cases since 2003, with the outbreak among birds itself having spread to 48 countries. According to data released by the World Health Organisation, as of January 23, a total of 352 people have been infected, of whom 219 have died. Indonesia has recorded the highest mortality rate of 80 per cent with 97 of the 120 infected having died. However, a January 17 review of the human situation by the Committee of the WHO on Clinical Aspects of Human Infection with Avian Influenza A (H5N1) Virus in New England Journal of Medicine, noted: Despite widespread exposures to poultry infected with avian influenza A (H5N1) viruses, influenza A (H5N1) disease in humans remains very rare.

The reviewers use the plural viruses because several strains of the virus are circulating across the world. They predominantly seem to come in two broad phylogenetic lineages, or clades as they are called, and their subclades. Whereas Clade 2 appears to have been circulating subclades 2.1, 2.2 and 2.3, Clade 1, which was predominant in the early phase of the outbreaks in Vietnam, Thailand and Cambodia, does not yet have any circulating subclades. While Clades 1 and 2.2 and 2.3 have been implicated in the outbreaks in South Asia and South-East Asia, only 2.2 has been the cause in West Asia, Europe and Africa. Clade 2.3 has been only recently introduced into South-East Asia from southern China, according to the WHO report. The highly virulent strain, which has caused a high rate of human infection and also a high mortality rate among humans in Indonesia, however, seems to be an entirely separate strain, classified as Clade 2.1, and the region has become endemic to this strain. The increase in worldwide human cases was, in part, owing to the spread of the Clade 2.2 virus across Eurasia and Africa. The origin of all these strains can be traced to Clade 0 from Hong Kong, where bird flu was first detected in 1997.

The strains circulating in India, according to H.K. Pradhan, former Director of the High Security Animal Disease Laboratory (HSADL) in Bhopal, have their origins in the major Chinese outbreak of December 2005 in the Qinghai lake due to Clade 2.2. Of course, the sequencing of the current outbreak in West Bengal is yet to be done. While it is suspected that the cross-border movement of poultry between India and Bangladesh is the reason for the present outbreak, this cannot be established because the sequence(s) of the Bangladesh virus is yet to be made available to the database of the World Organisation of Animal Health (OIE) or the WHO, which in itself could throw light on how the virus could have been introduced into Bangladesh.

What is, however, apparent is that human infections have been entirely avian in origin and they reflect strains circulating locally among poultry and wild birds. Bird flu viruses, the review points out, can be maintained, amplified and disseminated in live-poultry markets. While migratory birds may spread the highly pathogenic H5N1 viruses to new geographical regions, which was the possible cause of the Navapur/Nandurbar outbreak in Maharashtra in 2006, their importance as an ecological reservoir remains uncertain. While the spread of H5N1 viruses seems to be mainly due to the movement of poultry and poultry products to areas free from infection, recent Clade 2.2 outbreaks in sub-Saharan Africa, Egypt and Europe seem to suggest that the virus may have been introduced by wild birds.

Although limited human-to-human transmission has been seen in a few clusters of human infection, direct avian-to-human transmission of H5N1 remains the predominant mode of infection. The exact mechanism of infection of the cells in the human respiratory tract is yet to be deciphered. In contrast with the periodic human influenza outbreaks, due to the human-adapted H1N1 and H3N2 viruses, that circulate among human populations following sustained human-to-human transmission, such a mode for the bird flu virus seems to be highly inefficient. The pathogen and host factors that determine the mechanism are understood incompletely.

What is currently known is that the first step in infection involves the binding of hemagglutinin (HA) to certain receptors on the cell surface called sialylated glycans. These are basically sugar molecule chains combined to sialic acid. This chemical combination can occur via two modes of linkages: the alpha-2,3 link and the alpha-2,6 link. The infection in birds occurs as a result of the preferential binding of the virus to the alpha-2,3-linked receptors on avian cells. It is this fact that prevents H5N1 and other avian influenza viruses from readily infecting humans as the upper respiratory tract in humans predominantly expresses receptors of the other kind. Only infection in the upper respiratory tract can result in the shedding of virus particles through coughing or sneezing.

Viruses isolated from humans show that they acquire mutations that permit binding to both kind of receptors. That is, HA molecules manage to switch their preference to alpha-2,6-linked receptors. Even this explanation is insufficient for the inefficient human-to-human transmission that is seen in the current bird flu viruses. The WHO report suggests that perhaps changes in multiple viral genes not the sialic acid receptors alone are required to generate a potentially efficient human-to-human transmission mechanism, which could lead to pandemic bird flu among humans.

Recent work by Ram Sasisekharan and associates from the Massachusetts Institute of Technology shows that the topology, or shape of the sugary receptors on the cells, could be driving the underlying mechanism of infection. They point out that the evident conundrum in the case of bird flu was also seen in the human flu virus, H1N1, that caused the major 1918 outbreak: while a New York strain, which showed mixed alpha-2,3 and alpha-2,6 binding, transmitted inefficiently among humans, a Texan strain of the same virus with mixed binding characteristics transmitted efficiently. It is this fact that led them to look at other requirements governing the binding of the HA of influenza viruses to the cell receptors to make them suited for human adaptation, in particular the role of glycan diversity in the cells. At present, as evidence shows, avian flu viruses have predominantly alpha-2,3 specificity but have acquired certain mutations to enable some alpha-2,6 specificity as well.

Combining experimentation and techniques of data mining of extensive databases to study protein-glycan binding preferences, they found that the human upper respiratory tract showed diversity in the structures of the receptors that were presented to the virus, in particular their geometry and shape. The glycan receptors basically came in two topologies, cone-shaped and umbrella-shaped. What they found was that, while the cone-like topology was characteristic of both alpha-2,3- and alpha-2,6-linked glycan structures, the umbrella model was unique to the alpha-2,6 structure.

Given this geometric feature of the combining sites in human cells, the team was able to establish that what is required for an efficient adaptation of the HA is a mutation that confers not only a switch from an ability to combine with alpha-2,3 to an ability to combine with alpha-2,6, it needed to further mutate to be able to acquire the umbrella-shape in order to get entrenched in humans and effect efficient human-to-human transmission that is, as the authors write, mutation to not just linkage per se but to human-like receptors as well.

According to the authors, it is not inconceivable that a few mutations are able to confer the HA gene in the virus with this geometric specificity. With this improved understanding of why the virus is yet to get a foothold in human populations, the authors point out that the findings should enable better strategies for effective surveillance and potential therapeutic and prophylactic interventions to fight the scourge of bird flu viruses. Since each strain would be different in this respect, it would require Indian characterising of the circulating strains to be more detailed to capture this continually mutating feature of the virus.

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