# 5862
Earlier this month the three international agencies most closely associated with the tracking and control of bird flu (World Health Organization, FAO, OIE) issued a technical report on the current evolution of the H5N1 avian influenza virus.
The forward indicated that this report is intended for:
. . . persons generally familiar with influenza viruses and the epidemiology of influenza in humans and animals.
Which I suspect describes most of my readers. The document can be downloaded from:
FAO-OIE-WHO Technical Update: Current evolution of avian influenza H5N1 viruses
7 September 2011• Highly pathogenic avian influenza H5N1 viruses continue to circulate in poultry and cause disease, and remain a threat to human and animal health.
• Both animal and public health sectors at the national, regional, and international levels should maintain vigilance in regularly detecting, reporting, and characterizing animal influenza viruses, and in assessing and managing existing and evolving health
risks associated with these viruses.
Although often neglected by the mainstream media (unless they can use the word `mutant’ in a headline), H5N1 avian influenza continues to pose a threat to birds and to other species, including humans.
As you can see by the chart above, the number of human cases reported has slowly been increasing since their nadir of 2008. Still, they remain far below the numbers we saw in 2003-2006.
While that might seem like good news, it isn’t necessarily as comforting as it may appear.
Between budget cuts, pandemic fatigue, and other pressing public health concerns, surveillance and testing for the H5N1 virus remains quite limited in many parts of the world.
Best case, only the sickest of the sick - who have had direct contact with poultry - are likely to be tested for the virus.
Meaning that the numbers we see probably do not reflect the full burden of the disease. A fact that is true for practically every illness.
Source CDC
Whether we are talking influenza, Lyme Disease, Salmonella poisoning, or bird flu . . . official case counts are usually just the tip of the pyramid.
The concern is that if the virus adapts better to human physiology, it could begin to produce less virulent infections in some people, and spread more easily undetected.
While solid evidence of that happening is scant, in a recent seroprevalence study of 800 villagers in Thailand (who reported no history of a serious influenza-like illness), (5.6%) had antibodies to A/Thailand/ 676/005H5N1, and (3.5%) were shown to be seropositive to A/Thailand/384/2006 H5N1.
Suggestive, at least, that milder or perhaps even subclinical infections with the virus may be occurring outside our view.
And in the past couple of years we’ve also seen a trend in Egypt towards less virulent infections along with a mortality rate half that seen elsewhere, once again suggesting the virus might be adapting better to human physiology (see PLoS: Human-Type H5N1 Receptor Binding In Egypt).
Despite these recent studies, for now, the H5N1 virus does not appear to be ready for prime time. Full adaptation to a humanized pathogen could take years, or even decades.
Or it might not happen at all.
The tripartite report updates the situation in Cambodia, which has seen a resurgence in H5N1 cases this year (see Cambodia: 8th Fatal H5N1 Case Of 2011, stating:
Eight human cases of H5N1 infection have been reported from Cambodia in 2011, all of which were in people under 19 years of age and all of which were fatal. This is the highest number of human H5N1 cases reported in one year from Cambodia. The basis for the increased incidence and the high mortality remain unclear.
All of these cases represent sporadic infections likely related to exposure to sick poultry, and are not linked to each other, with the exception of a mother and her child. There is no evidence of sustained human-to-human transmission of H5N1 virus in Cambodia or any other country.
The report also addresses the recent announcement of a `mutated’ clade of H5N1 (see FAO Warns On Bird Flu), stating:
• Predictably, the avian influenza H5N1 viruses also continue to evolve, especially in areas where they circulate endemically in poultry. The slowly evolving viruses are periodically regrouped and assigned new clade names.
• One clade, recently named H5N1 clade 2.3.2.1 has evolved from clade 2.3.2 viruses previously circulating among poultry in the eastern Asia since 2005. This clade has been increasingly found in poultry in several countries, and in some areas has become predominant over previously circulating clades. Hence, clade 2.3.2.1 viruses are not “new”.
• An older clade of H5N1 viruses, named clade 1, is now being reported only from southern Viet Nam and Cambodia. To date (September 2011), only clade 1 viruses have ever been isolated from humans and animals in Cambodia.
• The clade “nomenclature” (i.e. method of naming) for H5N1 viruses is regularly addressed by the standing WHO/FAO/OIE H5N1 Evolution Working Group. Maintaining a standard H5N1 clade nomenclature is important for effective communications among scientists working in the animal health and public health sectors, among influenza researchers, and with the public.
The report concludes:
Virus evolution and nomenclature: Human health
• There is currently no evidence that any particular clade or strain of the H5N1 virus, including any newly evolved clade, is more transmissible or pathogenic in humans. Although three human cases of H5N1 virus infection have been associated with clade 2.3.2.1 viruses 2, there is no indication that clade 2.3.2.1 viruses pose any greater threat to human health than any of the other H5N1 viruses.
• As viruses spread more widely and intensively in poultry and wild birds, the likelihood of human exposures to infected birds increases. However, this does not increase the ability of the viruses to infect and transmit between people.
• Three clade 2.3.2.1 viruses have already been identified by WHO’s Global Influenza Surveillance and Response System (GISRS; the group of experts that studies animal and human influenza viruses that may impact human health) to be considered for inclusion in future human inter-pandemic H5N1 vaccine viruses if needed.3
Although we continue to see isolated human infections around the world, for now H5N1 is primarily a threat to poultry.
Despite ongoing evolution and mutation, the virus remains poorly adapted to human physiology, and only causes rare, sporadic human infections.
The concern, of course, is that over time that could change.
And so the world remains at Pre-pandemic Phase III on the H5N1 virus, and we continue to watch for signs that the virus is adapting better to humans.
