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H5N1 bird flu making a comeback in 2011

A deadly strain of the “bird flu” virus is currently spreading in Asia, with a focus in Cambodia, where all eight people infected so far this year have died.

A deadly strain of the “bird flu” virus is currently spreading in and beyond Asia, with a focus in Cambodia, where all eight people infected so far this year have died, according to an August 29, 2011 report from the Food and Agriculture Organization (FAO) of the United Nations (UN). The virus has also had a resurgence in Egypt, where a total of 151 people have been infected, including 32 cases in 2011, based on information from the World Health Organization (WHO).

Since appearing in 2003 with harbingers that included two dead tigers and two dead leopards fed fresh – but infected – chicken carcasses in a Thai zoo (WHO, PDF), this form of the virus, known as H5N1, has infected 565 people worldwide. Of these, 331 have died, reports the WHO, which provides continually updated information about new cases and case totals by country on its website. Indonesia, for example, has seen a steady number of cases since 2005 with a total of 178 cases and 146 deaths as of mid-August 2011.

While the “bird flu” seemed to have faded from the global stage as the panic around its debut in 2003 waned, it never really went away. In addition to persisting in Cambodia, it also retained its foothold in Egypt, where of 151 cases, 52 people have died, according to WHO. In fact, even after its global peak in 2006, when it occurred in 63 countries, the virulent strain continued in six countries: Bangladesh, China, Egypt, India, Indonesia, and Vietnam. In addition to human costs, the virus has also exacted an agricultural and economic toll, causing the culling of 400 million domestic birds and about $20 billion in damage worldwide since it emerged on the global infectious disease stage eight years ago.

Domestic birds came under growing threat again in 2008 as migratory birds brought H5N1 with them on their travels, exposing both domestic and wild birds on their routes. But natural migration may not be the only issue. In October 2004, for example, officials at Brussels International Airport seized two hawk-eagles smuggled from Thailand that tested positive for H5N1. Quoted in an FAO news release, Chief Veterinary Officer Juan Lubroth said,

Wild birds may introduce the virus, but people’s actions in poultry production and marketing spread it.

The spread has edged beyond South Asia, with the virus turning up in birds in Bulgaria, Romania, Israel, and Nepal, according to the FAO.

Poultry. Image Credit: Kashif Mardani, via Flickr.

Even more worrisome is that the virus is manifesting a key feature of any influenza virus: change. In this case, the changes translate into an ability to evade vaccine defenses developed against the old version of H5N1. Avian, or bird, viruses occur in two broad categories – the fairly mild version known as “low pathogenic,” and the much more deadly types known as “high pathogenic.” While there are three types of influenza viruses, known as A, B, and C, only the A viruses are relevant for bird flu. Experts class A viruses based on two proteins, hemagglutinin (H) and neuraminidase (N), each of which occurs in several forms that have been assigned numbers. If you recall the H1N1 “swine flu” virus, it consisted of hemagglutinin 1 and neuraminidase 1.

There are 16 version of H, and nine versions of N. Although any of the 16 possible hemagglutinins can make up an A subtype, the avian viruses of most interest are those with H5, H7, or H9, any one of which can pair up with one of nine possible neuraminidases. In the case of the high-pathogenic virus known as “bird flu,” H5 has paired up with N1 in a particularly virulent and deadly combination that has now changed oh-so-slightly, as influenza viruses will, to escape barriers against its invasion, including vaccines. While birds are susceptible to any of the A subtypes, it takes a special strain of A viruses to invade both people and birds.

It is there at the crossroads of people and birds that H5N1 has gained its virulent and deadly foothold, flourishing where the two mix. It seems that currently, the reported cases have involved bird-to-human transmission, although health authorities are likely keeping a keen eye out for cases that hint at human-to-human transmission, a whole different infectious disease crisis. According to the Centers for Disease Control and Prevention website, if high-pathogenic H5N1 “were to gain the capacity to spread easily from person to person, an influenza pandemic (worldwide outbreak of disease) could begin.” One reason for this fear is that humans, being humans, have little native resistance to a virus that’s spent much of its time in birds. Some other mammals also show little resistance. The same Thai zoo that saw four big cats die in 2003 experienced a second outbreak of H5N1 among its tigers in 2004, again because of fresh – but infected – chicken carcasses. Of the 441 tigers at the zoo, 147 died or were euthanized.

Right now, however, the concern is closer to the farmyard – or the backyard – than the zoo. Said Lubroth,

The general departure from the progressive decline observed in 2004 to 2008 could mean that there will be a flareup of H5N1 this fall and winter, with people unexpectedly finding the virus in their backyard.

While the resurgence of H5N1 “bird flu” is cause for concern, the FAO has noted that the six countries where the avian influenza strain has persisted all along – Bangladesh, China, Egypt, India, Indonesia, and Vietnam – are likely to face the biggest problems. Yet, Lubroth warned,

Preparedness and surveillance remain essential. This is no time for complacency. No one can let their guard down with H5N1.

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