What Causes Bird Flu

Bird flu is caused by a kind of influenza virus. Influenza viruses come in three distinct types, called type A, type B and type C, each with its own characteristics. Type C influenza viruses are fairly stable and cause relatively mild symptoms, while type B influenza viruses cause more severe, though relatively localised outbreaks. Type A influenza viruses are responsible for the deadly influenza pandemics that, every few decades, rampage around the world leaving a trail of sickness and death in their wake. It is to this last, most dangerous, type that bird flu, or avian influenza, belongs.

Type A influenza viruses may be further divided into subtypes – HA, of which there are fifteen, and NA of which there are nine. These subtypes may combine, resulting in a range of other subtypes, whose impact may be confined to a single species. For example, H1N1 is usually responsible for sickness in humans, while H3N8 infects horses. Birds are susceptible to at least fifteen different influenza A subtypes, of which by far the most dangerous is H5N1 – the strain of bird flu that has caused the deaths of millions of birds in recent months. Only recently have the subtypes that affect birds been observed in any other kind of animal (including humans) apart from pigs.

All type A influenza viruses, including bird flu, share the ability to quickly and easily change their genetic makeup and arbitrarily swap genes between themselves. As the virus reproduces, tiny changes take place in its genetic makeup that are then passed on to later generations. The cumulative effect of these changes, called antigenic drift, is that older strains of the virus are constantly being replaced by new strains that are unaffected by antibodies developed to combat earlier strains.

The second, lethal, kind of bird flu causes birds to become very ill extremely quickly and, because it is highly contagious, spreads rapidly to surrounding birds. It affects the birds’ respiratory tract, and attacks multiple organs and tissues, leading to massive internal haemorrhaging. So deadly is this form of bird flu that almost 100% of infected birds may die within 48 hours of contracting the disease.

Of potentially greater concern, however, is a process called antigenic shift, in which influenza A subtypes that normally infect different species, such as a human and a bird, combine to produce a completely new strain. Because the new virus is different from the strains that combined to produce it, there is no natural immunity to it. This enables the new virus to spread rapidly, causing extensive sickness and death.

Antigenic shift resulting in a virus that crosses the species barrier between birds and humans is often traced to locations where humans, chickens and pigs live in close proximity. Pigs can easily be infected with both human and bird flu viruses, making them an ideal venue for the two different kinds of virus to meet and exchange genes. Such a third party is not an essential component of antigenic shift, however; some bird flu viruses are capable of infecting humans who come into direct contact with infected birds, although they are not, currently, able to spread on to other humans, and it seems that this is how the vast majority bird flu cases in humans to date have occurred.

The more frequently humans are infected with bird flu through contact with infected birds, the greater the risk that one of those humans will be simultaneously infected with a human influenza virus; under those conditions there is the potential for the two viruses to merge and produce a deadly new strain that spreads as quickly amongst humans as bird flu currently does amongst birds. Although this is true of all bird flu strains, the deadly H5N1 strain has a well-documented propensity for both mutating quickly and commandeering large swathes of genetic material from viruses that infect other species via a process called reassortment.

In view of how far H5N1 has spread around the globe, the likelihood that this strain of bird flu will meet and combine with a human influenza virus, thereby creating the cause of a bird flu pandemic amongst humans, is disturbingly high. It is surely only a matter of time before the process begins – if it has not already begun.