Asian Bird Flu: Behind the Headlines

In the past several weeks, a new outbreak of avian influenza (more commonly known as “bird flu”) has been causing concern around the world. The number of humans affected by this outbreak has so far, thankfully, been very small. Still, the situation has been considered sufficiently grave that many nations and organizations are taking swift action to contain the disease, including killing large numbers of domesticated birds thought to have been exposed to this flu.

To better understand the nature of bird flu and the unique dangers it represents, one needs a fuller understanding of influenza in general—its varieties, its causes, its means of transmission, and its risks. This article seeks to provide that information, so that readers can better put into perspective the headlines on this health risk and the actions being taken against it.

The Flu: Some Background

Influenza (“the flu” for short) is an acute and potentially life-threatening infection of the respiratory tract. It attacks and damages the delicate tissues lining the lungs, causing them to become swollen and inflamed. In most cases, the lung tissues heal in a couple of weeks and the patient recovers. However, there is a significant risk of life-threatening complications such as pneumonia, in which sections of the lungs become so damaged and pus-filled that breathing can become difficult or impossible without medical intervention. The risk increases for anyone who has any kind of weakness in their respiratory or immune systems. This includes seniors, who often have a variety of pre-existing health problems, and children, whose immune and respiratory systems are not yet fully matured.

Influenza has been periodically hazarding the health of human beings for many centuries. Before its true cause was known, the cyclical outbreaks were thought to be caused by some sort of astrological influence of the stars and planets—thus the name “influenza.” Nowadays, scientists know that the flu is caused by one of several related strains of virus.

Viruses as Moving Targets

All viruses, including the influenza virus, periodically undergo changes that produce new strains, presenting continual challenges to the human immune system. Normally, the immune system recognizes an infectious agent by one or more characteristic proteins on that agent’s surface; it then manufactures the appropriate antibody for that agent, which destroys enough of the agent to stop the infection. However, each time the virus changes, it presents some new variant of its characteristic proteins, raising the odds that the patient’s immune system won’t be able to recognize it and thus making it that much harder for the body to fight off the infection.

Scientists have grouped the various strains of the influenza virus into three types, known as Types A, B, and C; avian influenza happens to be a Type A virus. Influenza type A viruses are further divided into subtypes based on variations in two proteins on the surface of the virus. These proteins, called hemagglutinin (H) and neuraminidase (N), play key roles in the life cycle of the virus, including its ability to infect healthy cells and to reproduce itself. (Influenza B virus is not divided into subtypes; influenza C virus typically causes a much milder form of the disease, and is thus less a focus of concern than the first two types.)

Type A influenza viruses undergo two different types of changes, known as antigenic drift and antigenic shift. Antigenic drift happens all the time, typically causing small changes that are just enough to confuse a patient’s immune system so that the patient can get re-infected with flu. In the much rarer antigenic shift, however, the change is abrupt and major, resulting in a whole new virus subtype with new forms of hemagglutinin and/or neuraminidase that human immune systems can no longer identify at all. Such a new subtype can produce the destructive epidemics for which influenza is feared.

From Birds to Humans

Avian influenza variants can infect several species in addition to humans, including birds, pigs, horses, seals and whales. These non-human variants of influenza viruses do not usually directly infect humans or circulate among humans, but on the rare occasions that the virus does succeed in making the jump between humans and other animals, the effects can be potentially devastating. If an animal should happen to become infected with both human and non-human variants of the virus, there is a chance that the two variants could mix, or reassort, resulting in a new antigenically shifted subtype of the virus that can infect humans, and for which the human immune system has no antibody protection.

Recent studies have suggested that the so-called Spanish flu epidemic of 1918, currently the deadliest outbreak of flu in recorded history, was an avian influenza virus which successfully made this trans-species jump. The 1918 outbreak was remarkable not only for the large number of deaths it caused—up to 40 million worldwide—but also because it inflicted such high mortality rates, reaching 70 percent in some communities. It also was unusual in that mortality was particularly high among young adults, the age group that is usually least impacted by the flu.

Research on the 1918 flu virus had previously been hampered by the difficulty in obtaining samples of the now-extinct viral strain. Viruses were not identified as the cause of influenza until the 1930s, and lung tissue samples taken in 1918 had been generally unreliable sources because the virus degrades easily. Recently, however, research teams from Britain’s Medical Research Council and the Scripps Research Institute in La Jolla, California were able to obtain usable viral RNA fragments from preserved tissue samples taken from World War I soldiers, and from the cadavers of Alaskan victims who were buried in permafrost.

From that RNA, the researchers were able to reconstruct the Spanish flu virus’s variety of the hemagglutinin protein, and to determine that it did resemble that of avian flu virus strains with some features of human flu virus strains. This is no doubt why this particular virus proved so deadly—its hemagglutinin was totally alien to the immune systems of most of the humans who encountered it.

It is exactly this kind of scenario that is causing health officials around the world to treat this latest outbreak of bird flu with such caution. So far, the majority of humans reported to be infected with this bird flu have been shown to have caught the disease from birds rather than another human, which means the virus has not changed sufficiently to be efficient at living within human beings. If human-to-human infection does increase, however, the risk of a serious epidemic begins to rise.

Asia as a Flu Incubator

The mechanics of how bird flu passes from birds to humans also goes a long way towards explaining why so many flu outbreaks over the years have originated in various Asian countries. Flu epidemiology is essentially a game of odds and opportunities: the more chances a virus has to jump from individual to individual, the more opportunities for the viral changes, small and great, to creep into the system, and the greater the odds of breeding a super-virus that can jump from one species to another with deadly effect.
Many Asian countries right now have very large populations living in densely inhabited areas. In many of these cultures, for a variety of historical and economic reasons, people raise large numbers of poultry for food and livelihood, living in close proximity with their livestock. This large concentration of people and birds provides innumerable opportunities for exposure to any virus present in the population, and thus increases the odds that outbreaks of avian influenzas will occur, and that a killer version of the virus will arise.

It is important to note that Asia is by no means the only region where flus can originate. Many flus can and do arise in other parts of the world. The 1918 Spanish flu epidemic, to give one notable example, is thought to have gotten its start in the European military camps of World War I, where soldiers lived in very close quarters under less than hygienic conditions, and kept farm animals in the camps as a source of food.

Conclusions

Researchers continue to search for effective vaccines for various flu strains. However, this latest outbreak of Asian bird flu underlines the point that fighting the flu is as much a public health policy issue as a medical research issue. As challenging as the medical search for cures may be, the public health issue may well be far more intractable. For instance, seeking some relief for the crowding of people and livestock in many Asian countries would require some drastic political and economic policy changes. Many of these countries would understandably find such changes politically and economically difficult, and would require much in the way of compassionate international support, both diplomatic and economic, to carry them out. In the meantime, the immediate tasks of containing the infections goes on, until such time as the underlying issues can be effectively addressed.