Avian Influenza Viruses
The natural hosts for avian influenza (influenza A) viruses are certain species of wild birds, usually shorebirds and waterfowl. In natural hosts the virus exists in the gastrointestinal and/or respiratory tract. Infected wild birds disperse the virus through defecation into bodies of water in which the virus can survive for several weeks. Most often wild birds will be affected by low pathogenic avian influenza (LPAI) whose presentation is asymptomatic or mildly symptomatic (e.g., ruffled feathers, drop in egg production, etc.).
Normally, avian influenza viruses do not infect people, so there is little to no immunity to these viruses in the human population. However, three influenza A virus hemagglutinin (HA, which may be shortened to H) subtypes can in fact infect both birds and people: H5, H7, and H9. Beginning in 1997, a H5N1 strain began infecting humans in Hong Kong, and thence spread to Europe and Africa. Since 2003, human H5N1 cases have been documented in Azerbaijan, Cambodia, China, Djibouti, Egypt, Indonesia, Iraq, Thailand, Turkey, and Vietnam. This H5N1 causes highly pathogenic avian influenza (HPAI), which is capable of causing high mortality rates among domestic birds and the rare infected human. Because the HPAI H5N1 is so exceedingly virulent, it is causing worldwide concern.
When a human is infected with the H5N1 strain, he will present with symptoms that are similar to human influenza, including fever, cough, sore throat, muscle aches, and eye infections. However, in over 50% of human cases, symptoms become worse, leading to pneumonia, severe respiratory distress, and other life-threatening complications. More than half of the people infected with the H5N1 virus have died.
The H5N1 virus is not capable of person-to-person spread at this time. In most of the known human H5N1 cases, the virus is believed to have been spread through direct exposure to infected poultry (e.g., domesticated chickens, ducks, and turkeys). However, in 2005, two Vietnamese individuals may have contracted HPAI caused by the H5N1 virus by consuming uncooked duck blood.
Presently, there is no vaccine for H5N1, although researchers throughout the world working on this problem. Scientists are also attempting to increase the present national vaccine production capacity, expand the existing supply, and develop new types of flu vaccines.
Besides vaccines, certain prescription medicines may be useful for alleviating the symptoms of avian influenza in humans. Further, researchers believe that antiviral medications, such as oseltamavir (Tamiflu) and zanamavir (Relenza), help to protect humans from avian influenza (i.e., prevention of illness and/or disease treatment). Unfortunately, due the influenza viruses' high rate of mutation, they can develop resistance against antiviral medications within a short time. For instance, the H5N1 virus is already resistant to amantadine and rimantadine, two antiviral medications commonly used to treat influenza.
The H5N1 virus, which now is endemic to parts of Southeast Asia, is also of significant global concern because of the high economic losses that result when domestic birds become infected with it. In Vietnam, one of the most affected countries, it is estimated that about 44 million birds, or 17% of the total poultry population, were killed in order to limit the spread of the H5N1 virus. The World Bank estimated that the cost to Vietnam's poultry industry was US $120 million (0.3% of GDP). In Indonesia, a survey conducted by the Food and Agriculture Organization indicates that more than 20% of the permanent industrial and commercial farm workers lost their jobs due to avian flu-related impacts. As of today, Cambodia, Indonesia, Laos, and Thailand are the countries that have suffered the most damage from the H5N1 influenza virus. The World Bank estimates that by mid-2005, avian flu had killed more than 140 million birds and caused losses valued at over US $10 billion.
The economic costs of H5N1 have mostly been related to domestic bird deaths, the culling of domestic birds to prevent disease spread, and the costs to governments for avian flu preparedness and prevention. Typically, countries shoulder the burden of economic losses solely, but the benefits from destroying the infected birds are felt at an international scale. Many argue that the international community should contribute aid to help offset the financial costs associated with avian influenza. No one country can protect itself against an influenza pandemic, but the actions one country takes to mitigate its spread can have important implications for the global community.