Avian Influenza on the Move
Influenza A (H5N1) virus - also called "H5N1 virus" - is an influenza A virus subtype that occurs mainly in birds. During 2003 and 2004, outbreaks of influenza H5N1 occurred among poultry in nine countries in Asia: Cambodia, China, Indonesia, Japan, Laos, Malaysia, South Korea, Thailand, and Vietnam. At that time, more than 100 million birds in the affected countries either died from the disease or were killed in order to try to control the outbreak.
Transmission of Bird Flu
Infected birds shed flu virus in their saliva, nasal secretions, and feces. In an agricultural setting, animal manure containing influenza virus can contaminate dust and soil, causing infection when the contaminated dust is inhaled. Contaminated farm equipment, feed, cages, or shoes can carry the virus from farm to farm. The virus can also be carried on the bodies and feet of animals such as rodents. Susceptible birds become infected when they have contact with contaminated excretions or surfaces.
In February of 2004, the CDC and the US Department of Agriculture (USDA) issued an order for a ban on the import of all birds from affected areas in Southeast Asia. This ban, with some modifications, remains in effect.
Risk to Humans from the H5N1 Virus
All influenza viruses have the ability to mutate, and scientists are concerned by the fact that the H5N1 virus is now able to infect humans. At this point, it is believed that most cases of bird flu infection in humans have resulted from direct contact with infected poultry or contaminated surfaces. Human infections of influenza A (H5N1) have been reported in Thailand, Vietnam, Cambodia, and Indonesia. The death rate for these reported cases has been about 50%, which is extraordinarily high. If the H5N1 virus were able to spread between people, an influenza pandemic - a worldwide outbreak of disease - could begin.
Pandemic viruses emerge as a result of a process called "antigenic shift," which causes a sudden, major change in influenza A viruses. These changes are caused by new combinations of the proteins on the surface of the virus, resulting in a new influenza A virus subtype. The appearance of a new influenza A virus subtype is the first step toward a pandemic, but the new virus subtype also must spread easily from person to person to cause a pandemic. If humans are already ill with another flu virus at the time they are infected with H5N1, the viruses could combine in such a way that an antigenic shift could occur.
There is little preexisting natural immunity to H5N1 infection in the human population, and if H5N1 gains the ability for efficient and sustained transmission between humans, high rates of illness and death would likely result. Infectious diseases including Severe Acute Respiratory Syndrome (SARS) and West Nile virus have shown us how easily, and how quickly, an emerging virus can travel across the world.
The most recent statistics from the World Health Organization (WHO) cite confirmed human cases of avian influenza A (H5N1) in Vietnam (91 cases with 41 deaths), Thailand (17 cases with 12 deaths), Cambodia (4 cases with 4 deaths), and Indonesia (5 cases with 3 deaths), a total of 117 cases and 60 deaths reported since January 2004. In addition, there are many suspected cases that have not been confirmed as H5N1 infection.
Influenza Viruses
Influenza viruses are described by a three-part naming system that includes the virus type, subtype, and strain. There are three major types - A, B, and C - and a number of subtypes which are classified based upon the surface coatings of the virus. These surface coatings determine whether the virus will affect humans, pigs, horses or birds, or more than one type of animal.
In addition to humans, influenza A viruses cause natural infections in birds, pigs and horses. Influenza B and C types usually affect humans. "Bird flu" viruses are the flu A subtypes that continue to occur mainly in birds. Wild birds worldwide carry the viruses in their intestines without getting sick, but these viruses are very contagious and can spread to the domestic bird population - including chickens, ducks, and turkeys - causing severe illness and often death.
Pandemic
No one can predict when a pandemic might occur. However, experts from around the world are watching the H5N1 situation in Asia very closely and are preparing for the possibility that the virus may begin to spread more effectively and widely from person to person.
An influenza pandemic occurs when a new influenza A virus emerges in the human population, causes serious illness, and then spreads easily from person to person worldwide. Pandemics are different from seasonal outbreaks or "epidemics" of influenza. Seasonal outbreaks are caused by subtypes of influenza viruses that are already in existence among people, whereas pandemic outbreaks are caused by new subtypes or by subtypes that have never circulated among people or that have not circulated among people for a long time. Past influenza pandemics have led to high levels of illness, death, social disruption, and economic loss.
Once a new pandemic influenza virus emerges and spreads, it normally becomes established among people and circulates for many years as seasonal epidemics of influenza. The US Centers for Disease Control and Prevention (CDC) and the WHO have large surveillance programs to monitor and detect influenza activity around the world, including the emergence of possible pandemic strains of influenza virus.
Influenza Pandemics of the 20th Century
During the 20th century, the emergence of new influenza A virus subtypes caused three pandemics, all of which spread around the world within 1 year of being detected.
- 1918-19 "Spanish flu" [A (H1N1)]: Caused the highest number of known influenza deaths: more than 500,000 people died in the United States, and up to 50 million people might have died worldwide. Many people died within the first few days after infection, and others died of complications later. Nearly half of those who died were young, healthy adults. Influenza A (H1N1) viruses still circulate today after being introduced again into the human population in the 1970s.
- 1957-58 "Asian flu" [A (H2N2)]: Caused about 70,000 deaths in the United States. First identified in China in late February 1957, the Asian flu spread to the United States by June 1957.
- 1968-69 " Hong Kong flu" [A (H3N2)]: Caused about 34,000 deaths in the United States. This virus was first detected in Hong Kong in early 1968 and spread to the United States later that year. Influenza A (H3N2) viruses still circulate today.
All these viruses contained genes that originated in birds.
Symptoms of Bird Flu in Humans
Symptoms of bird flu in humans have ranged from typical flu-like symptoms (fever, cough, sore throat and muscle aches) to eye infections, pneumonia, severe respiratory diseases (such as acute respiratory distress), and other severe and life-threatening complications. The symptoms of bird flu might depend on which virus caused the infection.
In small studies of people infected with H5N1 in Vietnam and Thailand, symptoms included fever, shortness of breath, cough, blood-tinged sputum, diarrhea, and abnormal blood and lung function tests.
Treatment of H5N1 in Humans
The H5N1 virus currently infecting birds in Asia that has caused human illness and death is resistant to amantadine and rimantadine, two antiviral medications commonly used for influenza. Two other antiviral medications, oseltamivir (brand name Tamiflu) and zanamivir, would probably work to treat flu caused by the H5N1 virus, though studies still need to be done to prove that they work.
Based upon limited data, the CDC has suggested that oseltamivir may be effective in preventing or treating avian influenza. Using this input, the Department of State has decided to pre-position the drug Tamiflu at its embassies and consulates in the Southeast Asian region for eligible US government employees and their families serving abroad.
A specific vaccine for humans that is effective against avian influenza has not yet been developed. The US government has made deals with several vaccine manufacturers to support the production and clinical testing of an investigational vaccine based on avian influenza A (H5N1). Efforts to produce a vaccine that would be effective against this strain of H5N1 virus are under way, but mass production and availability of such a vaccine is some time off.
Assessment of the Current Situation
The avian influenza A (H5N1) outbreak among birds in Asia is not expected to diminish significantly in the short term. It is likely that H5N1 infection among birds has become endemic to the region and that human infections will continue to occur.
The World Health Organization (WHO) is carefully monitoring the ongoing avian influenza outbreaks in parts of Asia, Russia and Kazakhstan. New evidence shows that H5N1 has now entered Turkey and Romania as well. The longer this avian influenza strain continues to circulate, the greater the possibility that people will be infected with H5N1, and therefore the greater the risk that the virus will adapt to people and trigger a pandemic. The timing and severity of a flu pandemic is uncertain, but most experts predict that one will occur.
Travel
It is possible that travelers returning from affected countries in Asia could be infected. Since February 2004, medical and public health personnel have been watching closely to find any such cases. The CDC currently advises that travelers to countries in Asia with known outbreaks of influenza A (H5N1) avoid poultry farms, contact with animals in live food markets, and any surfaces that appear to be contaminated with feces from poultry or other animals.
The CDC advises travelers to clean their hands often with soap and water or waterless alcohol-based hand-rubs to help prevent disease transmission. In addition, as a precaution, all foods from poultry, including eggs, should be thoroughly cooked. CDC further advises any travelers with fever and respiratory illness who are returning from countries affected by H5N1 virus to seek prompt medical attention.
People who are planning travel to a country that has reported the virus or who are concerned about avian influenza are advised to monitor the CDC and WHO websites for the latest information.
Eileen Early, RN, BSN
HealthLink Editor
There is a great deal of current and regularly updated data available on the Internet for those who want to keep up with the progress of the Avian Influenza A (H5N1) virus. Some of the sources used in this article are listed below.
Article Created: 2005-10-13
Article Updated: 2005-10-13
Each year, Medical College of Wisconsin physicians care for more than 180,000 patients, representing nearly 500,000 patient visits. Medical College physicians practice at Children's Hospital of Wisconsin, Froedtert Memorial Lutheran Hospital, the Milwaukee VA Medical Center, and many other hospitals and clinics in Milwaukee and southeastern Wisconsin.
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