Outbreaks of highly pathogenic influenza A (H5N1) occurred among poultry in eight countries in Asia (Cambodia, China, Indonesia, Japan, Laos, South Korea, Thailand and Vietnam) during late 2003 and early 2004. At that time, more than 100 million birds either died from the disease or were culled.
From December 30, 2003, to March 17, 2004, there were 12 human cases of confirmed H5N1 influenza in Thailand and 23 in Vietnam, including 23 deaths. No conclusive evidence of sustained human-to-human transmission was found.
On February 4, 2004, CDC and USDA issued an order for a ban on the import of all birds (Class: Aves) from affected areas in Southeast Asia.
Avian influenza viruses circulate among birds worldwide. Certain birds, particularly water birds, act as hosts for influenza viruses by carrying the virus in their intestines and shedding it. Infected birds shed virus in saliva, nasal secretions, and feces. Susceptible birds can become infected with avian influenza virus when they have contact with contaminated nasal, respiratory, or fecal material from infected birds. Fecal-to-oral transmission is the most common mode of spread between birds.
Most often, the wild birds that are host to the virus do not get sick, but they can spread influenza to other birds. Infection with certain avian influenza A viruses (for example, some H5 and H7 strains) can cause widespread disease and death among some species of domesticated birds.
Domesticated birds may become infected with avian influenza virus through direct contact with infected waterfowl or other infected poultry, or through contact with surfaces (such as dirt or cages) or materials (such as water or feed) that have been contaminated with virus. People, vehicles, and other inanimate objects such as cages can be vectors for the spread of influenza virus from one farm to another. When this happens, avian influenza outbreaks can occur among poultry.
Avian influenza outbreaks among poultry occur worldwide from time to time. Since 1997, for example, more than 16 outbreaks of H5 and H7 influenza have occurred among poultry in the United States. The U.S. Department of Agriculture monitors these outbreaks.
Low pathogenic forms of avian influenza viruses are responsible for most avian influenza outbreaks in poultry. Such outbreaks usually result in either no illness or mild illness (e.g., chickens producing fewer or no eggs), or low levels of mortality.
When highly pathogenic influenza H5 or H7 viruses cause outbreaks, between 90% and 100% of poultry can die from infection. Animal health officials carefully monitor avian influenza outbreaks in domestic birds for several reasons:
- the potential for low pathogenic H5 and H7 viruses to evolve into highly pathogenic forms.
- the potential for rapid spread and significant illness and death among poultry during outbreaks of highly pathogenic avian influenza.
- the economic impact and trade restrictions from a highly pathogenic avian influenza outbreak.
- the possibility that avian influenza could be transmitted to humans.
When avian influenza outbreaks occur in poultry, quarantine and depopulation (or culling) and surveillance around affected flocks is the preferred control and eradication option.
Influenza A viruses are found in many different animals, including ducks, chickens, pigs, whales, horses, and seals. However, certain subtypes of influenza A virus are specific to certain species, except for birds which are hosts to all subtypes of influenza A. Subtypes that have caused widespread illness in people either in the past or the current period are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2 subtypes have caused outbreaks in pigs and H7N7 and H3N8 viruses have caused outbreaks in horses.
Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, up until 1998, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs.
Avian influenza viruses may be transmitted to humans in two main ways:
- Directly from birds or from avian virus-contaminated environments to people.
- Through an intermediate host, such as a pig.
Influenza viruses have eight separate gene segments. The segmented genome allows viruses from different species to mix and create a new influenza A virus if viruses from two different species infect the same person or animal. For example, if a pig were infected with a human influenza virus and an avian influenza virus at the same time, the viruses could reassort and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus might then be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans.
This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur.
It also is possible that the process of reassortment could occur in a human. For example, a person could be infected with avian influenza and a human strain of influenza at the same time. These viruses could reassort to create a new virus that had a hemagglutinin from the avian virus and other genes from the human virus. Theoretically, influenza A viruses with a hemagglutinin against which humans have little or no immunity that have reassorted with a human influenza virus are more likely to result in sustained human-to-human transmission and pandemic influenza. Thus, careful evaluation of influenza viruses recovered from humans who are infected with avian influenza is very important to identify reassortment if it occurs.
While it is unusual for people to get influenza infections directly from animals, sporadic human infections and outbreaks caused by certain avian influenza A viruses and pig influenza viruses have been reported. (For more information see Avian Influenza Infections in Humans.) These sporadic human infections and outbreaks, however, rarely result in sustained transmission among humans.
Although avian influenza A viruses do not usually infect humans, several instances of human infections and outbreaks of avian influenza have been reported since 1997. Most cases of avian influenza infection in humans are thought to have resulted from contact with infected poultry or contaminated surfaces. However, there is still a lot to learn about how different subtypes and strains of avian influenza virus might affect humans. For example, it is not known how the distinction between low pathogenic and highly pathogenic strains might impact the health risk to humans. Of the documented cases of human infection with avian influenza viruses, illnesses caused by highly pathogenic viruses appear to be more severe.
A family slaughter chickens for food in Northern Vietnam.
The United States formally sounded the alarm over its own outbreak of bird flu as Vietnam said it had reversed an earlier decision to kill all poultry within a three-kilometre radius of a bird flu outbreak.
Because of concerns about the potential for more widespread infection in the human population, public health authorities closely monitor outbreaks of human illness associated with avian influenza. To date, human infections with avian influenza viruses detected since 1997 have not resulted in sustained human-to-human transmission. However, because influenza viruses have the potential to change and gain the ability to spread easily between people, monitoring for human infection and person-to-person transmission is important.
Four different influenza antiviral drugs (amantadine, rimantadine, oseltamivir, and zanamivir) are approved by the U.S. Food and Drug Administration (FDA) for the treatment and/or prophylaxis of influenza. All four have activity against influenza A viruses. However, sometimes influenza strains can become resistant to these drugs, and therefore the drugs may not always be effective. For example, analyses of some of the 2004 H5N1 viruses isolated from poultry and humans in Asia have shown that the viruses are resistant to two of the medications (amantadine and rimantadine). Monitoring of avian viruses for resistance to influenza antiviral medications is ongoing.