Frequently Asked Questions
Based on information from the World Health Organization
- What is avian influenza?
- Which viruses cause highly pathogenic disease?
- Do migratory birds spread the disease?
- What is special about the current outbreaks in poultry?
- Which countries have been affected by outbreaks in poultry?
- What are the implications for human health?
- Where have human cases occurred?
- How do people become infected?
- Is it safe to eat poultry and poultry products?
- Does the virus spread easily from birds to humans?
- What about the pandemic risk?
- What changes are needed for H5N1 to become a pandemic virus?
- What is the significance of limited human-to-human transmission?
- How serious is the current pandemic risk?
- Are there other causes for concern?
- Why are pandemics such dreaded events?
- What are the most important warning signals that a pandemic is about to start?
- What is the status of vaccine development and production?
- What drugs are available for treatment?
- Can a pandemic be prevented?
- What strategic actions are recommended by the World Health Organization?
- Is the world prepared?
Avian influenza, or “bird flu”, is a contagious disease caused by viruses that normally infect only birds and, less commonly, pigs. On rare occasions, avian influenzas have crossed the species barrier to infect humans.
In domestic poultry, avian influenza causes two main forms of disease. The so-called “low pathogenic” form commonly causes mild symptoms (ruffled feathers, a drop in egg production) and may go undetected. The highly pathogenic form is far more dramatic. It spreads rapidly through poultry flocks and has a mortality that can approach 100%, often within 48 hours.
Which viruses cause highly pathogenic disease?
Influenza A viruses have 16 H subtypes and 9 N subtypes. Only H5 and H7 subtypes are known to cause the highly pathogenic form of the disease. Fortunately, not all viruses of the H5 and H7 subtypes are highly pathogenic and not all will cause severe disease in poultry.
Normally, H5 and H7 viruses are introduced to poultry flocks in their low pathogenic form. When allowed to spread among domestic poultry, the viruses can mutate, usually within a few months, into the highly pathogenic form. This is why H5 or H7 virus in poultry is always a concern, even when the initial signs of disease are mild.
Do migratory birds spread highly pathogenic avian influenza viruses?
There are many unanswered questions regarding the role of migratory birds in spreading highly pathogenic avian flu. Wild waterfowl have probably carried influenza A viruses, with no apparent harm, for centuries. They are known to carry viruses of the H5 and H7 subtypes, but usually of the low pathogenic form. Considerable evidence suggest that migratory birds can introduce to poultry low pathogenic H5 and H7 viruses, which then mutate to the highly pathogenic form.
In the past, highly pathogenic viruses have been isolated from a few number of migratory birds, usually found dead near a poultry outbreak. Recent events make it likely that some migratory birds are now spreading the H5N1 virus in its highly pathogenic form.
What is special about the current outbreaks in poultry?
The current outbreaks of highly pathogenic avian influenza, which began in South-East Asia in 2003, are the largest and most severe on record. Never before have so many countries been affected at the same time, resulting in the loss of so many birds.
The H5N1 virus has proved to be especially menacing. Despite the death or destruction of an estimated 150 million birds, the virus is considered endemic in many parts of Indonesia and Viet Nam and in some parts of Cambodia, China, Thailand, and possibly also the Lao People’s Democratic Republic. Control of the disease in domestic poultry is expected to take many years.
The H5N1 virus is also of particular concern for human health, as explained below.
Which countries have been affected by outbreaks in poultry?
From December 2003 through early February 2004, poultry outbreaks caused by the H5N1 virus were reported in eight Asian nations (listed in order of reporting): the Republic of Korea, Viet Nam, Japan, Thailand, Cambodia, Lao People’s Democratic Republic, Indonesia, and China. Most of these countries had never before experienced an outbreak of highly pathogenic avian influenza.
In early August 2004, Malaysia reported its first outbreak of H5N1 in poultry, becoming the ninth Asian nation affected. Russia reported its first H5N1 outbreak in poultry in late July 2005, followed by reports in adjacent parts of Kazakhstan in early August. Deaths of wild birds from highly pathogenic H5N1 were reported in both countries. About the same time, Mongolia reported the detection of H5N1 in dead migratory birds. In October 2005, H5N1 was confirmed in poultry in Turkey and Romania.
Japan, the Republic of Korea, and Malaysia have announced control of their poultry outbreaks, and are now considered disease free. In other areas, outbreaks are continuing with varying degrees of severity.
What are the implications for human health?
The widespread persistence of H5N1 in poultry populations poses two main risks for human health.
1.) Direct infection when the virus passes from poultry to humans results in very severe disease. Of the avian influenza viruses that have crossed the species barrier to infect humans, H5N1 has caused the largest number of cases of severe disease and death in humans. Unlike normal seasonal influenza, where infection causes only mild respiratory symptoms in most people, the disease caused by H5N1 leads to rapid deterioration and high mortality. Primary viral pneumonia and multi-organ failure are common. In the present outbreak, more than half of the people infected with the virus have died. Most cases have occurred in previously healthy children and young adults.
2.) Even greater concern is that the virus – given enough opportunities – may change into a form highly infectious for humans and spreads easily from person to person. Should this happen, it could mark the start of a global outbreak (a pandemic).
Where have human cases occurred?
During the current outbreak, laboratory-confirmed human cases have been reported in four countries: Cambodia, Indonesia, Thailand, and Viet Nam.
Hong Kong has experienced two outbreaks in the past. In 1997, with the first recorded instance of human infection with H5N1, the virus infected 18 people and killed 6. In early 2003, the virus caused two infections, with one death, in a Hong Kong family with a recent travel history to southern China.
How do people become infected?
Direct contact with infected poultry, or surfaces and objects contaminated by their feces, is considered the main route of human infection. Most human cases have occurred in rural areas where households keep small poultry flocks, which roam about freely, sometimes entering homes or sharing outdoor areas where children play. Opportunities for exposure to infected droppings or to environments contaminated by the virus are abundant under conditions where infected birds shed large quanities of virus particles in their feces. Because many Asian households depend on poultry for food and income families may sell or slaughter and consume birds when disease symptoms appear in a flock, a practice that has proved difficult to change. Exposure to disease is considered most likely during slaughter, defeathering, butchering, and preparation of poultry for cooking.
Is it safe to eat poultry and poultry products?
Yes, though certain precautions should be followed. In areas free of the disease, poultry and poultry products can be prepared and consumed as usual (following good hygienic practices and proper cooking), with no fear of acquiring infection with the H5N1 virus.
Where the disease is prevalant, poultry and poultry products can also be safely consumed provided items are properly cooked and properly handled during food preparation. Normal temperatures used for cooking (70oC in all parts of the food) will kill the H5N1 virus. Consumers should know that all parts of the poultry are fully cooked (no “pink” parts) and that eggs, too, are properly cooked (no “runny” yolks).
Consumers should also be aware of cross-contamination. During food preparation, juices from raw poultry and poultry products should never be allowed to touch or mix with items eaten raw. When handling raw poultry or raw poultry products, people should wash hands thoroughly and clean and disinfect surfaces in contact with the poultry products Soap and hot water are sufficient for this purpose.
Raw eggs should not be used in foods that will not be further heat-treated as, for example by cooking or baking.
Avian influenza is not transmitted through cooked food. No evidence indicates that anyone has become infected following the consumption of properly cooked poultry or poultry products, even when these foods were contaminated with the H5N1 virus.
Does the virus spread easily from birds to humans?
No. Though more than 100 human cases have occurred, this is a small number compared with the huge number of birds affected and the numerous opportunities for human exposure, especially in areas where backyard flocks are common. It is unclear why some people, and not others, become infected following similar exposures.
A pandemic can start when three conditions have been met: a) a new influenza virus subtype emerges; b) it infects humans, causing serious illness; and c) it spreads easily and sustainably among humans. The H5N1 virus meets the first two conditions: it is a new virus for humans, and it has infected more than 100 humans, killing over half of them. No one will have immunity should an H5N1-like pandemic virus erupt.
All prerequisites for a pandemic have been met except one: the establishment of efficient and sustained human-to-human transmission of the virus. The risk that the H5N1 virus will acquire this ability will remain as long as opportunities for human infections occur. These opportunities will persist as long as the virus continues to circulate in birds, a situation that could endure for some years to come.
What changes are needed for H5N1 to become a pandemic virus?
The virus may spread among humans via two principal mechanisms. A “reassortment” event may evolve, in which genetic material is exchanged between human and avian viruses during co-infection of a human or pig. Reassortment could result in a full-blown pandemic virus, with a sudden number of rapidly spreading disease cases.
The second mechanism is a more gradual process of adaptive mutation, whereby the virus's ability to bind to human cells evolves during subsequent infections of humans. Adaptive mutation, starting among small clusters of human cases with evidence of human-to-human transmission, would probably give the world time to take positive action.
What is the significance of limited human-to-human transmission?
Instances of limited human-to-human transmission of H5N1 and other avian influenza viruses in association with outbreaks in poultry should not be a cause for alarm. In no cases has the virus spread beyond close contacts or caused illness in the general community. Data suggest that transmission requires very close contact with an ill person. Such incidents must be thoroughly investigated but – provided the investigation indicates that transmission from person to person is very limited – such incidents will not change the World Health organization's overall assessment of the pandemic risk. A small number of instances of avian influenza infection occurred among close family members. It may be impossible to determine if human-to-human transmission has occurred since the family members are exposed to the same sick animal and environmental sources as well as to each another.
How serious is the current pandemic risk?
The risk of pandemic influenza is serious. With the H5N1 virus now firmly established in large parts of Asia, the risk of more human cases will persist. Each additional human case gives the virus an opportunity to mutate, and thus develop into a pandemic strain. The spread of the virus to poultry and wild birds in new areas creates additional opportunities for human cases to occur. While nobody can predict the timing and severity of an outbreak, the probability that a pandemic will occur has increased over time.
Are there other causes for concern?
• Domestic ducks can excrete large quantities of highly pathogenic virus without showing signs of illness, thus acting as a “silent” reservoir of the virus, perpetuating transmission to other birds. Under these circumstances,it becomes harder to warn people to avoid risky agricultural practices.
• Compared with H5N1 viruses from 1997 and early 2004, H5N1 viruses now circulating are more lethal to experimentally infected mice and to ferrets (a mammalian model) and survive longer in the environment.
• H5N1 appears to be infecting and killing mammalian species previously considered resistant to infection with avian influenza viruses.
• The behaviour of the virus in wild waterfowl, may be changing. The spring 2005 die-off of approximately 6,000 migratory birds at a nature reserve in central China, caused by highly pathogenic H5N1, was highly unusual and unprecedented. In the past, only two large die-offs in migratory birds, caused by highly pathogenic viruses, are known to have happened: South Africa in 1961 (H5N3) and Hong Kong in the winter of 2002–2003 (H5N1).
Why are pandemics such dreaded events?
Influenza pandemics can rapidly infect virtually all countries at the same time.. Once the virus takes hold, pandemics are considered unstoppable, because they are caused by a virus that spreads rapidly by coughing or sneezing. Infected people can shed virus particles before symptoms appear, adding to the risk of spreading disease via asymptomatic air travellers.
The severity of disease and the number of deaths caused by a pandemic virus vary widely, and cannot be predicted. During past pandemics, rates reached 25-35% of the total population. Under the best circumstances, assuming that the virus causes mild disease, the world could still experience 2 million to 7.4 million deaths (projected from data obtained during the 1957 pandemic). Projections for a more virulent disease are much higher. The 1918 pandemic killed at least 40 million people. In the United States, the mortality rate during that pandemic was around 2.5%.
Pandemics can cause large numbers of people requiring or seeking treatment,overwhelming health services. High rates of absenteeism can also interrupt essential services, such as law enforcement, transportation, and communications. Because people will be fully susceptible to an H5N1-like virus, disease rates could peak rapidly in the community. Local social and economic disruptions may be temporary; however, problems may multiply in today’s era of globalization. Based on past experience, a second wave of global spread should be anticipated within a year.
All countries are likely to suffer emergency conditions during a pandemic, with opportunities for international assistance, at the same time governments focus on protecting people within their borders.
What are the most important warning signals that a pandemic is about to start?
The most important warning signal happens when clusters of patients with clinical symptoms of influenza, closely related in time and place, are detected, because this suggests human-to-human transmission is taking place. The detection of cases in health workers caring for H5N1 patients would also suggest human-to-human transmission. Detection of such events should be followed by immediate field investigation of every possible case to confirm the diagnosis, identify the source, and determine whether human-to-human transmission is taking place.
Studies of viruses, conducted by specialized World Health Organization laboratories, can corroborate field investigations by spotting genetic and other changes in the virus indicative of an improved ability to infect humans. WHO repeatedly asks affected countries to share samples of viruses with the international research community.
What is the status of vaccine development and production?
Vaccines against a pandemic virus are not yet available. Vaccines produced each year for seasonal influenza will not protect against pandemic influenza. Although a vaccine against the H5N1 virus is under development, no vaccine is ready for commercial production and no vaccines are expected to be widely available until several months after the pandemic takes hold.
Clinical trials are now under way to test experimental vaccines. Because the vaccine needs to closely match the pandemic virus, large-scale commercial production will not take place until the new virus has emerged and a pandemic has been declared. Current global production capacity falls short of demand expected during a pandemic.
What drugs are available for treatment?
Two drugs (certain neuraminidase inhibitors), oseltamivir (commercially known as Tamiflu) and zanamivir (commercially known as Relenza) can reduce the severity and duration of illness caused by seasonal influenza. The efficacy of these drugs depends on early administration (within 48 hours after symptom onset). For cases of human infection with H5N1, the drugs may improve survival rates, if administered early, but clinical data are limited. The H5N1 virus is predicted to be susceptible to the neuraminidase inhibitors.
Older versions of antiviral drugs, the M2 inhibitors amantadine and rimantadine, could potentially be used against pandemic influenza, but resistance to these drugs can develop rapidly, thereby limiting their effectiveness against pandemic influenza. Some H5N1 strains are fully resistant to these M2 inhibitors. However, should a new virus emerge through reassortment, the M2 inhibitors might prove to be effective.
For the neuraminidase inhibitors, main constraints involve a limited production capacity and a price that is prohibitively expensive for many countries. At present manufacturing capacity, it will take a decade to produce enough oseltamivir to treat 20% of the world’s population.
So far, most fatal pneumonia in cases of H5N1 infection has resulted from the virus, and cannot be treated with antibiotics. However, since influenza is often complicated by secondary bacterial infection of the lungs, antibiotics could be life-saving. The World Health Organization regards it as prudent for countries to ensure adequate supplies of antibiotics in advance.
No one knows. The best way to prevent a pandemic would be to eliminate the virus from birds, but it has become increasingly doubtful if this can be achieved.
If there are enough antiviral medications, sufficient, say, for 3 million people, the drugs could be used prophylactically near the beginning of a pandemic to reduce the risk that a fully transmissible virus will emerge, or at least to delay its international spread, giving the world time to augment vaccine supplies.
Success of this strategy depends on several assumptions about the early behaviour of a pandemic virus, which cannot be understood in advance. Success also depends on excellent surveillance and rapid response in the initially affected areas, combined with an ability to enforce movement restrictions in and out of the affected area. To increase the likelihood that early intervention using the WHO-supplied stockpile of antiviral drugs will be a success, surveillance in affected countries needs to improve, particularly concerning the capacity to detect clusters of cases closely related in time and place.
What strategic actions are recommended by the World Health Organization?
In 2005, WHO sent all countries a document outlining recommended strategic actions for responding to the avian influenza pandemic threat. Recommended actions aim to strengthen preparedness, reduce opportunities for a pandemic virus to emerge, improve the early warning system, delay initial international spread, and accelerate vaccine development.
No. Despite advanced warning of two years, the world is ill-prepared to defend itself during a pandemic. The World Health Organization has urged all countries to develop emergency preparedness plans, but only 40 or so have done so. WHO has further urged countries with adequate resources to stockpile antiviral drugs for use at the start of a pandemic. About 30 countries are purchasing large quantities of these drugs, but the manufacturer has no capacity to fill these orders immediately. If present trends continue, most developing countries will have no access to vaccines and antiviral drugs throughout the duration of a pandemic.