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Understanding Bird Flu: Avian Flu Answers

Posted on: Wednesday, 23 November 2005, 09:00 CST

- What is avian influenza?

Avian influenza, or "bird flu," is a contagious disease of animals caused by viruses that normally infect only birds and, less commonly, pigs. Avian influenza viruses are species-specific, but have, on rare occasions, crossed the species barrier to infect humans.

In domestic poultry, infection with avian influenza viruses causes two main forms of disease. The "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 flocks, affects multiple organs and is often fatal within 48 hours.

- What is special about the current outbreaks in poultry?

The current outbreaks of highly pathogenic avian influenza, which began in southeast Asia in mid-2003, are the largest and most severe on record. Never before in the history of this disease have so many countries been simultaneously affected, resulting in the loss of so many birds.

The H5N1 virus has proved to be tenacious. Despite the death or destruction of an estimated 150 million birds, the virus is now considered endemic in many parts of Asia. Control of the disease in poultry is expected to take several years.

- Which countries have been affected by outbreaks in poultry?

From December 2003 through February 2004, poultry outbreaks caused by H5N1 were reported in the Republic of Korea, Vietnam, Japan, Thailand, Cambodia, Lao People's Democratic Republic, Indonesia and China. In early August 2004, Malaysia became the ninth Asian nation to report an outbreak of H5N1 in poultry. Since then, Russia, Kazakhstan, Mongolia, Turkey and Romania have reported H5N1 outbreaks.

Japan, the Republic of Korea and Malaysia have announced control of their poultry outbreaks and are considered free of the disease.

- What are the implications for human health?

The widespread persistence of H5N1 in poultry populations poses two main risks for human health. The first is the risk of direct infection when the virus passes from poultry to humans, resulting in very severe disease. A second risk, of even greater concern, is that the virus _ if given enough opportunities _ will change into a form that is highly infectious for humans and spreads easily from person to person. Such a change could mark the start of a global outbreak, called a pandemic.

- Where have human cases occurred?

In the current outbreak, laboratory-confirmed human cases have been reported in five countries: Cambodia, China, Indonesia, Thailand and Vietnam.

Hong Kong has experienced two outbreaks in the past. In 1997, in the first recorded instance of human infection with H5N1, the virus infected 18 people and killed six of them. In early 2003, the virus caused two infections, with one death, in a Hong Kong family who had traveled to southern China.

- How are people infected?

Direct contact with infected poultry, or surfaces and objects contaminated by their feces, is presently considered the main route of human infection. To date, most human cases have occurred in rural areas where many households keep small poultry flocks, which often roam freely, sometimes entering homes or sharing outdoor areas where children play. Exposure is considered most likely during slaughter, defeathering, butchering and preparation of poultry for cooking.

- Does the virus spread easily from birds to humans?

No. Though more than 100 human cases have occurred in the current outbreak, this is a small number compared with the huge number of birds affected and the numerous associated opportunities for human exposure. It is not presently understood why some people, and not others, become infected following similar exposure.

- What about the pandemic risk?

A pandemic can start when three conditions have been met: a new influenza virus subtype emerges; it infects humans, causing serious illness; and it spreads easily and sustainably among humans. The H5N1 virus meets the first two conditions: it is a new virus for humans (H5N1 viruses have never circulated widely among people), and it has infected more than 100 humans, killing over half of them. No one will have immunity should an H5N1-like pandemic virus emerge.

But the third requirement has not been met: the establishment of efficient and sustained human-to-human transmission of the virus. The risk that the H5N1 virus will acquire this ability will persist as long as opportunities for human infections occur. These opportunities, in turn, will persist as long as the virus continues to circulate in birds, and this situation could endure for some years to come.

Source: World Health Organization

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