Bird Flu - The Risk of a Pandemic?

Summarized by Robert W. Griffith, MD
October 24, 2005 (Reviewed: October 31, 2005)

Introduction

Everyone knows what an epidemic is, but a pandemic? It's when an epidemic becomes widespread, and affects nearly all the available population. The last major one was the 1918 influenza pandemic, which killed 20 to 40 million people worldwide.

A flu pandemic occurs when a new flu virus appears against which the human population has no immunity. With the continuous increase in global transport and communications epidemics due a new flu virus are likely to quickly take hold around the world.

Here are some points about avian, or bird flu, that we've summarized from the Centers for Disease Control and Prevention (CDC)'s website (see first link below).

What is bird flu?

Flu viruses occur naturally among birds, and are very contagious, but although wild birds are often infected they don't usually get sick; domesticated birds, like chickens and ducks, can get very sick and often die. Bird flu doesn't usually infect humans, but a number of cases have occurred in the Far East since 1997.

Bird flu viruses are influenza A subtypes found chiefly in birds. The particular bird flu that is creating the fears of a human pandemic is labeled H5N1 - the H refers to a hemagglutinin protein and the N to a neuraminidase protein on the virus surface.

How does it spread?

Infected birds shed the virus in their body fluids and feces. Most infections in humans have resulted from contact with contaminated excretions; they have mostly been seen in poultry workers.

Human-to-human spread has been reported only rarely, and so far such transmission has not been reported to continue beyond one person. However, human infections have resulted in death.

Until recently, H5N1 bird flu has been confined to many countries in Asia, but now reports of infected birds have come from Turkey and Romania. Human cases have occurred in Cambodia, Indonesia, Thailand, and Vietnam.

How is bird flu in humans treated?

The H5N1 virus is resistant to amantadine and rimantadine, two antiviral medications often used to fight flu. Two other antiviral agents, oseltamavir (Tamiflu®) and zanamavir (Relanza®) are probably effective, but there is always the likelihood that the virus can develop resistance to these drugs, too.

Otherwise, Treatment is symptomatic i.e. the prevalent symptoms - cough, fever, sore throat, muscle aches - are managed by over-the-counter drugs. A very careful watch must be kept for complications, which are likely to be more serious than those seen in human flu.

How can it be prevented?

First, people in areas experiencing outbreaks of bird flu should avoid contact with poultry farms and animal markets unless they are wearing appropriate protective clothing. People should avoid all contact with dead migratory birds or wild birds showing signs of sickness.

There is, to date, no commercially available vaccine to protect humans from the H5N1 virus, although efforts are being made in a number of countries, including the USA; safety studies of a vaccine have begun. However, drug companies may not be able to produce enough vaccine in time to combat a human pandemic, if it occurs.

How likely is a pandemic?

A pandemic can start when three conditions have been met: a new flu virus subtype emerges; it infects humans, causing serious illness; and it spreads easily and is sustainable among humans. The H5N1 virus meets the first two conditions: it is a new virus for humans and it has infected over 100 humans, killing over half of them. No one will have immunity should an H5N1-like virus emerge.

The risk remains that the H5N1 virus will acquire the ability for human-to-human transmission. There are two ways this could theoretically happen: The first is a "reassortment" event, in which genetic material is exchanged between human and avian viruses during co-infection of a human or pig; someone with 'regular' human flu gets infected with bird flu, and the viruses 'mix' their genes. To prevent this happening it's been suggested that poultry workers in affected areas get 'regular' flu shots. Reassortment could result in a fully transmissible pandemic virus, resulting in a sudden surge of cases with explosive spread.

The second mechanism is a more gradual process of adaptive genetic mutation, so that the virus binds to human cells increasingly during subsequent infections of humans. Adaptive mutation, seen first as small clusters of human cases with some evidence of human-to-human transmission, would probably give more time to take defensive action.

Will a 'regular' flu shot protect one against bird flu?

There's no reason to think that human flu vaccine can offer protection against bird flu. However, a case can be made for giving such shots to poultry workers and other people at heightened risk for bird flu, to lessen the likelihood of reassortment (see above).

There's no reason, either, to abandon the idea of a yearly flu shot. Human flu regularly kills thousands of people each year, so that a shot (or the nasal spray, FluMist®) containing the annually selected strains is recommended for the elderly, the young, the ill, and health workers.

This brief article is based on our knowledge as of mid-October 2005. The subsequent weeks and months will doubtless change the situation considerably. You can get up-to-date information from the WHO at:
www.who.int/csr/disease/avian_influenza/en/

Source

• Centers for Disease Control and Prevention (CDC): Key facts about avian flu (bird flu) and avian influenza A (H5N1) virus. www.cdc.gov/flu/avian/gen-info/facts.ht

Related Links
WHO: Avian Influenza Frequently Asked Questions
CDC: Key Facts About Bird Flu
CDC: Preventing the Flu
Cold & Flu Guidelines: Influenza

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