Bird Flu (Avian Influenza)

Bird Flu (Avian Influenza) Main Page

Avian Flu (Avian Influenza)

Causes

Lying in bed with chills, a spiking fever and that run-over-by-a-truck feeling, you're not likely to care much about the habits and history of flu viruses. But epidemiologists do care — and with good reason.

Each winter, an average of 36,000 people die of influenza in the United States. And three or four times every century, a flu pandemic sweeps the globe, claiming millions of lives. That the flu can cause so much misery on both the small and the grand scale is a result of its ability to change quickly and unexpectedly, to outwit "best guess" vaccines, and to take the immune system by storm.

The ABCs of influenza viruses
All influenza viruses are divided into three types — A, B or C — depending on the virus structure. Type A is responsible for lethal influenza pandemics whereas type B causes smaller, localized outbreaks. Less common and more stable than other strains, type C has milder symptoms. Influenza B and C are usually found only in humans. But type A influenza infects both people and animals, including birds, pigs, horses, whales and seals.

Influenza A viruses are divided into subtypes based on two surface proteins:

• Hemagglutinin (HA)

• Neuraminidase (NA)

Fifteen distinct HA subtypes and nine NA subtypes exist, but they can combine to form a number of other subtypes, some of which normally are specific to a single species.

For example, subtypes H1N1, H1N2 and H1N3 usually cause influenza in humans, whereas H7N7 and H3N8 viruses cause disease in horses. At least 15 flu subtypes affect birds, the most virulent of which is H5N1. Until recently, avian subtypes have rarely been found in humans or in animals other than pigs.

Type A influenza viruses are further divided into strains, which are constantly evolving. And it is exactly this — the ability of influenza viruses to change their genetic makeup and to swap genes indiscriminately — that makes them so unpredictable and potentially deadly.

Faster than you can say antigenic
All living things change, but influenza A viruses change quickly, constantly and sometimes cataclysmically. This takes place in two ways:

• Antigenic drift. These are small, permanent, ongoing alterations in the genetic material of a virus. Because viruses aren't able to repair genetic errors that take place as they reproduce, new strains are continually replacing old ones. Once you have a particular strain of flu, you develop antibodies to it, but those antibodies won't protect you from new strains. In the same way, the flu vaccine you received last season won't ward off this year's bug.

• Antigenic shift. This occurs when influenza A subtypes from different species — a bird and a human, for example — trade and merge genes. The result is an entirely new strain, different from either of the parent viruses. Because no natural immunity to the new strain exists, it can spread quickly, causing widespread illness and death. And when one of the original subtypes is a human influenza virus, the new virus has the ability to spread easily from person to person and the potential to become a global epidemic.

How humans get bird flu
Migratory waterfowl, and ducks in particular, carry the viruses that cause bird flu. Often unaffected themselves, the host birds can spread the infection to susceptible species, especially domesticated chickens, turkeys and geese, resulting in severe epidemics that sicken and kill large numbers of birds — sometimes in a single day.

Avian viruses generally don't affect humans, but in 1997, an outbreak of bird flu in Hong Kong infected 18 people, six of whom died. Since then, human cases of bird flu have been reported in the Netherlands, Canada and throughout Asia. Most were traced to contact with infected poultry or surfaces contaminated by sick birds.

The genetic scrambling that occurs in antigenic shift explains how a disease that normally affects a bird or animal can suddenly turn up in humans. Often, flu viruses that cross the species barrier originate in areas where people live in close proximity to chickens and pigs. That's because pigs are susceptible to infection with both avian and human viruses and so are an ideal "mixing bowl" for genes.

But at least some bird flu viruses don't need a third party. Instead, they shuffle and rearrange their genetic material directly in humans. That seems to be the case in most instances of human-acquired bird flu: People become sick after direct contact with infected birds or bird-contaminated surfaces, not from contact with other animals.

Direct bird-to-human transmission works like this:

• Wild birds shed the virus. Infected migratory waterfowl, the natural carriers of bird flu viruses, shed the virus in their droppings, saliva and nasal secretions.

• The virus spreads to domesticated birds. Domestic poultry become infected from contact with these birds or with contaminated water, feed or soil. They may also catch the disease the same way humans contract conventional flu — by inhaling the airborne virus. Bird flu spreads quickly and lethally within a flock and is inadvertently transported from farm to farm on tractors and other equipment, on cages, and on workers' shoes and clothing. Heat destroys the virus, but it can survive for extended periods in cool temperatures.

• Markets provide pathways to humans. Open-air markets, where eggs and birds are often sold in crowded and unsanitary conditions, are hotbeds of infection and spread the disease into the wider community. Cock fighting, rampant throughout much of Asia, has also been implicated in the spread of bird flu — fighting roosters are often trucked long distances and smuggled across borders. At any point along the way, humans may pick up the virus through close contact with sick birds or contaminated surfaces. An ailing bird can shed the virus in its feathers as well as in droppings, and some people have contracted bird flu simply by touching an infected chicken or fighting rooster.

The ease of worldwide travel has the potential to spread bird flu around the globe, although that hasn't happened yet. Scientists don't think that migratory birds are carrying the virus from continent to continent because outbreaks haven't followed traditional flyways. Instead, outbreaks seem much more likely to spread locally through "wet markets," contaminated clothing and equipment, and smuggled birds.

(A) H5N1: Prelude to a pandemic?
Fifteen known subtypes of influenza A virus can affect birds. Although some strains are more deadly than others, even mild forms can quickly mutate into highly pathogenic types.

In the current epidemic, two influenza subtypes have proved especially dangerous — (A) H7N7, which sickened poultry workers in the Netherlands, and (A) H5N1, which has been responsible for the majority of human and avian deaths in Asia. Of these, (A) H5N1 is of particular concern for several reasons:

• Direct transmission. H5N1 became the first known bird flu strain to jump directly from birds to people when it surfaced in Hong Kong in 1997. It has since infected people in other Southeast Asian countries. Two other strains have caused illness in humans, but neither is as severe as H5N1.

• Virulence. The virus is especially lethal, killing close to 100 percent of susceptible birds and more than half of infected people. Birds who do survive can shed the virus for at least 10 days, greatly increasing the flu's spread.

• Rapid spread. Since 2003, hundreds of millions of birds have died, a loss that's ecologically and economically devastating. It's also alarming from a public health standpoint — widespread infections among birds may lead to more human disease.

• Genetic scrambling. (A) H5N1 mutates quickly and is notorious for grabbing large blocks of genetic code from viruses that infect other species, a process called reassortment. For that reason, it has particular potential to combine with a human flu virus, creating a new viral strain that spreads rapidly from person to person. The emergence of such a virus would mark the beginning of a potentially devastating pandemic.

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