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Saturday, January 24, 2004

A Bird In The Hand


Dr. Lester CN Simon

This is not a political article. The title came to mind because of the outbreak of avian influenza (bird flu), which has spread from birds to humans. It also represents a comical look at some of our sayings and nursery rhymes. For example, I never expected to see the day (or night) when the cow jumped over the moon. But cows have made the lunar leap. They have become stark raving, non-grazing lunatics, as Mad Cow Disease shows us.

The basic science of influenza tells us that there are three types of influenza viruses. Type A, type B, and type C. Wild birds are the natural habitat of influenza A virus. From wild birds it can spread to chicken, pigs, whales, horses, and seals. On the surface of the influenza A virus, there are two proteins called protein H (hemagglutinin) and protein N (neuraminidase). There are 15 types of protein H (H1, H2 ….H15) and 9 types of protein N (N1, N2…..N9). Hence there are 135 (15 x 9 = 135) subtypes of influenza A viruses. Two subtypes of influenza A found in humans are A (H1N1) and A (H3N2). Influenza B and influenza C viruses are found only in humans.

Pigs can be infected with two different subtypes of influenza A virus. In the pig, the genes responsible for making protein H and protein N can become assorted and then they can re-assort. This can result in the formation of a new virus. For example, subtypes H3N4 and H5N6 viruses in a pig can swap genes to produce a third virus subtype, H3N6.

The pigs are crucial because pigs are susceptible to infection from both avian and mammalian viruses, including human influenza subtypes. Pigs act as “mixing vessels” for the creation of new subtypes. Recent research suggests that humans can serve as “mixing vessels” just like pigs. The danger with new subtypes is that most people may have little or no protection since they were never exposed to the new subtype, either by natural infection or by vaccination. This may allow for an epidemic or a pandemic.

Over the past 100 years, there have been 3 flu pandemics. In 1918-1919 the Spanish flu was caused by influenza A (H1N1). About 20 million people died worldwide.
In 1957-1958 the Asian flu by influenza A (H2N2) caused 98,000 deaths. In 1968-1969 about 46,000 people died from the Hong Kong Influenza A (H3N2) pandemic.

The flu (influenza) and the common cold affect the respiratory system. They are caused by different viruses. The flu is worse than the common cold. People with the cold have a runny or stuffy nose. It may be difficult to tell the difference, but the flu is usually more intense, with fever, body aches, extreme tiredness and dry cough. The flu may be complicated by pneumonia and can result in hospitalization, especially in high risk groups such as the elderly and the chronically ill.

Bird flu (avian influenza) is caused by influenza type A. Bird flu runs a spectrum from mild illness to rapidly fatal disease in birds. Fifteen subtypes of influenza A infect birds. Subtypes H5 and H7 are highly contagious among birds, and are rapidly fatal.

Bird flu in wild birds can spread to domestic poultry such as chickens and turkeys. Live birds markets in Asia play an important role in spreading epidemics of bird flu.

The control of an outbreak of influenza A (H5N2) among the poultry population in USA during a 1983-1984 epidemic cost US$65 million. More than 17 millions birds had to be destroyed. More than 13 million birds died or were destroyed in Italy during the 1999-2001 bird flu epidemic caused by influenza A ( H7N1) epidemic.

Can humans get bird flu? Yes. The bird flu viruses more often infect other birds and pigs. But in 1997, bird flu caused by avian influenza A (H5N1) infected humans. This was the first documented case of transmission of avian influenza from birds to humans. It took place in Hong Kong where 6 of 18 humans infected, died.

This first case of direct transmission of influenza from avian to humans was alarming to health authorities worldwide. In Hong Kong, destruction, in 3 days, of some 1.5 million birds, the entire poultry population, averted a pandemic.

The alarm bells went off again in 2003 with an outbreak of avian influenza A (H5N1) in Hong Kong. It caused 2 human cases with 1 death. Then there was avian influenza A (H7N7) in the Netherlands in 2003 with the death of a veterinarian. Mild cases of avian influenza A (H9N2) occurred in 2 children in Hong Kong in 1999 and in 1 child in 2003.

The most recent alarm bells rang out loudly this month. First, there was an outbreak of avian influenza A (H5N1)in the poultry population in Vietnam in December 2003. Then laboratory tests confirmed the presence of avian influenza A (H4N1) in humans in Viet Nam. Children died. It is thought that they became infected by playing in yards where chickens were kept. They probably made contact with infected avian faeces, a good source of influenza A. Two birds in the bush may be better than one bird in the hand.

There is no evidence yet of direct human to human spread of the recent avian influenza A (H5N1). Nonetheless, H5N1 is of particular concern because it can mutate into subtypes rapidly. Its genes can re-assort with others easily to form new subtypes. WHO notes that H5N1 has the unique capacity to cause severe disease, with high mortality, in humans.

The relationship between humans and animals is in the spotlight as new diseases emerge. The vegetarians should not laugh. Do we know what is going into plants and vegetables for human consumption?

If we worry about meat and other cattle-derived products, pigs, chicken, turkey and other avian products, what will be left to eat? The little dog laughed to see such fun, and the dish ran away with the spoon.


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