"Superflu" a Threat as Ducks Emerge as Stealth Carriers

Bijal P. Trivedi
for National Geographic News
November 8, 2004

The bird flu that has killed millions of poultry in east Asia may be tougher and more dangerous to humans than previously suspected.

The avian influenza virus in question is the highly pathogenic (illness causing) A virus subtype H5N1. On some poultry farms it has killed 100 percent of infected birds.

Research from St. Jude Children's Research Hospital in Memphis, Tennessee, revealed that domestic ducks infected with H5N1 shed large quantities of virus in feces as well as through the respiratory tract. The 2004 strain of H5N1 survives longer in the outside environment than those from earlier outbreaks. But most disturbing is the fact that ducks carrying the virus do not die or show any symptoms of disease.

The World Health Organization (WHO) released these results prior to publication of the St. Jude report because of their significance for public health.

"H5N1 is now circulating widely in Asia. It is becoming increasingly pathogenic and we are discovering it can infect more and more types of mammals," said Klaus Stöhr, who leads the Global Influenza Programme at the World Health Organization in Geneva, Switzerland.

In its most recent incarnation the virus has killed humans. To date there have been 44 confirmed human cases of H5N1 flu in Thailand and Vietnam. Of these, 32 people died—a fatality rate of 75 percent. Survivors suffer from severe pneumonia.

"We believe we are closer than ever to the next pandemic [widespread outbreak]," Stöhr said. Statistically, pandemics tend to occur every 10 to 27 years. It has been 36 years since the last one struck, and Stöhr thinks the next is long overdue.

At present H5N1 cannot spread easily from person to person. There has been only one recorded instance of probable human-to-human transmission, although this cannot be confirmed experimentally, because scientists lack the appropriate biological specimens.


Experts fear that the H5N1 virus could acquire genes from human influenza viruses that will transform it into a "superflu," allowing rapid spread among people. The more people H5N1 infects, the greater the chance these human and bird viruses will intermingle and produce a pandemic strain.

Scientists classify influenza viruses based on the hemagglutinin (H) and neuraminidase (N) proteins that cover the surface of every influenza virus.

There are 15 forms of H proteins that have been identified on influenza Type A viruses; all 15 viruses are found in birds. But only H1, H2, and H3 viruses circulate widely in humans.

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