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What is bird-flu?

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 highly 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, distinguished by low and high extremes of virulence. The so-called “low pathogenic” form commonly causes only mild symptoms (ruffled feathers, a drop in egg production) and may easily go undetected. The highly pathogenic form is far more dramatic. It spreads very rapidly through poultry flocks, causes disease affecting multiple internal organs, and has a mortality that can approach 100%, often within 48 hours.

Influenza A viruses1 has 16 H subtypes and 9 N subtypes2. Only viruses of the H5 and H7 subtypes are known to cause the highly pathogenic form of the disease. However, not all viruses of the H5 and H7 subtypes are highly pathogenic and not all will cause severe disease in poultry.

On present understanding, H5 and H7 viruses are introduced to poultry flocks in their low pathogenic form. When allowed to circulate in poultry populations, the viruses can mutate, usually within a few months, into the highly pathogenic form. This is why the presence of an H5 or H7 virus in poultry is always cause for concern, even when the initial signs of infection are mild.

Origins of bird-flu

In 1878, in Italy, a serious disease affected chickens, causing high mortality. Decades later, after more progress in the field of pathology, it was determined that the origin of the fowl plague had been an influenza virus, more specifically a type A influenza virus, as it is the only one to affect domestic animals.

Avian flu was not to be associated with human disease again until World War I. The origin of the Spanish flu was traced back to a mutated form of the highly pathogenic H5N1 virus. It is estimated between 50 to 100 million people worldwide died following the pandemic which the unusually powerful strain of the virus provoked in 1918 and 1919. The virus found perfect conditions of infecting millions across the continent fast, as the close quarters and mass movement of soldiers during the war quickened the spread. Although the origin of bird flu could not be pinpointed to the Iberian peninsula, the name stuck as the illness received more press attention in Spain. It was a very aggressive type of bird flu,

affecting healthy individuals and leaving them too feeble to walk withing hours of the infection. Late 20th century research on biological samples taken from the victims of the disease discovered it was caused by a slightly altered form of the bird flu virus, that easily jumped from infected poultry to humans. The origin was unclear, but one theory indicated the United States as the initial hotbed.

Another group of researchers claim the bird flu origin can be found in Scotland. In 1959 a poultry farm in Aberdeen was confronted with a bird flu disease, which was later identified and isolated as the highly pathogenic virus which has claimed lives in Southeast Asia. However, its effects seem to be very mild in comparison to the ones the entire world is faced with right now. Scottish specialists name other flu outbreaks that have affected England in 1963, 1979 and 1991, to show the virus was very different to the current one bearing the same title. Also, unlike the latter, the 1959 Scottish virus proved to be incapable of moving from its origin in fowl to other species.

The latest bird flu outbreak to have sprung concern worldwide had its origins in Hong Kong, where in 1997, where 18 people contracted the illness and 6 eventually died. A large number of poultry was culled in an attempt to eliminate the virus, but a few years later it has reappeared and is spreading rapidly to other continents and rising the human death toll to hundreds.

How is it transmitted?

Infected birds pass on H5N1 through their saliva, nasal secretions, and faeces. Other birds may pick up the virus through direct contact with these excretions or when they have contact with surfaces contaminated with this material. Because migratory birds are among the carriers of the H5N1 virus it may spread to all parts of the world. Past outbreaks of avian flu have often originated in crowded conditions in southeast and east Asia, where humans, pigs, and poultry live in close quarters. In these conditions a virus is more likely to mutate into a form that more easily infects humans. H5N1 infections in humans are generally caused by bird to human transmission of the virus.

Until May 2006, the WHO estimate of the number of human to human transmission had been "two or three cases". On May 24, 2006, Dr. Julie L. Gerberding, director of the United States Centers for Disease Control and Prevention in Atlanta, estimated that there had been "at least three." On May 30, Maria Cheng, a WHO spokeswoman, said there were "probably about half a dozen," but that no one "has got a solid number." A few isolated cases of suspected human to human transmission exist with the latest such case in June 2006 (among members of a family in Sumatra). No pandemic strain of H5N1 has yet been found. The key point is that, at present, "the virus is not spreading efficiently or sustainably among humans."

There is also concern, although no definitive proof, that other animals — particularly cats — may be able to act as a bridge between birds and humans. So far several cats have been confirmed to have died from H5N1 and the fact that cats have regular close contact with both birds and humans means monitoring of H5N1 in cats will need to continue.

What are the various symptoms of bird-flu?

Symptoms of avian flu infection in humans depend on the particular strain of virus. In case of the H5N1 virus, infection in humans causes more classic flu-like symptoms, which might include: