Avian influenza affects most types of birds and often occurs in epidemics, particularly where birds are close together such as on poultry farms. It is highly contagious. The symptoms in birds can be quite variable from mild to fatal. The fatal disease, highly pathogenic avian influenza, is caused by influenza A virus types H5 and H7 and is characterized by rapid onset and rapid progression to mortality which is frequently 100%. While the disease is most often observed in domestic poultry, it is also found in migratory waterfowl which are the natural reservoir. These birds have a greater resistance to the highly pathogenic form than domestic birds to which the wild birds spread the virus.

The low pathogenic form can evolve into the highly pathogenic form when it enters a flock of domestic birds. Thus, initially, the disease is mild but later it results in high mortality. This happened in the United States in 1983 and in Italy in 1999 with a low pathogenic strain of H5N2 and H7N1 respectively. In both countries millions of birds had to be destroyed.

At the end of 2003 and the beginning of 2004, there were reports of a widespread avian flu outbreak in chickens  in Vietnam. The virus is influenza A, H5N1 which normally circulates in birds (both wild and domestic) in South-East Asia. There have also recently been outbreaks in poultry in Japan and South Korea. In Vietnam, but not so far in other countries, the virus has been transmitted to humans who came in contact with the infected birds and caused severe respiratory symptoms (initially cough, fever and sore throat and then viral pneumonia).  Twelve of the first fourteen patients in the current Vietnamese outbreak have died of the infection. A similar H5N1 avian flu outbreak occurred in Hong Kong in 1997 when eighteen persons were reported to be infected and six of them died. 

Molecular biological studies have shown that the virus isolated from infected humans is the same as that which infects birds and has not yet recombined with a human influenza virus. If this were to occur, the likelihood of human to human transmission would greatly increase. So far, it seems that all infected humans acquired the virus from birds rather than from another person.

Influenza viruses are segmented RNA viruses and have a high mutation rate because they do not proof-read their genome during replication. This leads to genetic drift. Because they have a segmented genome, they also undergo genetic shift as a result of the reassortment of segments when an infected animal harbors two types of influenza A virus. This gives rise to a virus that may have surface antigens that have not been seen in the human population recently and to which there is little immunity. This results in a highly lethal influenza pandemic. It has long been known that pigs can be infected by both human and avian influenza viruses and can give rise to such reassortants. It now appears that humans can also be doubly infected with the human and avian viruses so that reassortment occurs.

There is considerable concern about the H5N1 avian influenza that has recently infected humans because:

• it  has a high mutation rate
• it  undergoes reassortment with influenza viruses of other species
• it can cause severe (in fact, lethal) human disease
• infected birds excrete infectious virus for a week or more
• H5N1 variants can directly infect humans

This means that a reassortant of a human influenza A virus and the avian H5N1 is very likely to arise that (unlike avian flu) is easily transmitted from person to person. This may be the start of the next human influenza pandemic.

From CDC and WHO