Exposure of poultry to migratory waterfowl, which are the natural reservoirs of the virus, is the major point of transmission of the disease.

Infected birds shed virus in saliva, nasal secretions and feces. Avian influenza viruses spread among susceptible birds when they come in contact with contaminated nasal, respiratory and fecal material from infected birds. Fecal-to-oral transmission is the most common mode of spread. Aerosol transmissions can also occur. Transmission also takes place through mechanical vectors like rodents and garbage flies.

 It is estimated that 1 gram of feces contains enough virus to infect 1 million birds. International movement of poultry, poultry equipment, and people pose risks for introducing avian influenza into a country. Once introduced, the disease can be spread from bird to bird by direct contact. AI viruses can also spread through contaminated manure, equipment, vehicles, egg flats, crates, and people whose clothing or shoes have come in contact with the virus. The virus remains viable at moderate temperatures for long periods in the environment and can survive indefinitely in frozen material. In feces it can survive up to 150 days at 25°C.

Potential for an Influenza Pandemic

All influenza viruses are capable of mutation. Influenza virus can change its surface antigens very easily. This is due to “lack of genetic proof reading ability” of the virus. So the tiny errors occurring during replication becomes permanent changes in the genetic make up of the virus. This “antigenic drift” leads to the formation of a mutant strain of the virus. So also “antigenic shift “or gene swapping property of the virus can produce another mutant strain. This gene swapping usually causes a pandemic. As the virus is a new strain, human immune system will not have any pre existing immunity and the vaccines will not work against this strain.

So far 3 pandemics of avian influenza have occurred in the world. “Spanish Flu” in 1918 with H1N1 strain wiped out about 40-50 million people from the surface of earth and the “Asian flu” with H2N2 strain in 1957 killed about 2 million people. The last pandemic of avian influenza in 1968 with H3N2 strain was milder with death of approximately 1 million people.  

Once the fully contagious virus emerges, global spread of the disease is inevitable and the supply of vaccines and antiviral drugs will be inadequate especially in developing countries. So a sensitive surveillance system for the rapid detection of the pandemic virus is extremely important. WHO has issued a series of strategic actions for 6 distinct phases of pandemic threat. The situation we are facing is phase 3 – A virus new to humans is causing infections but does not spread easily from one person to another. This may result in the production of a strain that is infectious to humans. If this virus attains the capacity to spread among humans, a pandemic may result. There is little or no immune protection against them in the human population, as these viruses do not commonly infect humans.

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