The latest outbreak of the bird virus flu in the US has been reported from a highly biosecure Iowa egg farm housing 3.8 million birds, all of which will be slaughtered to control the spread.

Besides the H5N8, the new strain seen in the US - H5N2, is suspected to be a mutant descended from H5N8 plaguing poultry in Europe and Asia for years and possibly carried in by migrating ducks or geese.

The two strains have seen the extermination of more than seven million birds since late last year.

Most avian influenza viruses do not infect humans; however certain strains are infectious to humans. These include - H5N1, H7N3, H7N7, H7N9 and H9N2.

The most common type is the avian influenza sub-type H5N1 viruses, currently circulating in poultry in parts of Asia and northeast Africa.

Birds infected in India and Africa

Cases have been recently identified in Niger in Africa (following Nigeria last year) and Telangana in India. The latter saw over 160,000 birds being culled last week and over 200,000 eggs from poultry farms destroyed.

Moreover, it has also been confirmed that the virus has spread to poultry in the north eastern Indian state of Manipur.

The US strains are different from the H5N1 bird flu virus that has spread from birds to humans in the past, said an official with the US Centers for Disease Control and Prevention.

Spread to humans

The spread of the virus to humans is associated with direct or indirect contact with infected live or dead poultry but there has been no sustained human-to-human spread of H5N1 so far.

Any new flu virus in birds can mutate to become easily transmissible from human to human. "At this point we don't know very much about these viruses," said CDC officer Alicia Fry at a press conference.

CDC will monitor any person who has been exposed to the virus in the United States. At least 100 people have been monitored so far.

There have been nearly 650 cases of H5N1 human infections, reported from 15 different countries, since 2003, according to the US Department of Health and Human Services.

Germany, the Netherlands and the United Kingdom had first identified strains of H5N8 strains in outbreaks of avian influenza last year.

Deadly strain

Several outbreaks of the deadly strain H7N9, which can infect humans have been reported in China. The H7N9 strain of avian virus swaps genes with other types of flu viruses, giving rise to new strains. Any of the new strains could start a pandemic.

As chickens exhibit no outward signs of being infected by the H7N9 virus, it is difficult to monitor and control poultry populations. The strain is believed to be fast becoming endemic in the region.

Once it crosses borders like the H5 strains, it will be difficult to check its global spread.

The World Health Organization is particularly concerned about avian influenza given the unprecedented diversity and geographical distribution of the viruses currently circulating. The next pandemic could be worse than the 2009 swine flu outbreak that killed over 284,000, WHO has warned.