Avian flu – a second genetic subtype found in humans

Published in The Hindu on March 23, 2006

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Both the H5N1 virus subtypes still do not have the ability to spread from one human to another. Only one H5N1 subtype capable of infecting humans was present in 2003.

Even as India is managing to control the fresh outbreak of bird flu in Jalgaon, Maharashtra, scientists from the Atlanta, U.S., based Center for Disease Control (CDC) indicated the presence of H5N1 virus in two genetically distinct subtypes capable of infecting humans. The findings were presented at the recently held International Conference on Emerging Infectious Diseases in Atlanta.

It may be recalled that in 2003 there was only one genetically distinct H5N1 virus capable of infecting humans. The latest study based on samples taken from humans infected by H5N1 in 2005 has shown a second subtype capable of infecting humans. The finding was based on more than 300 H5N1 samples taken from both birds and humans from 2004 till the summer of 2005.

Avian genes

The only good news is that the scientists found the second subtype contained all genes that were of avian influenza origin. This underlines the fact that the second subtype was produced by mutation and not by re-assortment. Re-assortment can happen only when humans are infected with conventional influenza virus and the H5N1 virus at the same time and some amount of gene mixing happens between the two viruses to produce a new subtype.

A paper published in the journal Emerging Infectious Diseases in October last year brought to light the ability of the H5N1 virus to form distinct subtypes. However, both subtypes were not found to infect birds and humans alike. It was found that while one subtype was able to infect both humans and birds in Vietnam, Thailand, and Cambodia but only birds in Laos and Malaysia, the second subtype was found in viruses isolated exclusively from birds in China, Indonesia, Japan, and South Korea, the paper noted.

Now the identification of a second subtype in humans makes the possibility of keeping the virus under check more difficult. As the genetic diversity of the virus increases, the difficulty in finding ways of producing efficacious vaccines and drugs becomes greater too.

Vaccine challenges

Vaccines are developed based on the strain that is infecting humans. Now with one more subtype found, there is a need to develop another vaccine that can be effective against the new subtype has to be done. And the ability of the virus to mutate also makes vaccine development all the more difficult as one is not sure if the vaccine developed will still be relevant.

It is also possible that a vaccine developed for one subtype would be able to provide at least partial protection against the other subtype. The difficulty in developing an efficacious vaccine is just one of the critical issues. It has been found that genetic mutation by a virus gives it the ability to either develop into a type that can be easily transmitted between humans or less likely to do so.

No reason for panic

The emergence of a second subtype of the H5N1 as a result of mutation does not necessarily mean that human pandemic is round the corner; its capability to mutate cannot be taken lightly either. Whether a mutant capable of easily spreading between people will become a reality is not known. But the very ability of the virus to mutate makes could make this possible even if the chances are remote.

No conclusive evidence

It may be recalled that the virus rarely infected humans before 1997. Researchers are yet to conclusively prove that mutation has provided the virus the ability to infect humans relatively easily. Dr. Nancy Cox, Chief of the CDC’s Influenza Branch was quoted as saying to BBC that neither of the two subtypes of the avian flu had acquired the ability to be easily transmitted from one human to another.

In another development, a researcher at the University of Oxford in England has in an advance online paper published in the April issue of the Journal of Virology, shown the critical molecular factors that permit viruses to jump from one species to another. The researcher studied the rapid evolution of the B19 erythrovirus, a human parvovirus. B19 is a DNA virus unlike the avian flu virus that is a RNA virus.

According to the Howard Hughes Medical Institute press release, “… Viruses penetrate the interior of cells by binding to receptors on the host cell’s surface. The binding occurs in much the same way that a key fits into a lock. Sometimes proteins on the outer coat of a virus mutate enough to match the receptors on the cells of species other than ones that the virus usually infects.

This is what has happened with avian flu. “Once in a new species, the virus either dies out or is preserved and adds mutations that enable it to move from host to host within the new species. This can lead to widespread infection.”

More mammals infected

Already H5N1 has been found to infect other mammals excluding humans such as captive large cats such as tigers and leopards, the rare Owston palm civet and ferrets. Recently, the stone marten, a predatory nocturnal mammal with feeding habits similar to those of domestic cats was found infected with the avian flu virus in Germany.

Earlier three cats in Germany died due to H5N1 virus infection. In July last year, three Owston palm civets were found dead in captivity in Vietnam as a result of H5N1 virus infection. It has been found that during the second outbreak of avian influenza H5N1 in Thailand, probable transmission from one tiger to another was seen in a zoo.

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