April 2, 2009

Scientists have report that the H5N1 bird flu viruses may be adapting to pigs, as evidences showed that bird flu viruses isolated from pigs in Indonesia were less harmful to mice than were H5N1 viruses from chickens.

 

In the study, scientists from Japan and Indonesia collected viruses from chickens and pigs in Indonesia, grew them in laboratory cell cultures, and used them to infect mice, in which they found that the viruses from pigs were less lethal to mice than the viruses from chickens, according to their recent report in the Archives of Virology.

 

The report states that swine isolates were less virulent to mice than avian isolates, suggesting that the viruses weakened during their replication in pigs.

 

The report also noted that pigs are seen as a possible intermediate host that can help bird flu viruses adapt to humans, because the epithelial cells in pigs' trachea can be infected by both avian and human flu viruses.

 

According to authors from Airlangga University in Surabaya, Indonesia, the finding suggests that the virus may have become less harmful to mammals in general.

 

The authors reported that bird flu viruses that infect pigs are believed capable of adapting to them - gaining the capability to grow efficiently in swine cells - and thereby adapting to other mammals.

 

So far, H5N1 infections in pigs have been reported rarely or gone unnoticed because infected pigs show no signs of illness.

 

However, the authors wrote that they found H5N1 infections in pigs in Indonesia in 2005, 2006, and 2007.

 

They determined that the swine viruses were closely related to viruses in chickens found nearby, indicating H5N1 spread from chickens to pigs at least three different times.

 

They gathered three viruses from pigs and two from chickens on East Java in 2006 and 2007. They first determined that all the viruses grew well in embryonated eggs and in cultures of canine kidney cells, demonstrating that both avian and swine strains could grow in mammalian cell cultures. They then infected groups of mice with a range of doses of the five isolates.

 

The results showed that all three pig viruses were less virulent in mice than the chicken viruses were, as measured by how large a dose it took to kill half of the mice. Two of the pig isolates were "strongly weakened" in mice.

 

The authors interpreted of their findings that the decrease of pathogenicity in mice suggests that the H5N1 viruses may have lost their pathogenicity in mammals during replication in pigs.

 

They added that as H5N1 infections in swine increase the risk that a pandemic strain could emerge, the findings point up the need for continuous surveillance and management of H5N1 viruses in pigs.

 

On the other hand, virologist Richard Webby said the findings may mean that H5N1 viruses from swine will be less virulent in mammals generally, but it is not clear that the viruses have truly adapted to swine.

 

Webby said that in the authors' study, the two least lethal viruses were both from swine, but one swine isolate was lethal, which could mean that if these become adapted to mammals, they are potentially going to be less pathogenic.

 

He said that whether H5N1 viruses become more or less virulent when they adapt to mammals is a very important question, adding that the findings "might be to some extent reassuring."

 

He also said US Department of Agriculture researchers have infected pigs with H5N1 viruses and found that the viruses did not grow at all, but the researchers used viruses that did not come from pigs in the first place.

 

He noted that pigs often are fed broken eggs or even chicken carcasses, and such pigs might carry the virus in their snouts without becoming truly infected.