Sunday, May 07, 2006

How do low pathogenic avian viruses become highly pathogenic?

"Since 1997, more than 16 outbreaks of H5 and H7 influenza have occurred among poultry in the United States. Highly pathogenic strains can cause 90 to 100 percent deaths in poultry." Source - Institute of Science in Society (ISIS)

Recent studies into the spread of the highly pathogenic avian H5N1 virus have produced some startling conclusions...

For example: After testing hundreds of thousands of wild birds for the disease, scientists have only rarely identified live birds carrying the highly pathogenic H5N1. In a test of 13,000 wild birds in marshes within the bird flu infested provinces of China, the H5N1 virus was found in only 6 ducks.

They weren't prepared to draw solid conclusions, though: "Our data show that H5N1 influenza virus, has continued to spread from its established source in southern China to other regions through transport of poultry and bird migration."

According to ISIS: The reality is that nearly all the wild birds that have tested positive for the H5N1 disease were dead, and in most cases, found near to outbreaks in domestic poultry.

The World Health Organization (WHO) recently said that viruses from Turkey's first two human cases were virtually identical to those that killed 6 000 migratory birds in a nature reserve, Qinghai, in central China last year. But RSPB's conservation director Dr. Mark Avery insists that the trade in wild birds and the movement of poultry and poultry products, such as chicken manure used to fertilize fish farms, has led directly to the transfer of H5N1 across national boundaries.

"No species migrates from Qinghai, China, west to Eastern Europe," BirdLife's Dr. Richard Thomas said. "When plotted, the pattern of outbreaks follows major road and rail routes, not flyways."

About Avian Flus

Wild birds are the natural host for all known subtypes of influenza A viruses, according to ISIS. In wild birds and poultry throughout the world, influenza A viruses representing 16 HA and 9 NA subtypes have been detected in numerous combinations, such as H1N1, H3N3, H16N3 and so on. Typically wild birds do not become sick when infected.

Domestic poultry such as turkeys and chickens can become very sick and die from avian influenza, and some avian influenza A viruses also can cause serious disease and death in wild birds.

Avian influenza viruses are designated as low pathenogenic (LPAI) when they do not cause disease or only mild disease, and highly pathogenic (HPAI) when they do. The HA protein is synthesized as a single polypeptide precursor, which is cleaved into HA 1 and HA 2 subunits by proteases.

The switch from low to highly pathogenic avian virus is not fully understood, however... the switch appears to be associated with basic amino acid residues introduced into the HA cleavage site, which makes the protein easier to cleave and facilitates virus replication.

Influenza viruses evolve by small point mutations (antigenic drift) or large changes due to reassortment (antigenic shift), the mixing of genome segments from different viruses.

Ducks, geese, swans, gulls, terns and waders are the major LPAI virus reservoir, where the virus preferentially infects cells lining the intestinal tract and is excreted in high concentrations in their faeces. Influenza viruses remain infectious in lake water for up to 4 days at 22C, and for more than 30 days at 0C. Faecal to oral transmission is the most frequent route for transmitting viruses, both of high and low pathogenicity.

So, how do low pathogenic avian viruses become highly pathogenic?

The influenza A virus genome is in eight separate segments. The segmented genome allows the viruses from different species to mix and exchange segments to create new influenza viruses.

A pig infected with a human virus and a bird virus at the same time would allow the two viruses to exchange segments to create a new virus that retained most of the genes of the human virus but had the avian haemagglutinin and/or neuraminidase gene(s). The resulting new virus might be able to infect humans and spread from person to person.

If this new virus causes serious illness in humans, then a pandemic would result. This gene exchange could even take place in a human infected at the same time with human and avian flu viruses.

And that's my point...

Workers on the Jasper hog factory farm are leaving the building every single day -- but do they really know if they carry any avian virus, be it from one of the 2500 hogs, or from the tens of thousands of migratory birds that visit this region every year?

Picture it: One worker gets sick from one of the sick hogs while working inside. The worker showers, then leaves the building to go home. There's a bit of bird faeces on his truck window. He wipes it off, stuffing the rag back in his truck. He feels a sneeze coming on, grabs his hanky from his pocket....

The shower-in/shower-out policy may be effective in other areas of the world, but right next to a game reserve teeming with wildlife -- directly in the center of a massive wild bird habitat area -- am I the only person who sees any potential danger here?

And what happens when they start poking that untreated hog manure into the ground for all those birds and other wildlife to forage on...??

It may take years for anything to happen as drastic as a mutated virus. Or it may take only a few more months down the road. It kind of feels like living near a ticking time bomb, in my humble opinion.

Read more about avian flu and the science here.


Best Bird Flu Blogs team. said...

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Thank you for this and we hope that we will have many further contributions from you.

Best Bird Flu Blogs team.

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