Why the Bird Flu's So Dangerous- SF Chronicle

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Why the bird flu's so dangerous

Close watch on strain that could mutate into deadly human virus

Sabin , Chronicle Medical Writer

Monday, October 17, 2005

Like the biggest lout in a clan of bad neighbors, the H5N1 bird flu

virus is just the worst of a large family of avian influenzas that

regularly threaten poultry farms and sometimes menace human health.

The bird flu that has been making headlines in recent months --

spanning the globe from Indonesia to Romania -- has been a grave

concern for scientists since 1997, when it decimated chicken flocks

in Hong Kong and killed six people.

While the discovery that bird flu could jump directly to humans is

relatively new, avian influenzas have been troublesome for

agriculture at least since 1878, when Italian poultry were beset with

a disease labeled fowl plague.

Because of the way influenza viruses readily mutate and juggle genes,

there are now 144 possible subtypes of influenzas that can infect

birds. Out of this variety, a handful of combinations bearing names

such has H5N1, H5N2, H7N7 and H7N3 regularly pose a serious risk to

poultry.

Researchers fear that H5N1 could mutate into a form that transmits

easily among humans, triggering a worldwide pandemic that that could

kill tens of millions.

" We may sound like we are hyperventilating, but in our heart of

hearts we know this is a serious possibility,'' said Dr.

Pavia, director of the Task Force on Pandemic Influenza for the

Infectious Diseases Society of America.

Although the H5N1 virus does not pass easily between humans, at least

116 people have contracted the virus -- nearly all from close contact

with infected birds -- and 60 have died.

The 1918 Spanish flu, which caused 50 million deaths, appears to have

been a bird flu that became a human virus with just a few key

mutations. Pandemic flus in 1957 and 1968, which caused serious human

illness, were launched when human strains picked up genes from bird

strains.

Although the stakes have obviously grown higher, for decades bird flu

monitoring efforts were aimed primarily at protecting commercial

poultry flocks.

" Any avian influenza is bad news for a poultry house,'' said Hon Ip,

director of the National Wildlife Center virology lab in Madison,

Wis. " But not all of them cause the same degree of worry.''

Most avian influenzas -- such as those in wild birds -- cause few if

any symptoms. But around the world, more serious, " highly pathogenic "

strains emerge regularly.

Bird-killing viruses visited chicken flocks in the United States

twice in the 1920s, and in 1983 caused a disastrous outbreak in

Pennsylvania poultry resulting in the culling of 17 million birds.

Today an international network of biologists, affiliated with

governments and the United Nations, monitors poultry flocks and wild

birds for signs of flu. Since 1959, the worldwide networks have

recorded at least 22 outbreaks of highly pathogenic avian flu --

avian influenza capable of wiping out chicken, geese or turkey

flocks.

Dennis Senne, a microbiologist for the National Veterinary Services

Laboratories, in Ames, Iowa, said the federal avian flu monitoring

program has been screening 1.3 million birds annually for antibody

traces of exposure to avian influenza. The program is expanding.

" We feel we look pretty hard,'' he said. " Soon we will be approaching

3 million tests per year.''

Discoveries of deadly bird flus are relatively rare, compared to the

dozens of different strains of low pathogenic influenzas biologists

detect each year.

Ominously, bird flus that appear at first to be low pathogenic can

evolve suddenly into a more deadly strain. That is apparently what

happened in Pennsylvania in 1983, when a mild avian strain known as

H5N2 suddenly turned highly contagious and lethal to birds.

UC avian flu expert Dr. Carol Cardona noted that although a

strain of flu may be mild for birds, it could pose a threat to

humans. The only flu strains known to readily infect humans -- from

families of bird flu known as H1, H2 and H3 -- are not highly lethal

to birds.

Most of the deadliest bird flus come from families known as H5 and

H7. " When we see an H5 or H7, even if it is a low pathogenic, we jump

in with both feet,'' Cardona said.

In flocks where infection is found, all birds are killed to prevent

the spread of the virus to other farms. It is a strategy that has

kept bird flu largely under control in the United States.

Cardona believes the failure to cull flocks infected with H5N1 in

Asia has allowed the virus to morph into a particularly persistent

strain. " The thing that makes this guy in Asia so bad is that it has

been there so long. It is evolving, changing, looking for new

hosts,'' she said.

H5N1 comes from a line of viruses that started killing birds in China

in 1996. But H5N1 viruses have caused deadly outbreaks in birds as

far back as 1959, when one turned up in Scotland. Another H5N1 virus

killed birds in England in 1991.

H5N1 viruses have also been detected in the United States -- but

these identically named strains were low pathogenic and apparently

caused no illness in birds or humans. The virus most recently left

its footprint in Michigan in 2001, when turkeys routinely tested for

avian flu at a processing plant tested positive for antibodies to an

H5N1 strain.

According to microbiologist Senne, the virus itself was never found,

and the infected birds showed no signs of illness. Nor was there any

sickness at the farm they came from. No trace of the virus has been

seen since in Michigan.

Living samples of an apparently harmless H5N1 virus were also found

in 1986 in ducks killed by hunters at a marsh in northern Ohio. Every

year at the start of hunting season, Dr. Slemons of Ohio

State University obtains samples from the bottoms of freshly killed

waterfowl. " I ask the hunters if I can get a swab. They look at me

like, 'Boy, are you weird,' '' he said.

The strain he found is called A/Mallard/Ohio/184/1986 (H5N1). " Just

because it is an H5 doesn't mean that it's a problem,'' he said.

University of Minnesota veterinarian Dr. Halvorson isolated two

harmless strains of H5N1 in " sentinel ducks " in 1981 and 1985. The

ducks are stocked at Minnesota lakes and regularly tested for bird

flu. Avian influenzas pose a threat to the state's turkey industry --

one of the largest in the world.

Aside from their name, what these H5N1 viruses have in common with

the modern lethal cousins is a similar structure of spikes on their

outer surfaces. The " H " refers to the hemagglutinin protein.

Scientists have found 16 different types of H proteins in the avian

influenza family, identified by a succession of numbers such as H1,

H2, up to H16.

Similarly, the " N " refers to a second type of surface protein,

neuraminidase, of which there are nine varieties, and nine

identifiers such as N1, N2, up to N9.

Flu virus names, resembling license plates, are therefore mixtures of

H's, N's and numbers -- such as H5N1, H7N3, H9N2, or H7N7 -- that

tell scientists what kinds of proteins appear on the outer surface of

each virus.

One of the great mysteries in flu research is to find out what subtle

changes can turn a relatively harmless H5 strain into a killer. It

could involve mutation on the protein itself, proteins within the

core of the virus, or a combination of factors.

Halvorson said that the H5N1 strains found in Minnesota may bear some

resemblance to the dreaded strain on their outer surface, but are

almost certainly different in other respects. Biologists have noted

that avian flus in Asia are different from those in the United

States, suggesting that global transfer of bird flu genes is rare.

The good news for poultry farmers in America is that even low

pathogenic strains seldom infect their birds. " Over 99 percent of our

flocks are free of any influenza virus,'' Halvorson said.

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A sampling of avian influenza viruses

H9N2

Hong Kong, 2003**

H5N1

Hong Kong 1997*

H5N2

Pennsylvania, 1983

H7N2

Virginia, 2002

H5N8

Ireland, 1983

H7N4

Australia, 1997

*The feared bird flu spreading from Asia to Europe that has killed

more than 60 people.

**A mild bird flu that has also sickened a small number of people but

could become a pandemic strain.

What's in a virus name

Flu virus names are based on the types of hemagglutinin and

neuraminidase proteins on the viral surface. There are 144 possible

varieties of bird flu. Here are 10 examples of avian flu outbreaks

since 1966.

H5N9: Hemagglutinin protein (16 variations), Neuraminidase protein (9

variations)

Source: Chronicle research

http://sfgate.com/cgi-bin/article.cgi?

f=/c/a/2005/10/17/MNGRHF9FID1.DTL & hw=flu & sn=001 & sc=1000