|
Flus deadly numbers game
A new pandemic looming, says Ian Sample and it's not just humans at risk
The Guardian
29th January 2004
Please see listed pdfs
Characterization of Two Influenza A Viruses from a Pilot Whale - HINSHAW V.S. et al
Journal of Virology p. 655-656, May 1986 - pdf - 359KB
A Review of Potential Infectious Disease Threats to Southern Resident
Killer Whales - Gaydos J.K., Balcomb K.C. et al - pdf - 174KB
The nets being pegged up over chicken farms in Singapore are not to stop the birds making a break for freedom. The worry is what lurks outside the enclosures. Farmers fear wild birds could land among their chickens, and spread the latest strain of avian flu to throw Asia into turmoil. The virus has already forced South Korea and Vietnam to order mass cullings of domestic birds in the hope of thwarting the spread of the deadly infection. And for good reason. According to the World Health Organisation, the epidemic of avian flu is unprecedented, having ripped through bird populations in at least nine countries, leaving millions dead.
Health officials are getting edgy. Avian flu, they say, could do more than wipe out countless chickens. It could trigger the next "big one" - a human flu pandemic that spreads around the world, killing huge numbers of people, as in 1918 when flu claimed a reported 40-50m. Is such a doomsday scenario likely or, like Sars, is avian flu little more than a good scare story?
So far, the virus has ticked two of the three boxes that most concern health officials. First, it has shown it can jump from birds to humans. Second, it is highly dangerous when it does so - of at least eight people in Vietnam to catch avian flu, six have died. The virus has yet to tick the third box: to date, there are no confirmed cases of the flu being spread from person to person. If that happens, the chances of a pandemic rocket.
For bird flu to become a real threat, it has to undergo chance mutations that make it thrive in humans. As the name suggests, influenza viruses are originally found in wild birds, mostly ducks and shore birds. These viruses are so well adapted to birds, having co-evolved with them over millions of years, that they are not typically able to survive in other animals.
Occasionally, a mutation allows the virus to take hold in another species. The annual flu epidemics that strain health services are thanks to viruses that jumped into humans years ago. Influenza has also jumped from birds into pigs, horses, seals and whales. In birds, the organism causes a gastro- intestinal infection, so the disease is usually spread when water is contaminated with infected faeces.
Flu virus can mutate in two ways, one slow track, one fast track, bringing about changes in how it behaves. Slow track mutations happen every time the virus infects a cell, copies itself and churns out masses of new virus. "In effect, every single virus released from a cell will have at least one mutation," says Laurence Tiley, a molecular virologist at Cambridge University. One genetic change will unlikely be enough to make a bird virus a big threat to humans. Any number of mutations might be needed for the virus to take hold in humans, helping it better infect cells, replicate inside them, or evade the immune system.
But virologists say if enough people become infected, it is a matter of time before the virus mutates into a strain that not only can infect people, but also be spread from person to person. "Every time somebody gets infected, the virus is going to mutate and the more people get infected with it, the more chance there is of that happening," says Tiley. "It's just a numbers game."
There's no guarantee that chance mutations alone will lead to a strain of flu that could tear through the human population. It's not the only way the virus can mutate, however.
The big concern now is fast-track mutation: that a person carrying a human flu virus also catches avian flu. If that happens, the viruses can trade genes leading, in the worst case, to a lethal and highly infectious virus, which no one has immunity to. "This really is the fear today, that the avian flu virus will get into people who already have the human flu virus," says Francois Meslin of the World Health Organisation.
Mixing avian and human flu viruses - a process called "antigenic shift" - to produce a particularly nasty version can happen without both viruses getting into humans. Pigs can catch flu from humans and birds, making them effective mixing pots.
One difficulty is understanding what genetic changes could make avian flu virus a global killer. The virus could need genes to make it replicate more rapidly, so you cough or sneeze more virus out. It could need genes that make you sneeze harder or more often. "It's really the $1bn question," says Jacqueline Katz of the US Centres for Disease Control (CDC) in Atlanta. "We really don't understand what would make it transmissible between humans."
Teams from the CDC and the WHO are trying to isolate viruses from patients and birds in Asia in the hope of finding a treatment. For now, though, the first line of defence is a blunt tool - simply slaughtering birds so there are none around to catch the virus. But slaughter is needed on a huge scale. "The spread of the virus is unprecedented. The extent to which slaughtering is necessary this time is mind boggling. We're talking millions and millions of birds," says Katz.
Health officials are faced with few alternatives. Tried and tested quarantine measures that contained the Sars virus are likely to be insufficient against a highly infectious influenza virus. A vaccine will be painfully slow to manufacture. Antiviral drugs are expensive and in short supply. Health officials will take few chances.
The animals which also suffer
Chickens Avian flu in chickens comes and goes with depressing regularity, but the 1997 outbreak in Hong Kong was different. For the first time, influenza virus from chickens was found to jump directly into humans, dashing theories that the avian virus could only infect humans if it first jumped into another animal, such as a pig or a turkey. Although the number of people who caught the disease was just 18, the virus killed six of them. Last year, while attention was focused on Sars, the Netherlands was fighting to contain an outbreak of avian flu. More than 11m chickens were slaughtered and many workers caught eye infections from the virus.
Horse Influenza was discovered in horses in 1956, and since then, two distinct strains of the virus have been identified. Both cause similar symptoms - a dry, hacking cough, muscle soreness and loss of appetite - and the infection is often compounded by pneumonia. So-called equine 1 influenza has not been found in horses for more than 15 years and may have completely died out, but the second virus strain remains and periodically causes widespread infections. In 1989, there was a severe outbreak of horse flu in Jilin, northern China, which killed many of those infected. Equine flu remains one of the most common respiratory tract infections of horses stabled at racetracks.
Pig Virologists call pigs "mixing vessels" because they can catch flu from humans as well as birds. If a pig harbours both at once, the viruses can swap genetic material, producing potentially more dangerous viruses. Some scientists suspect that at least two of the flu pandemics of the past century, in 1957 and 1968, were caused by viruses that were part avian, part human. They could have been created in pigs infected with both viruses at once. Influenza was first seen in pigs during the 1918 flu pandemic that killed 40-50m people around the world. The virus makes the animals cough, although they often show signs of depression, develop a fever and a runny nose, too.
Whale In 1986, scientists discovered a pilot whale with influenza. It was thought to have picked up the virus from contaminated water. The disease makes whales waste away and have difficulty moving around. There are no known cases of humans catching the flu. "The chances are pretty remote. Whales don't suffer from it a lot and we don't go around having whales breathe all over us," says Laurence Tiley, a virologist at Cambridge University.
Further information
Characterization of Two Influenza A Viruses from a Pilot Whale - HINSHAW V.S. et al
Journal of Virology p. 655-656, May 1986 - pdf - 359KB
A Review of Potential Infectious Disease Threats to Southern Resident
Killer Whales - Gaydos J.K., Balcomb K.C. et al - pdf - 174KB
Top
|
