dcsimg

published on 1 December 2005 in life

H5N1

H5N1 torpedoed
The term pandemic indicates a disease that affects an enormous number of persons worldwide. Much is being said these days, about the avian flu, let us try to learn more about it.
H5N1 seems a navy battle order, instead it is the name of the virus that provokes the avian flu. This disease is a severe infection that affects wild and domestic animals and is caused by type A influenza virus.  The onset of the disease may be mild or very severe, in the latter case the animal invariably dies in a few days.
H5N1 has a horrific appearance : H5 and N1 are two pointy proteins  that  protrude from the capsid (for further information, refer to the Special Report on “virus”)  making the shape of  the  virus look  like a medieval steel club.
Who is affected
Till today, the principal victims of this virus have been chickens : in Asia over a hundred million died because of the disease or because of the prevention measures carried out by man. Among birds the symptoms of the disease vary according to the species. Chickens  fall  very sick, they seem weak and listless and die rapidly, while ducks with this disease  do not show any precise symptoms, and look perfectly healthy. Among birds, the virus spreads through saliva, their droppings  and organic liquids.
Why is the world afraid?
So why worry so much about a disease that affects chickens? The answer lies in a particular characteristic of the virus  that provokes this disease, H5N1, which  like the others in its category, can also attack whales, pigs, dolphins, horses and man. It has been verified that the pathogenic agent that provoked the three pandemics in the last century was a typical animal virus.
Changing species
According to some scientists, H5N1 is not the fruit of a combination of different strains of virus, but a simple animal virus that in some ways is “trying” to spread to human beings. .
Perhaps this virus is changing rapidly but casually and it might become really dangerous if it manages to transfer from man to man and not only from chicken to man.
For the time being, fortunately, its attempts have not been very effective.

Old and new virus
A step back in the past : the case of the “Spanish flu”  

In 1918,  the last year of the terrible battles of the First World War, many people died for reasons that had nothing to do with weapons. The cause was a flu like disease that affected the lungs and led to death in just a few days. Notwithstanding its name, the disease did not come from Spain. Actually  due to the fact that Spain at the time was a neutral country, and  therefore did not censor information, news of the disease spread from there even if the virus was present in most of Europe. The Spanish flu killed a total of 50 million people worldwide.
Scientists have studied  extensively in order to understand the causes of such a large human catastrophe In 1996 the pathologist Jeffery Taubenberger isolated fragments of a virus in the lung tissue of a soldier who had died in September 1918. He discovered something that confirmed what had been suspected for a long time : the virus of the Spanish flu came from some unknown animal, and  because it had changed, it had succeeded in  attacking man.
A new  virus
The study of the Spanish flu undoubtedly shows that an influenza virus that affects animals is more dangerous than a typical human virus.  Let us try to understand why. Common influenza affects millions of people around the world every year, but it never has such lethal results. It is provoked by viruses that have accompanied man for a long time and that our organism has learnt to recognize and defeat. In other words the virus of the flu that keeps us home from school or work for a few days every year, is  known by our immune system (for further information, read  the special report on viruses). We catch influenza more or less every year because the viruses change rapidly, in other words they change their characteristics slightly so as to cheat our immune system for  a few days, but they never change so radically that we are caught unprepared to face them.
However, if a virus that usually attacks animals does succeed in infecting man, this would be an absolute novelty for the defences of our  immune system which would not have the time to learn to recognize it and set up a suitable defence system.  The immune system’s delay in triggering  into action would give the virus all the time to provoke serious damages in the organism.

Where does it come from?
The first case of infection  by the avian influenza virus H5N1in man, dates back to 1997 in Hong Kong. In 2004,  other infections were reported in Vietnam and in Thailand, and in only few weeks the infection spread to Japan, South Korea, China and Indonesia. Why did this virus first appear in those areas? Poultry is a very important food for the populations in the Far East. Every year millions of hens, ducks and geese are bred there. It is estimated that in the Chinese province of Guangdong alone, where the virus probably developed,   hundreds of millions of chickens are bred. The conditions of overpopulation of the animals and the hot and humid climate of those areas facilitated the evolution of new strains of the virus. Furthermore, these are areas that are densely populated by human beings who live in close contact with the poultry they breed. If there had been only chicken farms, then perhaps the “avian” problem would not exist, or better it would only concern the plumed animals. Instead, in those areas there are also large pig farms which are very often bred along with the poultry.
The  “pig” hypothesis
The role of pigs in the development of avian influenza was perhaps a determining factor. A virus that attacks birds specifically cannot infect man, unless there is a change in its small genetic heritage . This type of change can take place as a consequence of an exchange of genetic material between  different strains (the technical term to indicate the “families”) of viruses. In other words two different viruses can come into contact and exchange some gene thus giving origin to a new viral form with intermediate characteristics between the two original viruses.
The mixing of viruses does not occur habitually because viruses do not reproduce by coupling as in the case of animals, but they enter an organism and use the cell synthesis systems to create replicas of themselves (for a more detailed description, read the Special Report on Viruses).
And it is in the cells of a host organism that the mixing of viruses may take place. For this to happen,  the host cells must be vulnerable to the attack of two different viruses . In fact, pigs can be affected by both  avian and human viruses. Probably the avian and human viruses mixed in the cells of a pig thus creating the hybrid that is able to attack man.
Spreading the virus
Fortunately only few cases of avian flu have been reported in man. In October 2005,  the total number of cases worldwide was 117, with 60 deaths, distributed in some areas of Indonesia, Vietnam, Thailand and Cambodia.
After spreading to almost the whole of Asia,  the virus started to move westwards. In summer it reached Russia and in autumn some wild animals were reported to be infected in the delta of the Danube in Rumania. Now checks are being carried out on some suspect cases  noted in poultry farms in Greece and Turkey. How does the virus travel and how does it spread so far? It is simple, it flies, following the migratory routes. Also wild birds, in fact, can be infected by the influenza, but unlike their domestic cousins they move, and fly freely from one country to another, often covering large distances.
For example the birds that migrate from Siberia, where H5N1 has recently been identified, can bring the virus to the areas around the Caspian Sea and the Black Sea. From these areas, through the Balkans, the epidemic could spread to the whole of Europe.
The virus could also pass directly from the wild fauna to man through game animals. For this reason in some areas in North West Germany, hunting  migratory animals was  prohibited for most of last autumn. .
Man can also be responsible for the diffusion of viruses, for example, by transporting infected animals or products.

What to do to defend ourselves
Fortunately the potential risk of a pandemic is well known by governments worldwide. Since the first appearance of this disease, a number of measures have been taken in order to prevent and fight the virus. Unfortunately the more drastic form, to limit the diffusion of the disease is to cull the infected animals, or those at risk, because they are living in areas  in which the diffusion of the virus is strong.  In the Asian countries where the virus appeared, hundreds of millions of birds were culled. These are always difficult and painful operations, because  the concerned areas are often very poor and  for the local farmers and breeders, the chickens represent an essential source of revenue. The governments therefore have undertaken to indemnify those who are forced to cull their poultry and to carry out  vast health checks on animals and humans. FAO and OIE (World Organization for Animal Health), for example, have developed a programme that involves an investment of 100 million dollars to survey, diagnose and control the disease. Knowledge is the best form of prevention. The constant study and collection of data are essential, as always, to know and unveil the mysteries of this virus and to prepare the necessary defences. One thing is certain, the disease can be caught by direct contact with the infected animals, while there is no risk involved in eating eggs or meat that are well cooked, because heat kills the virus.
Is there a cure?
A vaccine does not exist, because the virus that may provoke the pandemic is still unknown. In fact,  a virus that is able to specifically affect human beings,  fortunately,  still has not been encountered. Vaccines for common flu are not efficacious against the avian flu, however they may be very useful as they would prevent a simultaneous attack of the human virus and the virus of animal origin.

written by Andrea Bellati

Sources
OMS (World Health Organization)
FAO (Food and Agriculture Organization of the United Nations)
National Geographic – numero di ottobre 2005

With the sponsorship of the Italian Ministry of Education, Universities and Research
 
Eni S.p.A. - P.IVA 00905811006