Flu is a disease of birds caused by influenza virus type A, which may be low or high pathogenicity. Spread throughout the world, avian influenza can infect virtually all species of birds, albeit with very different manifestations, from the lightest to the highly pathogenic forms that produce epidemics and contagious acute. If caused by a highly pathogenic form, the disease appears suddenly, followed by rapid death in almost 100% of cases. The fear of a new pandemic, arising from a passage of avian viruses to humans, has sparked a series of extraordinary measures of prevention throughout the world.
A virus unstable
Nature reserves of different subtypes of avian influenza viruses are wild ducks, identified as a source of infection for poultry farming (chickens and turkeys), are particularly susceptible to the disease. In Asian countries, a prominent role in the spread of the virus been identified in the sale of live poultry markets. Moreover, viruses can be transmitted from farm to farm by mechanical means, tools and contaminated instruments, machinery, feed, cages, clothing or even operators.
Viruses of low pathogenicity can, after circulation for short periods in a poultry population, mutate into highly pathogenic viruses. For example, as reported by WHO during the 1983-1984 epidemic in the United States, H5N2 virus initially caused low mortality but became, within six months, highly pathogenic, with a mortality close to 90%. To control the epidemic, in that case, it was necessary to demolish more than 17 million birds, at a total cost of nearly $65 million.
You know at least fifteen subtypes of influenza viruses that infect birds, although all highly pathogenic influenza epidemics were caused by type A viruses of subtypes H5 and H7. H9 subtype viruses are usually of low pathogenicity. Depending on type of protein combined with the virus (from N1 to N9), the virus acquires a different name (H5N1, H7N2, etc.).
All type A influenza viruses are known for their genetic instability, because they are subject to numerous mutations during DNA replication and lack of correction mechanisms. This phenomenon, defined as “genetic drift”, it generates changes in antigenic composition of these viruses. One of the main activities of the influenza surveillance, therefore, is devoted to monitoring these changes, the basic condition for the choice of an appropriate vaccine composition. In addition, type A viruses can undergo replenishment of its genetic material, a process defined as “genetic shift”, which means that new products are viral subtypes other than parent, and therefore capable of inducing the disease in subjects who were previously vaccinated against the parental strains.
Risk of infection for humans
Since the beginning of this epidemic in parts of Southeast Asia, which was launched in 2003, WHO launched an alert to all international institutions to work together to implement plans and preventive actions to reduce the risk of passing the avian virus to humans. Essential condition for viruses that are normally housed animals become pathogenic to humans is that in the process of re-assortment acquire genes from human viruses, which make them so easily transmissible from person to person. Cases of bird flu in humans recorded in 2003 and 2004 are cases of direct transfer from infected poultry to people.
Of the 15 subtypes of avian virus, H5N1 circulating since 1997, has been identified as the most worrying because of its ability to rapidly mutate and acquire genes from viruses infecting other animal species. Birds that survive the H5N1 issued for a period of 10 days.
Since early 2003, H5N1 has carried out a series of jumps of species, acquiring the ability to infect cats and mice, thus becoming a public health issue far more worrisome. The ability of the virus to infect pigs has long been known, and then the mixing of humans, pigs and poultry is well regarded as a high risk.
In recent outbreaks, starting in 2003, has been documented ability of this virus also directly infect humans, causing acute forms of influence that in many cases led to death. The main risk, which raises fears the emergence of a new pandemic after the three that occurred during the twentieth century (1918, 1957, 1968), is that the coexistence of the avian virus with the human, in a person infected with both facilitates the recombination of H5N1 and allows us to transmit the human population.