Influenza A-H1N1 Virus Evolution

Since a long time ago, experts had known that influenza viruses have the capability to trade genetic components from one to the other. It seems the conditions of the current outbreak are very unique because it is caused by a new strain of influenza type A H1N1 virus which is a reassortment of several versions of influenza viruses that commonly infects a range of different species (human, avian, and swine) without any contact history with swine.
Influenza virus evolution is very dependent on swine which acts as a main reservoir for this virus. Swine also plays an important role in the ecology of human influenza viruses. H1N1 and H3N2 influenza viruses are endemic in swine populations worldwide and the disease is considered as the most hazardous respiratory disease in swine.

Epidemiology
The history of influenza A viruses started when an influenza pandemic hit Spain in 1918. Genetic studies indicate the pandemic virus, influenza type A H1N1, was very likely transmitted from swine to human, but other experts say the virus that caused the 1918 pandemic was pure an avian influenza virus that had infected human.
After that, the H1N1 influenza virus started to naturally circulate among swine populations in North America, Europe, Asia, and South America. The virus was stable for at least 60 years without showing any antigenic or genetic change. This virus was then referred to as classic H1N1.
The idea that human influenza viruses could be transmitted to swine was proven to be possible with the isolation of human-like H3N2 viruses from swine in Taiwan in 1970. Several years later, H3N2 viruses also regularly occurred in swine populations in Asia, Europe, North America, South America, and Africa. 
Then an avian-like H1N1 virus was found in swine populations in Europe in 1979. This virus had different antigenic and genetic properties from classic H1N1 viruses. All gene segments forming this virus prototype were from avian.
In 1998, H3N2 viruses containing a mixture of gene segments similar to those from human influenza viruses and classic swine viruses were isolated from swine in the United States of America. This finding indicates that gene reassortment of circulating influenza A virus in swine populations in the United States has occurred.

Virus evolution
Signs that influenza A viruses around the world are starting to rapidly evolve and create new variants every year has been seen in the last 10 years. The evolution indicates that swine are mixing vessels for the reassortment of human and avian viral genes.
From tracing H3N2 viruses isolated in 1998 to 1999 in the United States, experts found double reassortant viruses that consisted of human and swine viral genes and triple reassortant viruses that consisted of human, swine, and avian viral genes.
Introduction of H1N1 viruses from avian to swine occurred in the Southeast region of China and since 1993 this virus has been commonly found circulating in swine populations in Southeast Asia along with classic H1N1 viruses. In 2008, double reassortant H3N2 viruses containing human and avian viral genes and triple reassortant H3N2 viruses containing human, swine, and avian viral genes were found.
The development of triple reassortant viruses through genetic drifts has great implications for both humans and animals. Another triple reassortment virus was isolated from swine in the United States in 2006. This virus contained the same human viral genes as those in the human H2N2 isolate which caused the Asian influenza pandemic in 1957.

Transmission to human
Since the 1918 influenza pandemic until now, influenza A virus transmission to human could be said to be quite limited. Infection of H3N2 and H1N1 viruses in human were sporadically detected since 1950. Fifty cases of influenza A zoonoses were recorded in the United States of America (32), Czech Republic (6), Netherlands (4), Russia (3), Switzerland (3), Canada (1), and China - Hong Kong (1).
The current situation reminds experts of the 1976 incident in the United States. At that time, 200 soldiers in the Fort Dix military training center in New Jersey fell ill; 4 people suffered pneumonia and one person died. The virus identified was influenza A-H1N1, a direct descendent of the 1918 flu pandemic virus.
In 2007, a triple reassortant H3N2 virus was isolated from a 7 month old baby in Canada. Since there was no history of contact with swine, the virus was believed to have transmitted from human to human.

New zoonoses
Researchers said the development of influenza A viruses that are dramatically different from previous viruses could become a new zoonosis threat to mankind. Even though there are still many unsolved mysteries regarding the evolution process of influenza viruses, experts predict the tendency of increased transmission from human to human could potentially lead to a future pandemic.
There are a number of factors that accelerate the development of influenza viruses into a new zoonosis. First, swine often have contact with human and poultry, particularly in China, the United States, and Brazil. Second, same virus strains could infect and spread in both human and swine populations, such as the 1918 H1N1 or 1968 H3N2. Third, humans could be infected by viruses from birds, but influenza A viruses from swine are known to spread form human to human, such as in Fort Dix.
Fourth, reassortment of influenza viral genes often occurs in swine. Fifth, swine have receptors for both human and avian viruses on the surface of its cells. Sixth, a triple reassortment virus from swine has been reported to cause disease in two children in Europe.

By TRI SATYA PUTRI NAIPOSPOS (OIE Regional Coordination Unit for Southeast Asia in Bangkok)