Medical Information

• What is Influenza?
• What is Pandemic Influenza?
• Prevention Strategies
• Vaccines
• Antiviral Drugs

What is Influenza?

Influenza is a highly contagious viral disease of the respiratory tract. The disease is caused by a group of closely related subtypes of virus, and different subtypes infect different animal species. It derives its public health significance from the rapidity by which epidemics evolve and spread amongst the community, and associated widespread morbidity and serious complications such as viral or bacterial pneumonia which may be fatal.

The influenza virus contains two surface proteins, haemagglutinin (H) and neuraminidase (N), which are involved in the infection of the host and production of new virus. The H protein is involved in attaching the virus to the cells it infects, while the N protein assists the virus in detaching from the cell in which it is produced. For influenza A, 16 distinct forms of H have been identified (designated H1 to H16) and 9 distinct forms of the N (designated N1 to N9). Influenza A viruses have infected many different animals, including ducks, chickens, pigs, whales, horses, and seals. However, certain subtypes of influenza A virus are specific to certain species, except for birds, which are hosts to all known subtypes of influenza A. Subtypes that have caused widespread illness in people either in the past or currently are H3N2, H2N2, H1N1, and H1N2. H1N1 and H3N2 subtypes also have caused outbreaks in pigs, and H7N7 and H3N8 viruses have caused outbreaks in horses.

Influenza A viruses normally seen in one species sometimes can cross over and cause illness in another species. For example, until 1998, only H1N1 viruses circulated widely in the U.S. pig population. However, in 1998, H3N2 viruses from humans were introduced into the pig population and caused widespread disease among pigs. Most recently, H3N8 viruses from horses have crossed over and caused outbreaks in dogs.

For the influenza B virus, only one H and one N have been identified.

 

Epidemics of influenza

The influenza virus has very high mutation rates and the surface antigens are prone to variation. Small mutations give rise to 'antigenic drift' which results in the emergence of new strains of influenza A and B. Essentially this involves small changes to the viruses that are already circulating around the world. Every year, this process results in widespread epidemics, usually between late autumn and early spring. During these seasonal epidemics, attack rates usually depend upon age, reflecting whether the person has been exposed to the circulating strain before.

 

Current Status of Avian Influenza

Avian influenza is an infectious disease of birds caused by the influenza A virus. Although some species are more resistant to infection than others, all birds are believed to be susceptible to avian influenza viruses. Infection can cause a wide variety of symptoms in birds, from mild illness to a rapidly fatal disease resulting in severe epidemics. The latter is known as 'highly pathogenic avian influenza'. Sixteen subtypes of influenza virus are known to infect birds, but to date all outbreaks of the highly pathogenic form have been caused by the subtypes H5 and H7.

The natural reservoir of avian influenza viruses are migratory waterfowl, most notably wild ducks, and these birds are also the most resistant to disease. Domestic poultry, including chickens and turkeys, are particularly susceptible. Domestic flocks coming into contact with wild migratory waterfowl have been implicated as a frequent cause of outbreaks and live bird markets have also played a significant role in the spread of epidemics.

Usually avian influenza viruses do not cause disease in species other than birds and pigs. In Hong Kong in 1997 the first documented infection of humans with an avian influenza virus occurred. Infection with the H5N1 strain caused severe respiratory disease in 18 humans, of whom 6 died. This coincided with an epidemic of highly pathogenic avian influenza caused by the same strain in poultry in Hong Kong.

It was determined that close contact with live infected poultry was responsible for human infection. Genetic studies showed that the virus had jumped directly from birds to humans. The rapid destruction of Hong Kong's entire poultry population was thought to have reduced further opportunities for direct transmission to humans, and may have averted a pandemic.

These events in 1997 marked the first time that an avian influenza virus was transmitted directly to humans and caused severe disease with high mortality. In February 2003, another outbreak of H5N1 avian influenza in Hong Kong occurred, causing 2 cases and 1 death in members of a family who had recently travelled to southern China.

Recently, two other avian influenza viruses have caused illness in humans. An outbreak of the highly pathogenic H7N7 avian influenza strain, which began in the Netherlands in February 2003, caused mild illness in 83 humans and the death of 1 veterinarian. Mild cases of avian influenza H9N2 (which is not highly pathogenic in birds) in children occurred in Hong Kong in 1999 (two cases) and in mid-December 2003 (one case).

The most recent cause for concern commenced in December 2003/January 2004, when laboratory tests confirmed the presence of H5N1 avian influenza virus in human cases of severe respiratory disease in Vietnam. Since then there have been several waves of disease, affecting multiple countries, mainly in Asia. In October 2005 evidence of spread into Central Asia and eastern Russia, and Europe has occurred with confirmed bird cases in Romania, Ukraine, Turkey and western parts of Russia., and human cases occurring in Turkey and Iraq. The disease new appears to be well established, and is likely to appear in birds in these and other countries into the future. This has been the pattern throughout 2006 with continuing occasional 'bird to human' transmission occurring in various developing countries.

The following countries are considered mild-high risk due to human to human transmission of avian or novel influenza strains or widespread bird to human transmission:

No countries are considered high risk at present.

 

The H7 Avian Virus

The H5N1 virus is but one of the many influenza A viruses which can infect various animals including humans.

It is the most serious so far identified affecting poultry and wild birds, and is essentially an avian disease. However, experience has shown that it can be transmitted to humans by close and direct contact with infected birds. There is also evidence of limited human-to-human transmission but the strain is functionally adapted to affect birds primarily and not other animals.

In 1999 a mild strain of avian virus, H9N2, caused a mild illness in children in Hong Kong where the first instances of human infection with the more serious H5N1 strain of avian influenza had been recorded two years previously.

2003 saw the start of the waves of H5N1 infection affecting countries in Asia, Indonesia, the Middle East, North and West Africa and Europe that have captured the attention of scientists, lay people and the press; speculation that a change or mutation affecting this virus' ability to infect humans more easily could result in a human pandemic has fortunately so far remained no more than a theoretical, though valid, possibility.

In 2003 an outbreak of H7N7 avian influenza virus infections occurred in the Netherlands and poultry handlers either developed conjunctivitis from this infection or had no symptoms though developing antibody evidence of being infected. In this outbreak a veterinary surgeon died from respiratory infection.

There was a massive outbreak of H7N3 infection in poultry in the Fraser valley of Canada 's province of British Columbia in 2004 and although birds were culled to control spread of the outbreak H7 virus infections have generally been regarded as of low pathogenicity i.e not causing a high death rate. Nevertheless an H7N7 outbreak which started on May 22 nd this year in Oxfordshire in the United Kingdom has been categorised as highly pathogenic.

Research scientists at the Centres for Communicable diseases in Atlanta, USA have now reported that mild bird flu strains circulating in North America i.e H7 strains have gained some ability to infect humans by increasing their ability to stick to proteins on the surface of human lung cells, a key point which may allow spread from one human to another. The H7N7 strain from the Netherlands outbreak lacked this ability.

Time and further research will tell if perhaps the H7 virus strains are the ones more likely then the H5 ones to mutate to a form which threatens to result in human infections and a possible pandemic.

 

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