H5N1 is one of many variants of the sub-group of influenza viruses known as avian influenza viruses. It first emerged in Hong Kong in 1997 and has since been reported, in an even more lethal strain, in many other Southeast Asian countries. To date it has been responsible for more than 30 human deaths and many officials worry it could be the cause of the next major influenza pandemic.
The Influenza Virus
The influenza virus is classified by type, subtype and strain. There are three types of influenza virus, classified as A, B and C. Influenza types B and C are normally only found in humans. While the type B virus can be responsible for an epidemic neither is considered a likely cause of a pandemic. Neither the type B nor the type C are classified further into subtypes though the type B, along with type A, are broken up into different strains.
Influenza Type A – or Avian Influenza Viruses
The influenza type A virus can infect humans, birds, horses, pigs and many other wild animals. Influenza type A viruses are first classified by subtype based on two proteins on the surface of the virus hemagglutinin (HA) and neuraminidase (NA). The 15 different HA subtypes can combine with the 9 different NA subtypes to give us classifications such as the one under discussion here, H5N1, as well as others currently in circulation amongst people (H1N1, H1N2 and H3N2). There are many other subtypes that are still found only in animals.
The H5 and H7 subtypes are further classified as highly pathogenic avian influenza (HPAI) or low pathogenic avian influenza (LPAI). These distinctions are based on genetic differences in the viruses and are responsible for the different mortality rates amongst domesticated poultry populations – 90-100% for the HPAI vs. no illness to mild illness for the LPAI. It is not known yet whether the distinction between these two has any correlation with an avian strain making the leap to humans or the attendant mortality rate within human populations.
From Birds to Humans
The avian influenza virus is present in wild bird populations around the world though it is most commonly fatal only in domesticated bird populations. Since 1997 there have been more than 16 outbreaks of the H5 and H7 subtypes amongst domestic bird populations just in the US alone. Generally of the LPAI variety, these outbreaks are still closely monitored to make sure there is no leap to the HPAI, carrying with it the threat of a leap to humans, the likelihood of quarantine or culling of the flock and potential trade restrictions.
While all birds can be carriers of the type A virus, many subtypes of the virus are specific to certain species (H1N1, H1N2, H2N2 and H3N2 have caused illness in people, H1N1 and H3N2 in pigs and H7N7 and H3N8 in horses). It is in the leap from species to species that the potential for harm to humans is found. The influenza virus is a segmented genome with 8 separate gene segments. When two different types of virus are present in the same animal these segments may swap places in a process known as antigenic shift, producing a new type of virus.
It is in cases of antigenic shift and new types of the virus that we are most concerned. Most type A viruses will not infect humans. It was not until 1997, with the H5N1 outbreak in Hong Kong in which 6 people died and 18 were hospitalized, that a case of the virus traveling directly from birds to humans was documented. Since then there were two documented cases in China in 2003 and most recently there have been outbreaks in Thailand and Vietnam in which cases are still being reported. While there is some cause for concern the good news is that the virus does not seem to be able to support human to human transmission at an efficient rate yet. This is most likely related to the fact that the current incarnation of H5N1 seems to be made up entirely of avian genes. Were the virus to undergo antigenic shift in a human or pig and pick up human genes as well it could be the start of a very dangerous pandemic.
Avian influenza, bird flu, is of course most easily recognized by flu-like symptoms including fever, cough, sore throat or muscle aches. It can also cause conjunctivitis (eye infections), respiratory problems or pneumonia (including viral pneumonia).
Treatments and Prevention
There are currently four antiviral drugs approved by the Food and Drug Administration. Two of them, amantadine and rimantadine, have proven ineffective against the current H5N1 type A virus. Together with the other two (oseltamivir and zanamivir) however, we have a fair arsenal with which to battle influenza. As avian influenza viruses which travel easily between humans are not that common today prevention is also accomplished with simple safety procedures and healthful guidelines related to the handling of poultry and avian contaminated surfaces. Unfortunately, it is in the Third World and in factory farms and slums where education and money are lacking, it is in these places that a human to human virus is likely to develop.
Summary and Sources
It was the article H5N1: The Monster at the Door by Mike Davis which got me started on this topic. I learned a great deal more about the virus from the CDC’s website at http://www.cdc.gov/flu/avian/index.htm. My layman’s impression is that there is no specific danger today. We have had three influenza pandemics in recent history, the Spanish flu in 1918-19, the Asian flu 1957-58 and the Hong Kong flu in 1968-69. The worst was the Spanish flu, responsible for 20-50 million deaths, with an unknown cause. The second two are both the products of antigenic shift, in which human influenza genes were combined with avian influenza genes. Given that the deadliest of pandemics (the Spanish flu) only infected from 2-5% of humans, and given that the current strain of H5N1 has so far proved 70% fatal, it is in the threat of this virus picking up human genes, and therefore the ability to travel human to human, that most alarmists find their fodder. While true that a pandemic of that size would be literally catastrophic it still seems to be a case of worrying about ‘might be’ instead of ‘will do’. For more information on this topic check out the CDC’s website or the website of the World Health Organization at http://www.who.int/en/.