Emerging and re-emerging diseases Every year for the past thirty years a new infectious agent, or an old one thought to be under control, has emerged to cause disease in humans. Nearly two thirds of infectious diseases in humans come from animals, and they are known as zoonoses or zoonotic infections. Viruses are particularly good at jumping the species barrier because they are able to infect a wide range of animals, including humans. Some notable examples of viruses that cause zoonoses include rabies, Ebola, Marburg, Lassa, hanta, morbilli and influenza. So why do known viral diseases continue to cause problems? The answer is that whenever there is a change in the environment or the virus, a new opportunity is created to cause disease in humans. The trigger could be localised increases in human population, removal of forests, climate change, humans settling in new geographic areas or humans encountering exotic animals and the viruses they carry. Viruses that cause colds and flu The influenza virus is an example of a zoonotic infection that re-emerges year after year. In humans, 'the flu' is caused by either influenza A or B viruses. Influenza B viruses infect only humans and although they can cause flu epidemics, do not cause pandemics (world-wide epidemics). Influenza A viruses can infect birds – including chickens, ducks and wild birds – and mammals, including humans, pigs, cats, horses and whales to name a few. ‘Avian flu’ or ‘bird flu’ is caused by avian influenza A viruses that can also infect humans. We tend to think of colds and flu as the same thing – an infection that makes you suffer from a running nose, sore throat, fever, aches and pains. Although the symptoms are similar, a head cold and the flu are caused by different viruses. About half of all cases of the common cold are caused by Rhinovirus, the rest being caused by Coronavirus or Respiratory Syncytial Virus (RSV). For most of us, the flu causes irritating symptoms for about a week. But it can be a very dangerous disease. World wide, the flu infects between 3 and 5 million people every year, causing 300,000 to 500,000 deaths. In Australia, up to 3500 cases of the flu are confirmed every year, mostly in the very young, the old, or people with underlying medical conditions. Why can you get the flu more than once? The influenza virus infects cells of the nose, throat and bronchi of the upper respiratory tract. Once inside a cell, the virus takes control of the function of the cell to make more copies of itself. Eventually, the cell loaded with virus particles dies and the virus is released to spread to other cells. During an infection, the immune system only ‘sees’ the outside surface of the virus particle (Box 1: An ever-changing virus). If the immune system doesn’t recognise the virus from previous infections, the virus multiplies and spreads to other cells, and you get another dose of the flu. The reason the immune system doesn’t recognise the virus is because its outer surface is constantly changing in subtle ways. New flu types come along every year as minor variants of existing viruses. A slightly different vaccine is made each year to try to keep up with the changing virus. This is why doctors recommend that particular groups of people in the population are vaccinated each year for the flu. The H5N1 bird flu story – so far Occasionally, a radically different virus emerges which has the potential to cause a pandemic (Box 2: Past flu pandemics). In December 2003, an outbreak of bird flu known as H5N1 occurred in poultry in eight Asian countries at the same time. Like other influenza A viruses, H5N1 is able to infect humans that have close contact with infected birds or water. The disease in humans is particularly virulent, causing death in about half of the adults and children who show symptoms of the flu, although the number is decreasing with time.
Some of the symptoms caused by H5N1 bird flu are unlike those of other types of flu. Notably, a sore throat, a runny nose and other symptoms we usually associate with the flu are not always seen. Indeed, some children in Vietnam who were thought to have died from encephalitis (brain inflammation) probably died from H5N1 bird flu. A growing number of people are infected by the virus but show no signs of illness, becoming instead carriers of the disease. The variety of symptoms – including the unusual or none at all – means that cases of bird flu could go unnoticed for some time, giving the virus a chance to spread. Why is H5N1 bird flu such a concern? Three conditions need to be met for a pandemic to occur. Firstly, we need the presence of a new virus to which the population has little or no immunity. The second condition is that the virus has the ability to infect humans and to be passed on to other humans. And the third condition is that there is no effective vaccine to prevent the spread of the disease in humans. If all of these conditions are met by a flu virus, or any other type of virus, conditions are perfect for a pandemic to occur. Authorities are worried about H5N1 because it has many of the characteristics of a virus that can cause a pandemic. Most cases of bird flu are caused by people being infected by birds or water contaminated with the H5N1 virus. Authorities have reported a number of cases where infection is likely to have occurred due to close contact between family members with the flu. So far, H5N1 has not shown signs of being very easily transferred from one person to the next via casual contact. However, a more infectious virus may be created if a person with the flu also catches bird flu H5N1. The different strains may recombine to create a more infectious virus. The concern is that you might get a virus like H5N1, that is already lethal, becoming extremely contagious, which would make it easier for it to spread from person to person. Fighting the flu Because H5N1 is so widespread, a global reporting system is required to limit the spread of outbreaks of the flu in humans (Box 3: Controlling an outbreak). In 1999, WHO published a world preparedness plan, which includes a network of 112 influenza centres, constantly monitoring reported cases of the flu to determine likely sources of infection and to contain potentially pandemic strains. Australia has the ‘Australian Action Plan for Pandemic Influenza’ and ‘Ausvetplan’ in place to respond to a potential flu pandemic reaching Australia. There are two approaches to fighting the flu in humans. They are prevention of infection by vaccination (Box 4: Vaccines for pandemics) and antiviral medications. People in direct risk of infection from animals include workers in the poultry industry and farmers and their families in Asian countries. Medical professionals or family members nursing infected people are also potentially at risk. Flu vaccines are not normally stockpiled because, until a pandemic starts, you don’t know exactly which one to make. But for H5N1, the WHO is recommending that vaccines that are an imperfect match of flu be made because it is seen to be a serious threat. An imperfect vaccine could give some protection, so although you might fall ill, you are unlikely to die from the infection. Antiviral drugs could be used in the early stages of a pandemic, until vaccines are available. While H5N1 is resistant to the antivirals called amantadine and rimantidine, it is sensitive to neuraminidase inhibitors such as Relenza and Tamiflu (Box 5: Antivirals bring hope). Tests for the effectiveness of antiviral drugs show that they need to be taken in the early stages of infection and they tend to decrease the severity and duration of the infection. Most health officials believe that a pandemic is due to occur some time soon, and that it is a case of when, not if. In which case, it would be wise to be ready for it. Boxes Related Nova topic: Immunisation – protecting our children from disease
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Posted July 2005.
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