Influenza Virus

00:01 So, the subject of this lecture are two other infectious diseases that affect the lungs which are of great importance. One is Influenza A and the other is Tuberculosis. So, discussing Influenza A in more detail- The importance of this disease is that it causes every year winter epidemics. In most people who have not got pre-existing disease or are relatively young it will be the cause of bronchitis often with marked systemic symptoms and cause what we call flu, and the patient maybe is off work for a couple of days but then recover.

00:32 However, these winter epidemics also can cause exacerbations of chronic lung and cardiac disease in patients who have those problems, and that’s a very important cause of morbidity and mortality. In addition, Influenza A occasionally causes pandemics. The most famous being the one that occurred after World War I, which was estimated to have killed 20 million people.

00:55 And the most recent one was the swine flu pandemic a few years ago. Influenza A is not just infection of humans, it also infects animals such as pigs and is very prevalent in birds; geese, duck and chickens for example. It’s an RNA virus, it’s coded in the surface membrane, and in that surface membrane are embedded two proteins: haemagglutinin and neuraminidase. Those are important because the structure of those proteins defines the type of virus you have, and there are only three main structural types, which affect humans, that is H1N1, H1N2, H3N2. And most of the influenza that affects humans are some variations on those three types. As I mentioned the human types cause winter epidemics and occasional pandemics. In addition to the non-human types of influenza A virus, you occasionally get infected with avian influenza. Now these are viral types that affect birds, but can spread from those birds to infect a human and they cause quite an aggressive disease.

02:00 And you’ve probably heard about avian influenza through the news because there is a lot of discussion about this as a cause of severe viral infections. Fortunately, this is a sporadic infection and does not spread from the infected person to another infected person, as yet.

02:16 And the most famous example is H5N1. So what are the clinical manifestations of the influenza virus? Well these are created by the fact that it’s a virus that infects and kills respiratory epithelial cells. If somebody is infected with Influenza A, they will cough out virus particles. Those are inhaled by the next person to get infected and then they will invade the epithelial cells, replicate within the epithelial cells, and then eventually kill the epithelial cells, releasing the viruses to infect more epithelial cells in the same person, and also to be coughed back out into the environment to cause infection. And because of this ability to transmit between people readily, that is why the virus causes epidemics and pandemics. The actual infection of the respiratory epithelium itself causes several distinct clinical problems- firstly, as I've mentioned already it causes a bronchitis, cough, phlegm production, some systemic symptoms, with feeling ill and fever, but not an infection deep in the lungs, just in the tracheal bronchial tree, and with that you may get a sore throat and the upper respiratory tract symptoms of a runny nose, etc. Occasionally the virus also causes gastroenteritis symptoms because it infects the gastrointestinal system and very rarely it can spread to cause encephalitis.

03:36 Within the lung, there are three main problems, which cause deaths. One is that the infection with the virus unfortunately weakens the immune system against bacteria, and that means that Influenza A infection will allow a bacteria to get into the lungs and cause a secondary bacteria pneumonia and that is the main method by which the pandemics have caused death in the past. In addition, the virus itself can spread to affect the alveoli and cause what we call a primary viral pneumonia, and these are generally speaking very aggressive and they are associated with a high mortality, but they are relatively rare. The normal winter epidemics, unfortunately if they affect somebody who has got chronic heart, lung or kidney disease will cause the underlying disease to deteriorate, to get worse. So Influenza A is a common cause of exacerbations of COPD, heart failure and renal impairment, and that’s associated with a higher degree of morbidity and mortality.

04:40 So what’s the management in Influenza A? For most people, actually we just clinically recognize they have Influenza A they take a couple of days off work they get better.

04:50 If you need to, if the patient is badly affected, coming into a hospital for example, then you’d like to confirm the presence of influenza A, and that can be done by nasopharyngeal swab which can be tested using immunofluorescence, antibodies against influenza A’s antigens, or the polymerase chain reaction to try to amplify the presence of the Influenza A DNA, and that will identify patients who have active infection, and it is a very good test. But if it’s only really necessary in the high risk patients or they will be hospitalized.

05:21 If you have somebody with Influenza A and you want to treat them, the neuraminidase inhibitors which target that surface protein, the neuraminidase will help the patient recover from infection more quickly than if they were not given the treatments itself. It also probably reduces the chance of the infection being transmitted to other people. Because of this risk of superadded bacterial infection of the lung, some patients will need antibiotics if there is a suspicion, they may be developing a bacterial pneumonia. A very important point about Influenza A is that its highly contagious, and if somebody is in hospital for influenza A, they need to be isolated so they do not spread the disease to other people, and because of this ability of the viral infection to upset the body’s physiology and cause exacerbations of underlying disease, it’s very dangerous if Influenza A is allowed to spread to other people within the hospital. Lastly, and probably most critically is that Influenza A is a disease that can be prevented by vaccination in general. And every year patients who are at risk of disease or severe disease, the elderly and those of underlying chronic disease such as COPD or heart failure should be vaccinated to prevent them from getting that year's version of influenza A.