Tuberculosis: Epidemiology, Cause and Pathogenesis

00:01 what’s the epidemiology of tuberculosis? Well, 30% of the population of the world are infected by tuberculosis. Fortunately, most of those patients have what we call as latent infection, that is the bacteria is present in the body but it’s not causing active disease. Of those 30% of the population of the world, it's estimated about 10% will actually have reactivated disease from that latent disease over a lifetime and what that means that the dormant bacteria stopped replicating and actually start to cause an infection.

00:41 At any one time there are about 10 million people across the world with active tuberculosis infection and that causes death in about 10% of them. So we are talking one million deaths per year and that makes tuberculosis the first or second commonest cause of bacterial infectious death across the globe. What’s tuberculosis caused by? It’s caused by the organism Mycobacterium tuberculosis. Now this is quite an unusual bacteria.

01:07 So unlike the bacteria that cause Pneumococcal pneumonia, it is quite slow growing, it takes several weeks to culture it in the laboratory. It has a very unusual cell wall full of lipid, and there is no environmental source, you cannot catch tuberculosis from the environment you catch it from somebody else who is already infected with the bacteria. There are a couple of sub-species of Mycobacterium tuberculosis- Mycobacterium bovis, Mycobacterium africanus, and there is also the BCG. Now that is the Mycobacterium bovis that has been cultured in the laboratory and is now not pathogenic, it doesn’t cause disease, or rarely causes disease and we use that as a vaccine to try and prevent tuberculosis infection. As I’ve already mentioned there are environmental mycobacteria which are related to M. tuberculosis and these are called the non-tuberculosis mycobacteria and there is quite a lot range of different species and they can cause a similar disease to tuberculosis, sometimes, but they are far outweighed by patients infected with M. tuberculosis. So the pathogenesis of tuberculosis, like Influenza A requires being infected by somebody with active disease.

02:19 So If somebody has active lung disease with tuberculosis they will cough and in the cough, there will be infected droplets carrying the bacteria that can be inhaled by somebody else.

02:30 Once the bacteria is inhaled, it gets into the lung, and there it actually invades your normal mechanism for protecting the lungs against bacterial infection which are the alveolar macrophages. It actively seeks out and invades these macrophages because it's able to prevent the normal macrophage killing mechanisms. So it diverts the normal phagolysosome pathways and that allows it to survive in the macrophage and it can be latent in that macrophage for decades. In addition, the macrophages because they move, will allow the bacterium to spread across the body, and this is one of the reasons why sites of immune function such as the lymph nodes often get infected with tuberculosis. Long term persistence within the macrophages is what latent disease is. In addition, there is a certain inflammatory response to this infection, which causes a very distinctive histological appearance called granulomas and that is one of the hallmarks of tuberculosis infection, is the presence of granulomas in the infected tissue. So to talk about the disease, there's two main forms- there's pulmonary tuberculosis, which is TB affecting the lungs, and there is extra-pulmonary tuberculosis which is TB affecting outside the lungs.