Site author Richard Steane
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Lung section at autopsy showing tubercles

Tuberculosis is a bacterial disease

Tuberculosis - often shortened to TB - is a disease due to infection by bacteria of the genus Mycobacterium.
Mycobacterium tuberculosis is the main causative organism, but there are others e.g. Mycobacterium bovis.
Interestingly, leprosy is caused by a different species of Myobacterium.
It is mainly known as a disease of the lungs (pulmonary TB).
Inside the lungs the bacteria become enclosed inside structures called tubercles, which explains the name.
However other parts of the body can be also affected if the bacteria leave the lungs.
Mycobacterium tuberculosis.jpg

There are effectively two forms of the disease: the active form and the latent form

Active form of tuberculosis: Symptoms

The disease has been given a variety of names: Unlike the causative organisms for many bacterial diseases, Mycobacterium grows quite slowly.

Tuberculosis is usually spread by droplet infection

but it can sometimes also be transmitted from a mother to her baby via the placenta
When an (actively) infected person coughs, sneezes or spits (or even talks, sings, laughs or kisses!), bacteria are forced out in the aerosol produced. This can be breathed in, and potentially spread the infection widely.

However the greatest risk occurs when larger numbers of people - young and old - are crowded together in a confined space, as occurs in many slum dwellings. A very comparable situation occurred in the UK in the industrial areas until the beginning of the 20th century.

Coughs and Sneezes

a 1945 "health propaganda trailer"
made by the Ministry of Information
for the Ministry of Health

Anti-spitting poster
An anti spitting campaign poster

The latent form of tuberculosis

Not all who are infected get active TB. The body's immune system may deal with the bacterium directly, but it is known that TB bacteria are usually not killed by being consumed by white cells: phagocytes (macrophages), and the bacteria may become dormant inside them for a long time. This results in a latent (hidden) infection. Balls of tissue composed of several type of cells from the immune system (with B and T lymphocytes on the outside) develop into granulomas which are the basis for the tubercles. Cells in the centre of these structures die and this gives them a cheese-like texture.

This form of the disease is not infective: sufferers cannot spread the TB bacteria, as the immune system has contained the infection. It is also said to be asymptomatic, because none of the symptoms listed above are seen.

It depends on many factors whether the disease progresses further, to become active TB. Anything that challenges the immune system is likely to cause problems:

TB and milk

Cows can be infected with a different species of Mycobacterium (Mycobacterium bovis) especially if housed in insanitary conditions near to cities. They can pass it on to humans who drink the milk. Infection with Mycobacterium bovis is practically indistinguishable from infection with Mycobacterium tuberculosis.

In the 1930s around 40% of dairy cattle in the UK were infected with the bacterium Mycobacterium bovis, and 0.5% of cows produced contaminated milk. Approximately 2000 human deaths a year are believed to have been caused by drinking contaminated milk or by being in close contact with infected cows.

The risk of TB from cows was greatly reduced when milk was pasteurised (heat treatment to kill or reduce numbers of bacteria) and a screening program was started for cattle. Any cows that test positive are culled, and 'movement orders' are used to reduce the spread of the disease.

euro_badger.png It has been found that badgers can pass tuberculosis to cows (presumably having caught it from them in the first place). Even though badgers are a protected species, there is much pressure from the farming community to kill badgers if they are found to be carrying TB.

DEFRA set up a research programme to see how effective TB control by culling badgers could be, but it was terminated before its planned conclusion. Although killing badgers did seem successful in some circumstances, it had the opposite effect if all the badgers in an area were not killed, because the survivors were likely to migrate to new areas and spread the disease in a way that would not have happened if thet were left alone!

Some questions

Move the mouse pointer near to the > prompt for possible answers

Droplet infection is an example of what form of transmission?
>horizontal transmission

What is the name given for the transmission of TB from mother to baby?
> vertical transmission

B and T lymphocytes produce antibodies to, or attach specifically to, invading micro-organisms. Which of the body's lines of defence are they?

>third line: specific white (blood) cells

Web references

Clicking on each link below will bring it up as a new page

Tuberculosis From Wikipedia, the free encyclopedia

I've got TB - but how? When Tim Wilson volunteered to be a guinea pig in a medical research programme, he was shocked to learn that he was carrying tuberculosis [Some interesting personal perspectives on TB]

Bovine TB: What is bovine tuberculosis?

Bovine tuberculosis - The past and present problem [INSTITUTE FOR ANIMAL HEALTH]

Badger culling has mixed effects

Badger culls among anti-TB plans [BBC]

Badger cull 'not cost effective' BBC news story

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