Monday, 15 September 2008

How infectious is TB?

TB, caused by the bacterium Mycobacterium Tuberculosis, has reached epidemic proportions in many poor, overcrowded regions of the world, like sub-Saharan Africa and India. But how does it's infectiousness compare with that of meningococcus (Neiserria meningitidis)?

The short answer (and contrary to what might initially be expected) is this: much more infectious.

When a person infected with active TB coughs, he will expectorate numerous droplets. If the TB patient's disease is eroding into the airways, some of these droplets will contain active M. tuberculosis organisms. The very big droplets will fall to the floor harmlessly within seconds, and if the medium sized droplets (which stay afloat a while longer) are inhaled, they are merely trapped in the upper respiratory tract, from which they are cleared without causing infection. The smaller droplets, on the other hand, evaporate almost immediately, leaving 'naked' organisms that stay afloat in the room (or whatever) until something blows them away.

In this way, TB spreads, and it is actually quite a rare way for respiratory pathogens to be transmitted. Yet it is devastatingly effective.

First the good news: as with meningococcus, casual social contact doesn't seem to increase your risk of contracting the disease. But anything more than this, and you are at significant risk. For instance:

When active tuberculosis developed in a school-bus driver, 30 percent of the children who rode his bus had a positive tuberculin skin test. After one student contracted active disease in a school that had a large, open dormitory, 19 percent of the others converted to tuberculin positivity, regardless of how close their beds were to that of the index patient.

The closer the contact, the worse of course. Sharing a home with someone who has active TB entails about 1:3 chance of getting the disease.

So, contrary to expectations (and media hype), TB is a far more contagious disease than meningococcal meningitis.

However, you must bear in mind that these figures are for TB infection, not active TB disease. Only a small percentage of people who have latent TB infection will end up with active TB disease. In the vast majority, the infection is soon controlled and quarantined without symptoms or residual dysfunction. If this didn't occur, M. tuberculosis would never be able to sustain such high infectivity odds - we would all be dead.

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