Infectious Disease: Question 10

Worked solution

Part (a): Community-level control measures

Because tuberculosis spreads through airborne droplets from person to person, controlling it within a community relies on more than just treating people who are already diagnosed:

• Screening or testing close contacts of a diagnosed patient, such as household members, co-workers or classmates, so that anyone else who has become infected can be identified and started on treatment before they unknowingly spread the bacterium to others.
• Isolating infectious patients, for example keeping a newly diagnosed, actively infectious patient away from crowded indoor spaces until treatment has reduced the number of bacteria they release, which lowers the chance of them passing the infection on to new people.

(Other valid measures include improving ventilation in shared spaces to disperse airborne droplets, or contact tracing more broadly to map and interrupt chains of transmission.)

Part (b): Why completing the course matters

A course of antibiotics for tuberculosis typically lasts several months because Mycobacterium tuberculosis can be slow to eliminate completely; symptoms often improve well before every bacterium in the body has been killed. If a patient stops taking the antibiotic as soon as they feel better, some bacteria are likely to still be present, and among these survivors are more likely to be any bacteria that are naturally less susceptible to the antibiotic (compared with the ones killed earliest in treatment).

Once the antibiotic is withdrawn, these surviving bacteria can multiply again, so the patient’s infection can return. Because the bacteria that survived an incomplete course are more likely to include less-susceptible individuals, the tuberculosis that returns can be harder to treat with the same antibiotic, and in some cases develops into a multidrug-resistant strain (MDR-TB). Treating MDR-TB requires a longer course of different, often more toxic and more expensive, second-line drugs, and in the meantime the patient may also transmit this harder-to-treat strain to other people.

Final answers

• (a) Screening/testing close contacts of a diagnosed patient, and isolating infectious patients until they are no longer able to transmit the bacterium (ventilation and wider contact tracing are also valid).
• (b) Symptoms can improve before all bacteria are killed; stopping early leaves survivors, including less-susceptible bacteria, able to multiply again, so the infection can return and become harder to treat, potentially as a multidrug-resistant strain (MDR-TB).