Tuberculosis

Tuberculosis (TB) is currently one of the deadliest infectious diseases caused by bacteria. According to the World Health Organisation (WHO) in 2020, tuberculosis was the second leading cause of death by an infectious disease after COVID-19. Decades after being identified, this disease remains at the centre of a serious public health crisis. It is present in all countries and age groups, however, it is completely curable and preventable with incidence rates falling 2% each year. Ending the tuberculosis epidemic by 2030 is one of the health targets in the Sustainable Development Goals.

• What is tuberculosis?
• The causes of tuberculosis
  • Bovine tuberculosis
  • Mycobacterium tuberculosis
  • How is tuberculosis transmitted?
• The symptoms of tuberculosis
  • Tuberculosis tests
• The treatment of tuberculosis
• The tuberculosis vaccine

What is tuberculosis?

The two responsible bacterial pathogens for tuberculosis are Mycobacterium tuberculosis and Mycobacterium bovis. These pathogenic bacteria infect human lung tissue living inside human cells but can go on to spread throughout the whole body and infect the lymph nodes, gut, kidney, spine, brain, and even bone tissue.

An estimated 25% of the population is thought to have a tuberculosis infection, but not everyone infected will become sick. The vast majority of people with tuberculosis have a latent tuberculosis infection. This means they harbour tuberculosis bacteria but are not sick and cannot transmit tuberculosis to others. People can have lifetime inactive tuberculosis infection without ever exhibiting symptoms. However, it can develop into tuberculosis disease when the immune system is unsuccessful in fighting off the tuberculosis bacteria.

People with tuberculosis disease have symptoms and can spread the bacteria to others around them, although it usually requires prolonged exposure. Therefore the tuberculosis bacteria incubation period before symptoms onset varies from a few weeks up to several years depending on the ability of the immune system to fight off the bacterial pathogen.

In other words, tuberculosis infection can develop into tuberculosis disease. People with tuberculosis disease can spread the tuberculosis bacteria to others through the air via airborne droplets containing the tuberculosis pathogen.

The causes of tuberculosis

Tuberculosis infection is primarily caused by inhaling droplets of liquid in the air containing the tuberculosis bacteria Mycobacterium tuberculosis, i.e. it's an airborne disease. These droplets are released from other infected individuals while coughing or sneezing. Most tuberculosis bacteria enter the body through the respiratory system and go on to attack the lungs, multiplying and causing tuberculosis disease if left unchecked by the immune system.

Mycobacterium tuberculosis

Mycobacterium tuberculosis has a particular appearance under the microscope that makes it stand out: it has a waxy coating around its cell membrane due to the presence of mycolic acid. It is this waxy coating that makes it difficult to stain with the traditional Gram staining, so the acid-fast stains are used instead to identify it under the microscope.

Fig. 2. Mycobacterium tuberculosis under the microscope. Do you notice the waxy appearance of the bacterial colonies?

The immune system finds attacking Mycobacterium tuberculosis quite difficult. This is because M. tuberculosis is adapted to defend itself from it, especially from macrophages. Macrophages will try to engulf and destroy the bacteria, however, M. tuberculosis fights back.

M. tuberculosis cells can join together to "attack" a single macrophage. The macrophage still engulfs the group of bacteria, but because of the number of bacteria, the macrophage is overpowered and dies. The bacteria then reproduce within the dead macrophage, which is "eaten" by another macrophage with the bacteria still inside. And so the cycle continues, with the bacteria reproducing.

M. tuberculosis also has the ability to remain dormant within the host cells, avoiding the immune system. When the body's conditions are better for M. tuberculosis' survival, the bacteria will activate and start reproducing again.

How is tuberculosis transmitted?

Tuberculosis is spread when anyone with tuberculosis disease sneezes, spits, or coughs the tuberculosis bacteria from their respiratory system into the surrounding air. If people inhale these particles, they become infected with tuberculosis bacteria.

The symptoms of tuberculosis

The symptoms of tuberculosis disease may start off mild for many months and progressively worsen. The slow onset of tuberculosis symptoms complicates efforts to control the spreading of the disease because it discourages contagious people from seeking treatment early on. People with active tuberculosis infection are at their most contagious stage two to four weeks after becoming infected.

Tuberculosis tests

Diagnosis of tuberculosis is very important for controlling the spread of the disease. Tuberculosis infection, including latent infection, can be confirmed through the Mantoux tuberculin skin test (TST) or blood tests.

Tuberculosis disease usually requires additional diagnosis. WHO guidelines recommend tuberculosis disease diagnosis be done using rapid molecular diagnostic assays that rely on nucleic acid amplification, such as polymerase chain reaction (PCR) tests. These tests confirm tuberculosis disease by detecting the presence of the tuberculosis bacteria’s genetic material in a clinical sample.

Patients with active tuberculosis infection either suspected or confirmed should be ideally isolated from others during their most infectious stage to prevent further contagion.

The treatment of tuberculosis

Despite killing millions, tuberculosis is a treatable and curable disease. As with most other bacterial infections, treatment involves the use of antibiotics that kills the tuberculosis bacteria. Active tuberculosis infection is treated with a multi-month regimen of several antibiotic drugs like isoniazid and rifampicin. This combination of drug therapeutics usually cures up to 95% of all tuberculosis patients.

Tuberculosis treatment can last anywhere from at least 6 to 9 months or longer and must be completed in full even if the patient begins to feel better before finishing their treatment. A long treatment time is required to ensure all bacteria are killed.

Stressing the importance of patients completing their treatment regimens is essential to successfully reducing tuberculosis incidence. Prematurely stopping the antibiotic treatment gives any remaining living bacteria a chance to mutate and evade the antibiotic action later. This originates drug-resistant strains that go on to infect others, further complicating the effort to stop tuberculosis outbreaks.

Treatment for resistant strains requires the use of new antibiotics and treatments, often more expensive and less potent resulting in even longer multi-year treatment regimens.

Nevertheless, according to the WHO, since the year 2000, tuberculosis treatment has saved upwards of 60 million people! Without access to these treatments, it’s estimated that almost half of HIV-negative people with tuberculosis disease and almost all HIV-positive people with tuberculosis disease would die.

General tuberculosis prevention efforts can be very important in reducing tuberculosis incidence. General strategies like contact tracing, implementing rapid tuberculosis diagnosis, and programs that follow up with tuberculosis patients and encourage them to continue their treatments are all important to tackle tuberculosis disease.

The tuberculosis vaccine

Another possible preventive measure to tackle tuberculosis disease is the tuberculosis vaccine, BCG. BCG is a moderately effective vaccine, protecting up to 80% of children who receive it. As such, BCG is mainly used in children or young adults considered to be at risk.

At-risk groups can include those living in areas with high tuberculosis incidence or with family members that become ill with tuberculosis disease. The BCG vaccine is typically not given to people over the age of 35 as its efficacy significantly decreases with higher age groups.