Tuberculosis (TB) is currently one of the deadliest infectious diseases caused by bacteria. According to the World Health Organisation (WHO) in 2020, TB 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 TB epidemic by 2030 is one of the health targets in the Sustainable Development Goals.
What is the meaning of tuberculosis?
Tuberculosis (TB) is a bacterial infection. It mostly affects the lungs which is the primary site of infection.
For this reason, this disease is often described as pulmonary TB.
The two responsible bacterial pathogens for TB 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.
Fig. 1: Pulmonary TB
An estimated 25% of the population is thought to have a TB infection, but not everyone infected will become sick. The vast majority of people with TB have a latent TB infection. This means they harbour TB bacteria but are not sick and cannot transmit TB to others. People can have lifetime inactive TB infection without ever exhibiting symptoms. However, it can develop into TB disease when the immune system is unsuccessful in fighting off the TB bacteria.
People with TB disease have symptoms and can spread the bacteria to others around them, although it usually requires prolonged exposure. Therefore the TB 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.
Tuberculosis is an infectious disease caused by the bacteria Mycobacterium tuberculosis or Mycobacterium bovis which starts in the lungs. TB infection can develop into TB disease. People with TB disease can spread the TB bacteria to others through the air via airborne droplets containing the TB pathogen.
TB can affect anyone and is a leading cause of death in the developing world killing millions of people every year.
The causes of tuberculosis
TB infection is primarily caused by inhaling droplets of liquid in the air containing the TB bacteria Mycobacterium tuberculosis, i.e. it's an airborne disease. These droplets are released from other infected individuals while coughing or sneezing. Most TB bacteria enter the body through the respiratory system and go on to attack the lungs, multiplying and causing TB disease if left unchecked by the immune system.
TB infection can also be the result of ingesting meat or milk contaminated with the bacteria Mycobacterium bovis. This less common form of TB occurs in cattle but can also spread to humans that consume their contaminated meat or milk. This rarely happens in countries where cattle are routinely screened for disease and their milk is pasteurised which kill any present bacteria. However, the infection can occur in countries where unpasteurised milk and undercooked meat are still consumed.
The symptoms of tuberculosis
Active TB infection can leave individuals severely debilitated if left without treatment. Symptoms of TB disease include:
As part of the immune system’s response against the pathogenic bacteria, people with TB disease also commonly experience fever, loss of appetite, and consequent weight loss.
The symptoms of TB disease may start off mild for many months and progressively worsen. The slow onset of TB symptoms complicates efforts to control the spreading of the disease because it discourages contagious people from seeking treatment early on. People with active TB infection are at their most contagious stage two to four weeks after becoming infected.
Diagnosis of TB is very important for controlling the spread of the disease. TB infection, including latent infection, can be confirmed through the Mantoux tuberculin skin test (TST) or blood tests.
The Mantoux tuberculin skin test consists of injecting tuberculin (a mix of antigens from the bacteria that cause TB) into the forearm. After some time, the swelling around the area of injection is measured. A positive or negative diagnosis depends on the size of the swelling.
TB disease usually requires additional diagnosis. WHO guidelines recommend TB 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 TB disease by detecting the presence of the TB bacteria’s genetic material in a clinical sample.
Other methods of TB disease diagnosis involve:
- The collection of sputum (mucus) samples from the lung of the patient. The sample is then analysed under a microscope to assess whether bacteria are present.
- A chest X-ray can also be done to observe the effects of TB disease on the lungs.
Patients with active TB infection either suspected or confirmed should be ideally isolated from others during their most infectious stage to prevent further contagion.
How is tuberculosis transmitted?
TB is spread when anyone with TB disease sneezes, spits, or coughs the TB bacteria from their respiratory system into the surrounding air. If people inhale these particles, they become infected with TB bacteria.
The TB bacteria can be suspended by tiny droplets of liquid in the air for several hours.
Once the TB bacterial pathogen enters the body, the immune system of most healthy people is able to kill the bacteria and prevent TB disease. In other cases, the immune system is incapable of killing the bacteria but prevents the Mycobacterium from spreading, causing latent TB infection. As mentioned before, individuals with latent TB infection do not have any symptoms and are not infectious to others. If however, the immune system is incapable of fighting the invading pathogen, the TB bacteria will spread usually first within the lungs and eventually to other parts of the body. Infected people will then develop active TB infection or TB disease and display symptoms within a few weeks or months, infecting others and continuing the transmission cycle. Over the course of a year, people with active TB infection are thought to infect on average 5-15 other people.
Individuals with latent TB infection can also fall ill with TB disease if their immune system becomes weakened and is no longer capable of containing the dormant Tuberculosis Mycobacterium. There are several factors that can contribute to weakening the immune system and to the development of active TB disease:
alcohol overuse,
smoking,
diabetes,
or even malnutrition
Can all increase the risk of falling ill with TB disease after becoming infected.
TB-infected individuals have, according to the WHO, a 5-10% lifetime risk of developing TB disease.
Fig. 2: TB transmission
HIV infection is one of the most relevant risk factors for TB illness. People with HIV/AIDS have a compromised immune system that makes them extremely vulnerable to active TB infection. TB is often the first opportunistic infection that HIV-positive patients suffer from. People living with HIV are multiple times more likely to fall ill with TB. TB and HIV make up a very lethal combination, each one contributing to the other’s progressive deterioration. As a result, the HIV pandemic has historically been accompanied by a TB pandemic.
To learn more about HIV/AIDS and how it attacks our immune system check out our article on HIV.
The treatment of tuberculosis
Despite killing millions, TB is a treatable and curable disease. As with most other bacterial infections, treatment involves the use of antibiotics that kills the TB bacteria. Active TB 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 TB patients.
One of the reasons there was a resurgence of TB disease in the last decades across the world was the added difficulty in providing effective treatment. Overuse and inappropriate use of antibiotics have led many bacteria to become resistant to these drugs. Like all other organisms, bacteria mutate and find new ways to evade the antibiotic’s function. TB therapeutics is no exception, with the appearance of Mycobacterium resistant strains to first-line anti-TB drugs.
Multidrug-resistant forms of TB (MDR-TB) and even Extensively drug-resistant forms of TB (XDR-TB) have emerged as serious public health threats. The emergence of drug-resistant forms of TB bacteria has been partly fuelled by inappropriate usage of TB antibiotics including incorrect prescriptions, poor-quality drugs and especially patients stopping their treatment before completion.
TB 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 TB 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 TB 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.
To learn more about how antimicrobial drugs kill bacteria and the emergence of antibiotic resistance, check out our article on Antibiotics!
Nevertheless, according to the WHO, since the year 2000, TB treatment has saved upwards of 60 million people! Without access to these treatments, it’s estimated that almost half of HIV-negative people with TB disease and almost all HIV-positive people with TB disease would die.
The tuberculosis vaccine
General TB prevention efforts can be very important in reducing TB incidence. General strategies like contact tracing, implementing rapid TB diagnosis, and programs that follow up with TB patients and encourage them to continue their treatments are all important to tackle TB disease.
Another possible preventive measure to tackle TB disease is the TB 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. These groups can include those living in areas with high TB incidence or with family members that become ill with TB disease. The BCG vaccine is typically not given to people over the age of 35 as its efficacy significantly decreases with higher age groups.
Tuberculosis - Key takeaways
- Tuberculosis (TB) is an infectious disease caused by the bacteria Mycobacterium tuberculosis or Mycobacterium bovis. TB is a leading cause of death in developing countries killing millions of people.
- TB is mainly transmitted through the air when a person with TB disease coughs up the TB germ and these are inhaled by others via airborne droplets.
- TB infection is different from TB disease. The infection may progress into a disease (active infection) if the immune system is incapable of fighting off the pathogen. People who are infected with the TB bacteria but do not have the disease have a latent TB infection.
- TB disease symptoms include cough with sputum and blood, chest pains, sweating, shortness of breath fatigue fever, loss of appetite, and weight loss. TB treatment requires a multi-month antibiotic regimen that kills the bacteria in almost all TB patients.
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