Tuberculosis: one of the most virulent pathogens worldwide

You may be not unfamiliar to the infectious disease tuberculosis (TB), an ancient disease whose history can be traced back to thousands of years ago. The disease is still one of the top 10 causes of death worldwide, although numerous valuable progress has been made in its prevention, screening, diagnosis, and treatment. The pathogen that causes TB can easily spread from one person to another. So, it’s important to get a better understanding of TB in order to prevent yourself from getting affected by the disease and avoid spreading the disease to other people if you are already infected.

What causes TB? 

The pathogen that causes TB is a bacterium called Mycobacterium tuberculosis (Mtb). This bacterium is an acid-fast bacterium and has an unusual, thick, waxy cell wall. Because the tough cell wall prevents passage of nutrients into and excreted from the cell, Mtb has a remarkably slow growth rate. On the other hand, Mtb’s unique cell wall helps it survive in harsh conditions, such as varying levels of acidity and the detrimental effects of free radicals. Furthermore, the unique architecture of the cell wall makes Mtb infection relatively difficult to treat with antibiotics. The genome of Mtb codes for about 200 transcription factors, which regulate gene expression and protein production. The complex transcription factor network helps the bacterium adapt to changing environment.

How is TB spread?

The disease TB mainly affects your lungs, and the pathogen Mtb is spread from one person to other by the airborne route. When a patient with an active form of TB coughs, speaks, sneezes, spits, laughs or sings, many small droplets are expelled from the respiratory tract. These small droplets may contain the bacterium Mtb and remain suspended in the air for up to several hours. If a susceptible person breathes in these small droplets, he/she may get infected with Mtb. You may get TB via household contacts and other close contacts of someone with active TB. This is more common than that you get the disease from a stranger. Active TB can be contagious while latent TB is not, and it is therefore not possible to get TB from someone with latent TB.

Signs, symptoms, and complications

Not all people infected with Mtb will develop the illness TB. In fact, most Mtb-infected people do not feel sick or show any symptoms. This is the so-called latent TB. By contrast, some (about 5-10%) of the infected persons will develop TB disease and may develop further complications. This is known as active TB. People with weakened immune systems, such as elderly adults and HIV-infected individuals, are more susceptible to active TB than those with healthy immune systems.

Generally, it takes months to years after first exposure for TB symptoms to appear. The common symptoms of an active TB may include:

  1. Tiredness
  2. weight loss
  3. Fever
  4. Night sweats
  5. Chills
  6. Loss of appetite
  7. Coughing
  8. Chest pain
  9. Bloody sputum
  10. Shortness of breath

Of these, coughing significantly exacerbates the highly infectious nature of the disease — an uninfected person may get infected by breathing in bacteria-containing droplets released by an infected person when coughing.

TB primarily affects the lungs and can also spread beyond the lungs. If the infection spreads to other organs such as the kidney, spine, eye, liver, and spleen, the symptoms will depend upon the type of organ affected. If left untreated, active TB can be fatal. Some of the complications of TB include spinal pain, joint damage, brain inflammation, liver problems, kidney problems, and heart disorders. According to estimates, TB affects approximately 2 billion people worldwide and kills about 1.8 million annually. More than 95% of these deaths occur in the developing world. Among the most vulnerable are children and those with HIV/AIDS, who have weakened immune systems. HIV, the virus that causes AIDS, weakens an individual’s immune system, making it less able to fight the TB bacteria Mtb. Co-infection of HIV and Mtb is extremely hard to treat. That’s why a significant proportion of TB deaths are among people with HIV.

Diagnosis

There are two major types of tests, TB skin test and TB blood test, to aid in the diagnosis of TB.

The TB skin test is the most commonly used and is performed by injecting a tiny amount of tuberculin purified protein derivative (PPD) into the inner surface of the forearm of a person who is suspected of having Mtb infection. Several days later, the arm will be rechecked to see if the size of skin reaction is large enough to be read as positive. The TB skin test is the standard method of screening for infection with Mtb, but there can be about 20% false negative results, which may lead to misdiagnosis or an absence of diagnosis. So if a person is tested negative for the TB skin test, a second test may be needed.

The TB blood test is also known as interferon-gamma (INF-γ) release assay (IGRA), which is an ex vivo blood tests that detect INF-γ release from a patient’s T-lymphocytes after stimulation by antigens found on the Mtb complex. Interferons are a group of cytokine that released by host cells in response to the presence of certain antigens, including bacteria such as Mtb. The TB blood test requires only one visit, whereas the TB skin test needs a second visit to interpret test results.

A positive TB skin test or TB blood test only means that an individual has been infected with Mtb. It cannot clearly tell whether the individual has latent TB infection or active TB disease. To determine whether the individual has TB or not, additional tests such as a chest x-ray and a sample of sputum are required.

How to prevent Mtb infection and TB?

Humans have been struggling against TB for thousands of years, although it was only until 1882 researchers first identified that TB is caused by the bacterium Mtb. Currently, there is one vaccine available to prevent TB. That is the bacilli Calmette-Guérin (BCG) vaccine, which was first used in 1921. Besides, many candidate vaccines are being developed. In children, BCG reduces the risk of getting Mtb infection by 20% and the risk of infection progressing to active TB by about 60%. BCG is an important preventive strategy against but is not quite good enough. TB was still the top infectious killer in 2016. Why TB is so prevalent may be in part because it can spread easily through the air from person to person.

In addition to getting vaccinated, there are other things that you can do to reduce your risk of getting infected with Mtb and developing TB disease. For example, Leading a healthy lifestyle can help you maintain a strong immune system so that you are less likely to be affected by active TB disease; Scheduling a TB test if you have been exposed to someone with TB to determine whether you are infected with the bacterium or not. If you are infected, it’s necessary to confirm with the doctor whether you need treatment. Appropriate medical care can prevent potential complications of TB. Antibiotics, such as isoniazid, rifampin, ethambutol, and pyrazinamide, are the most common medications used in TB treatment.

Author Bio

Caroline Liu is a writer and works at Cusabio. She is focused on writing contents on topics like infectious diseases.

Author: Robin Gupta