Tuberculosis (TB): Mycobacterium Tuberculosis Infection

最後更新日期 十一月 22, 2021.

Tuberculosis (TB) is an airborne bacterial infectious disease that primarily affects the lungs but can involve any part of the body. TB was the most reported infectious disease aside from COVID-19 in Hong Kong in 2020. TB is both curable and preventable but can be fatal if it is not treated properly. Read more to learn about preventing TB infection. 

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What is Tuberculosis (TB)?

Tuberculosis (TB) is an infectious disease caused by infection with the bacterium called Mycobacterium tuberculosis that most often attacks the lungs, causing pulmonary tuberculosis. 

Most healthy people who are infected by TB bacteria do not experience any symptoms, as the immune system can contain the growth and spread of the bacteria. This kind of infection is called latent TB infection (LTBI) or inactive TB and is asymptomatic and non-infectious. People with latent TB have only a 5-10% chance of developing active TB in their lifetime. 

Active TB (also called TB disease) is a condition in which your immune system cannot stop TB from growing and spreading. Some people may develop TB disease soon after the infection due to pre-existing conditions that weaken the immune system, such as HIV/AIDs. Others may never develop TB disease in their lifetime or only develop it years after the initial TB infection. Not only can TB make you sick and spread to others, but the bacteria can also attack other parts of the body, such as the lymph nodes, bones, kidneys, brain, and spine. 

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Signs and Symptoms of Tuberculosis (TB)

While latent TB generally causes no symptoms, signs and symptoms of active TB depend on which part of the body is affected. Common signs and symptoms of pulmonary tuberculosis (TB) in the lungs are:

  • Persistent cough that lasts more than 3 weeks
  • Coughing up blood or sputum
  • Chest pain, or pain when coughing or breathing

Other typical signs and symptoms of TB disease include:

  • Fatigue
  • Fever and chills
  • Sweating at night
  • Loss of appetite
  • Unintentional weight loss

When TB bacteria spread to other parts of the body (extrapulmonary TB), signs and symptoms vary according to which organ is affected. For example, TB in the gastrointestinal system may cause abdominal pain and vomiting; TB in the genitourinary system may cause haematuria (blood in the urine); TB in the bone can cause bone pain.

Causes of Tuberculosis (TB): How does TB spread?

A bacterium called Mycobacterium tuberculosis is the causative agent of TB. It spreads through the air when a person with active pulmonary TB coughs or sneezes out the bacteria which is then inhaled by others. Although it is contagious, you will not catch TB very easily because it usually requires prolonged exposure to become infected. You are more likely to get TB infection from people who live or work with you rather than from casual exposure in public.

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Risk Factors of Tuberculosis (TB)

TB is an infectious disease that can affect anyone. The following factors increase the risk of TB infection:

  • Regions: Some areas are reported to have a higher rate of TB, such as Asia, Russia, South Africa, and South America. Traveling or living in these areas can increase the risk of TB infection.
  • Close contact: Living or working with an infected person increases the risk of TB. 
  • Substance abuse: Alcohol abuse, tobacco smoking and illicit drug use contribute to the development of TB disease. 
  • High-risk environmental settings: People who work or reside in homeless shelters, prisons, hospitals, and nursing homes are at higher risk of TB infection. 

An individual with a healthy immune system can normally deal with TB infection. However, people with weakened immunity are much more likely to develop active TB and become sick. Conditions that may weaken our immune system include:

  • Medical conditions: These include HIV/AIDs, diabetes, severe renal disease, and certain cancers (e.g. leukaemia, lymphoma).
  • Medications: Drugs used to treat autoimmune disorders (e.g. Crohn’s disease, rheumatoid arthritis), oral steroids (e.g. prednisolone), and immunosuppressants (e.g. cyclosporin, tacrolimus) can suppress the immune system.
  • Other conditions: Patients who are malnourished or undergoing chemotherapy or radiotherapy often have suppressed immunity. 
  • Age: The immune systems of newborns, children and the elderly tend to be weaker than those of healthy young adults.

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Diagnosis of Tuberculosis (TB): TB Tests

As latent TB is asymptomatic, people with the above-mentioned risk factors should have a regular check-up even if there are no signs and symptoms. Many different TB tests are available for diagnosing TB. These may include:

  • Physical examination: Physicians carry out auscultation to listen to your lungs with a stethoscope. Abnormal breathing sounds may be detected if you are suffering from TB in your lungs.
  • TB skin test: Also known as tuberculin skin testing (TST) or a Mantoux test. Tuberculin is injected into the skin of your forearm. After 48-72 hours, you must return to the doctor’s office to check if there is a small red bump on the applied skin, indicating that your immune system is sensitized to tuberculosis, suggesting prior exposure.
    The result depends on the size of the bump which is examined by healthcare professionals. If the result of TB skin test is positive, you are likely to be infected with TB, and additional tests are required to determine if it is latent or active TB.
    A TB skin test is not, however,  100% accurate. People with previous BCG vaccination may have false-positive results, while a recent infection (within 8-10 days) can result in a false-negative reaction. Since most people born in Asia received the BCG vaccine at birth, false positives are common here.
  • TB blood test: Also known as interferon-gamma release assays (IGRAs). A blood sample is taken to analyse how your immune system reacts to TB bacteria. This is more accurate than the TB skin test, but is considerably more expensive. It requires visiting the doctor only once, and is not affected by prior BCG vaccination.  If the result is positive, you may be infected with TB, and additional tests are required to see if it is latent or active TB. 
  • Imaging tests: Chest X-ray or CT scan of the lungs can be used to assess for pulmonary TB when a positive skin or blood TB test result is found. 
  • Sputum tests: Sputum culture and smear (microscopy) tests can be taken to confirm the diagnosis of TB bacteria. If TB bacteria are seen on the smear test, you have active TB and are likely to be infectious to others. A TB culture, where the lab tries to grow TB bacteria from the sample, is the gold standard and may be positive even when the smear test is negative. You are likely to be less infectious to others in this case than when the smear test is positive. Culture can also isolate which strain of TB you have and check for drug resistance. However, it takes up to 1-2 months for these results to be ready. 
  • In recent years, PCR tests for tuberculosis have become increasingly available and can be done on a variety of body fluids including sputum. These tests can detect minute quantities of the bacteria’s DNA, but can also have false-negative and false-positive results.
  • Other TB tests: For TB that is outside of the lungs (extrapulmonary TB), other TB tests might be necessary, such as biopsy, endoscopy and body fluid examination

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Complications of Tuberculosis (TB)

Untreated TB can be fatal. Active TB may affect not only the lungs but also spread to other body parts. Typical clinical examples of extrapulmonary TB include: 

  • Lymph node TB
  • Gastrointestinal TB
  • Spinal TB
  • Miliary TB
  • Central nervous system TB
  • Genitourinary TB
  • Cardiac TB
  • Skin TB

One must be aware of any signs and symptoms of TB, and avoid any delay in treating tuberculosis. Consult a physician right away if you have risk factors for TB infection or experience any symptoms of tuberculosis.

Tuberculosis (TB) Treatment

Both latent TB and active TB should be assessed and managed properly as they can become serious and fatal. The main treatment for tuberculosis (TB) is administration of antibiotics with a combination of drugs in order to prevent drug resistance. The choice of antituberculosis agents, drug combination and duration of treatment depends on many factors, such as 

  • whether the TB is latent or active 
  • drug resistance and susceptibility
  • life stage (e.g. children, pregnancy, breastfeeding)
  • overall health condition (e.g. HIV/AIDs, kidney or liver diseases)
  • site of infection in your body

The most commonly administered anti-TB drugs are:

In Asia, latent TB is treated only in certain circumstances; your doctor will help assess if it is appropriate for you. Latent TB requires a single agent taken for 9 months, or sometimes two agents taken for a shorter time. Active TB is always treated and usually requires all of the above four drugs in combination for at least 6-9 months. When you suffer from drug-resistant TB, additional drugs might be needed, such as fluoroquinolones (e.g. levofloxacin) and second-line injectable agents (e.g. amikacin, streptomycin). The duration of treatment for drug-resistant TB can be up to 20 months or even longer, depending on the type of TB strain. 

Points to note:
As TB antibiotic treatment can interact with many other drugs, you should consult your doctor or pharmacist before taking any new medications, including Chinese herbal medicines. These antibiotics may damage the liver, eyes or other organs. You will be monitored by your doctor and should comply with regular follow-up. It is important to seek medical help immediately if you experience the following adverse effects:

After taking the medicines for 2-3 weeks, you are no longer contagious and you might start to feel better, but do not stop taking any of the prescribed medicines. Continue taking the medications as prescribed on time as indicated by your doctor. Keep in mind that completing the entire course of anti-TB treatment is extremely important because an incomplete course may lead to the development of drug-resistant TB, which requires more difficult and prolonged treatment.

To make sure you take the medication correctly as prescribed, a program called directly observed treatment (DOT) has been introduced. During DOT, the patient takes the given dosage of medications under the strict supervision of healthcare workers. DOT is beneficial not only because treatments will not be missed, but also because the healthcare professionals can monitor your response, check for side effects, and support and answer questions to help ensure you can complete the course. 

If you forget to take the pills, ask your healthcare provider for advice. Along with DOT, here are some tips to help you remember to take the anti-TB drugs:

  • Take the medicines at the same time every day.
  • Ask your family or close friends to remind you to take the pills.
  • Mark on a calendar to record your drug administration progress.
  • Arrange your pills in a weekly pill dispenser and take them accordingly.

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BCG Vaccine in Hong Kong

Bacillus Calmette–Guérin (BCG) vaccine is a live attenuated vaccine that is used to prevent TB infection. However, the BCG vaccine offers only partial protection against tuberculosis (TB), and it is more efficacious in children than in adults. It works best against disseminated TB and TB meningitis, which are sometimes seen in infants and young children, and less well against the common forms such as pulmonary TB seen in older individuals.

BCG vaccination is included in the Hong Kong Childhood Immunisation Programme (HKCIP), which offers free of charge BCG vaccination for all newborns in Hong Kong. It is also recommended for children residing in Hong Kong who are aged below 15 and have never received BCG vaccine before.

You can find more information about child immunization here

Preventing the spread of Tuberculosis (TB)

Only active pulmonary (lung) or laryngeal (voicebox) TB can spread easily to others. Your doctors will prescribe you some medications, but it still takes a few weeks before they work to prevent the spread of TB bacteria. Here are some ways to stop TB from spreading to your family and friends: 

  • Take your Anti-TB medications exactly as prescribed
  • Wear a surgical mask
  • Cover your mouth with a tissue whenever you cough, laugh or sneeze. Put the tissue in a closed bag and throw it away properly
  • Do not go to work or school without your doctor’s permission, especially during the first few weeks of tuberculosis (TB) treatments
  • Ventilate the room well, because TB can linger and spread more easily in small, closed spaces 
  • Avoid close contact with your family members and friends. TB can be spread by both direct and indirect contact.

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Seek Help!

If you need more help, please contact the following organizations:

 

Dr. Sarah Borwein is a Canadian trained General Practitioner who co-founded the Central Health Group and has been practicing family medicine in Hong Kong for over 15 years. After obtaining Certification in Family Medicine from the College of Family Physicians of Canada, she completed a Master's degree in Infectious Diseases from the London School of Hygiene and Tropical Medicine. She worked as a staff physician at the Beijing United Family Hospital where she was Director of Infection control during the SARS outbreak in China. A French speaker, Sarah is the advising and referral doctor for the French Consulate in Hong Kong. She is the site director for GeoSentinel (an international disease surveillance network) in Hong Kong and is past President of the Asia Pacific Travel Health Society. In addition, she sits on the Centre for Health Protection's Scientific Committee on Vector-borne Diseases, which advises the Hong Kong Department of Health on this type of illness.

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