Very low risk of getting TB via casual contact

The human immune system in action. This coloured scanning electron microscope image shows a white blood cell (dyed red) in the act of destroying tuberculosis bacteria (yellow).
The human immune system in action. This coloured scanning electron microscope image shows a white blood cell (dyed red) in the act of destroying tuberculosis bacteria (yellow).PHOTO: MAX PLANCK INSTITUTE FOR INFECTION BIOLOGY/VOLKER BRINKMANN

Tuberculosis has come under the spotlight, with news that the National University Hospital is screening 178 children who had been cared for by a nurse with TB. So far, one of the children is found to have the latent form of the disease. Here are the facts on TB.

Q What is tuberculosis?

A It is an airborne disease caused by bacteria (Mycobacterium tuberculosis). This commonly presents in the lungs. However, it can also affect other parts of the body, such as the lymph nodes, bones, joints, kidneys and brain.

TB is treatable and curable. About one-third of the world's population have latent TB, which means they have been infected by TB bacteria but are not ill and cannot transmit the disease.

Q How common is TB in Singapore?

A In the past few years, we have seen about 3,000 new cases of active TB disease in Singapore each year, which means they already have symptoms and can be contagious.

Q How infectious is TB?

A The risk of contracting TB through casual contact is very low; the highest risk is over prolonged periods in close contact.

In most healthy people, the immune system is able to fight the bacteria that cause TB. People with latent TB have a 10 per cent lifetime risk of falling ill with the active form of the disease.

However, those with compromised immune systems have a much higher risk of falling ill.

Q What does the screening entail?

A It involves clinical assessment for symptoms, chest X-rays, skin tests and/or blood tests. In children, the type of screening is determined by age and health.

When results are available depends on the type of test performed. Chest X-rays can be reviewed on the same day, while blood tests can take a few days, and the results of skin tests take 48 to 72 hours. While waiting for the results, patients who are well and have no symptoms do not require isolation, and do not have to change their daily activities.

Q What is latent TB infection?

A For most people who breathe in TB bacteria and become infected, the body is able to fight and "wall off" the bacteria so that they become inactive. This is called latent TB infection, which is not a disease state. People can have latent TB without knowing about it. It can lie dormant in the body and become active only many years later.

Fortunately, the vast majority of people (90 per cent) with latent TB do not ever develop active TB. Very young children, however, are less able to contain the germs and active TB disease can develop quite quickly in them.

Q How is latent TB treated, and does it prevent future infections?

A Treatment usually consists of a single antibiotic taken for six to nine months. Someone who has completed treatment successfully is very unlikely to develop another episode.

Q How is this different from active TB?

A In some people, TB bacteria overcome the immune system's defences and multiply, resulting in the progression from latent TB infection to active TB disease. This happens in 10 per cent of those who are infected.

Some people develop active TB soon after infection, while others develop it later when their immune system becomes weak. Such patients can sometimes be infectious and may spread the illness to others.

Q What are the symptoms of active TB?

A The symptoms of TB depend on the organ affected, and can include a persistent cough that lasts three weeks or longer, prolonged fever, night sweats, fatigue, weight loss, chest pain and coughing up blood or sputum.

Q How is active TB treated?

A This usually consists of several antibiotics taken for six months. This may last longer, depending on the location, extent and severity of the disease.

While being treated, the patient is not contagious. Once his doctor says he is fit to go back to work or school, he can resume his usual activities.

Q Can a child with active TB disease transmit it to his parents or caregivers?

A Young children with active TB disease are not considered to be infectious.

Q What if I have been vaccinated with BCG?

A All children in Singapore undergo BCG vaccinations as part of the National Childhood Immunisation Programme.

The BCG vaccine mainly protects young children against disseminated TB (disease in multiple parts of the body) and TB meningitis (disease in the fluid around the brain). The average protection is up to 90 per cent.

Sources: Professor Sonny Wang, Emeritus Consultant, Department of Respiratory and Critical Care Medicine and Director, Tuberculosis Control Unit, Tan Tock Seng Hospital; Dr Leong Hoe Nam, Infectious Diseases Physician, Mount Elizabeth Novena Hospital; Dr Chan Si Min, Consultant, Division of Paediatric Infectious Diseases, National University Hospital

A version of this article appeared in the print edition of The Straits Times on December 11, 2015, with the headline 'Very low risk of getting TB via casual contact'. Print Edition | Subscribe