Tuberculosis is a disease caused by an infection with the Mycobacterium tuberculosis bacillus, a type of bacteria that can affect any part of the body but most commonly the lungs. It is one of the most common infectious diseases in the world today but most people have latent TB which is essentially a dormant infection and is therefore asymptomatic. However, should the ideal conditions arise, specifically diminished immune defenses, it can rapidly become an active infection.
Throughout most of the 20th century there was a decline in new TB cases in the United States largely driven by better living conditions and lifestyle factors. Towards the end of the century, there has been a slight increase in new TB cases mainly due to HIV co-infection. More recently drug-resistant strains of the TB bacillus – MDR-TB (multi drug-resistant tuberculosis) and XDR-TB (extensively drug-resistant tuberculosis) – have also posed a new problem increasing the contagious time period despite treatment. Globally, TB still remains one of the leading infectious causes of deaths particularly in developing countries and nations with a high HIV prevalence.
What is pulmonary tuberculosis?
Pulmonary tuberculosis, sometimes commonly referred to as TB of the lung or lung TB, is an infection of the lung with the Mycobacterium tuberculosis bacillus. With almost one-third of the world’s population having latent TB, the term pulmonary tuberculosis or lung TB denotes an active infection. The initial period of infection is known as the primary infection, which then progresses to the dormant phase (latent or inactive TB). Here the bacillus is sealed off from the healthy lung tissue in a tubercle. It can remain in this state for long periods of time. Eventually the active disease may develop but only in some individuals with one or more risk factors. This is almost always related to diminished immune function. The bacillus then becomes reactivated and rapidly destroys lung tissue thereby reducing pulmonary function.
Causes of Pulmonary Tuberculosis
TB is caused by the Mycobacterium tuberculosis bacillus. Humans are the only reservoir of this bacillus.
TB is transmitted from an infected person to a non-infected person by droplet spread. This means that the airborne droplets containing the bacilli may be coughed out by an infected person, passed through the air and inhaled by a non-infected person. A single cough may expel as many as 3,000 infective droplets although only about 10 bacilli are needed to cause an infection (1). The bacilli are eventually deposited into the terminal airways and then into the lung tissue. Although rare, it is possible for the bacilli to enter via the skin or gastrointestinal tract.
The airborne droplets can remain suspended in the air of a closed room for hours. Once it settles on any surface, it is difficult to become re-suspended into infective particles. It tends to attach to dust particles, which if airborne and subsequently inhaled, is usually unable to reach the terminal airways and then the lung air sacs. This, however, does not rule out the possibility of dust particles with infective droplets being able to transmit tuberculosis.
Latent TB Infection
Once the lung tissue becomes infected, a large number of immune cells known as macrophages then aggregate at this site. Fibrous tissue walls off the bacilli and it is then contained within these sites known as the tubercles. This natural protective mechanism prevents the infection from extending any further. At this point the infection is dormant (latent or inactive) – there is no continuing destruction of the lung tissue and this state can be maintained for years or even decades. In a small minority of cases associated with severely diminished immune activity, this protective mechanism fails to restrain the bacilli and the primary infection may progress into an active infection. With the formation of a tubercle, the TB bacilli cannot be transmitted to others (non-contagious).
Active TB Infection
When the protective walling off process fails to restrain the bacilli within the tubercle, it starts spreading through the lung. It destroys lung tissue (granulomatous necrosis) and causes the formation of large abscess cavities. This destruction and subsequent fibrosis reduces the functional lung capacity. This means that the destroyed lung tissue and thickening of the respiratory membrane affects the gas exchange in the lungs – ultimately carbon dioxide cannot leave and oxygen cannot enter the bloodstream as efficiently as it would in the healthy lung. The bacilli may spread to any other part of the body or the latent infection elsewhere can become active.
Although the cause and pathophysiology of tuberculosis is well understood, the reason why the latent infection becomes active in certain cases is not always as clear. Most risk factors have been conclusively identified but it does not mean that every person with one or more of these risk factors will develop TB.
- Living with a person with tuberculosis (TB).
- Health care professionals and related workers in close contact with TB patients.
- Living in crowded or closed environments.
- HIV infection.
- Malnutrition associated with any number of factors including poverty, neglect, chronic malabsorption syndromes and so on.
- Intravenous (IV) drug use.
- Poorly controlled and long term diabetes mellitus.
- Immunosuppressive treatments like TNF inhibitors and corticosteroids.
- Lung and airway diseases like silicosis.
Signs and Symptoms of Tuberculosis
There are three important phases that need to be considered when assessing the clinical presentation. The initial infection is known as the primary infection. In rare instances there are acute symptoms of an infection that are non-specific and resolve spontaneously. Primary infection is usually asymptomatic (without signs and symptoms) and then progresses to the dormant phase (latent infection). There are no symptoms during this latent period and this phase can persist for years or decades. However, in most instances of active TB, it tends to arise within 2 years of infection. These symptoms include :
- Night sweats
- Weight loss
- Lack of appetite
- Chest pain
The most common symptom of TB is a persistent cough. A cough is non-specific for TB but with the presence of other symptoms and a case history indicating risk factors, TB should be suspected. However, other radiological and laboratory investigation is needed for a conclusive diagnosis. Initially, the cough is minimally productive with small amounts of yellow to green sputum. As the condition progress, the cough becomes significantly more productive and the mucus may be blood streaked. Coughing out copious amounts of blood (hemoptysis) is seen in later stages of the disease. The chest pain may be exacerbated by or only occur with coughing.
- Tuberculosis. Emedicine Medscape
Article reviewed by Dr. Greg. Last updated on December 3, 2011