Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis. It is not hereditary in the same manner as genetic disorders like cystic fibrosis. The misconception often arises because TB can spread among individuals living in close proximity, such as within households, and because a person’s genetic makeup can influence their susceptibility to developing the disease after exposure.
The Cause and Transmission of Tuberculosis
Tuberculosis is caused by the bacterium Mycobacterium tuberculosis, which primarily affects the lungs. It spreads through airborne droplets released by a person with active pulmonary or laryngeal TB. When an infected individual coughs, sneezes, or speaks, tiny infectious particles, known as droplet nuclei, become suspended in the air. These droplets can remain airborne for several hours, allowing others to inhale them and become infected.
A distinction exists between latent TB infection and active TB disease. In latent TB, a person is infected with the bacteria but does not feel sick and cannot transmit the disease to others. Conversely, active TB disease means the person is ill and can spread the bacteria through respiratory droplets. Not all forms of TB are infectious; for example, many forms of extrapulmonary TB (TB outside the lungs) are not considered contagious.
The Genetic Link to Tuberculosis Susceptibility
While tuberculosis itself is not inherited, a person’s genetic makeup can influence their likelihood of developing active disease after being infected with Mycobacterium tuberculosis. Studies, including twin studies, have indicated that genetic factors play a significant role in determining an individual’s susceptibility or resistance to TB, with heritability estimates ranging from approximately 36% to 80%. Variations in certain genes can impact how effectively an individual’s immune system responds to the bacteria.
Genes involved in the immune response, such as Human Leukocyte Antigen (HLA) genes and cytokine genes, have been linked to TB susceptibility. Variations in these genes can alter the body’s ability to contain the M. tuberculosis infection, making some individuals more prone to developing active disease. A genetic predisposition does not guarantee TB development; exposure to and infection with the bacterium are still necessary.
Tuberculosis Transmission from Mother to Infant
Transmission of TB from a mother to her infant can occur, which might be confused with heredity. An infant can acquire tuberculosis from a mother with an active infection during pregnancy or at birth. This is termed congenital tuberculosis, and it occurs when the bacteria directly transmit to the baby, not through genetic inheritance.
Transmission can happen through transplacental spread through the umbilical vein, leading to infection in the fetal liver and lungs. Alternatively, the infant may aspirate or ingest infected amniotic fluid during development or birth, causing primary infection in the lungs or gastrointestinal tract. This direct transfer of the Mycobacterium tuberculosis bacterium to the infant reinforces that TB is an infectious condition.