Mycobacterium Tuberculosis Pathogenesis and its spread

Mycobacterium tuberculosis is a rod-shaped bacterium with a thick, waxy cell wall. Like other bacteria, it carries its genes in DNA. The waxy, lipid-rich wall helps the bacteria survive and multiply inside human immune cells.

This bacterium causes tuberculosis (TB) in humans. TB is a serious disease, but it can be cured with antibiotics. It can affect infants, children, adults and older people.

People with weakened immune systems are at higher risk of getting sick, including people with HIV or those on immune-suppressing treatment. The World Health Organization's TB fact sheet also lists diabetes, undernutrition, tobacco use and heavy alcohol use as risk factors.

Not everyone who is infected gets sick. In latent (inactive) TB infection, the germs stay in the body without causing symptoms, and the person cannot spread them. Without treatment, latent TB can turn into active TB disease later.

Active TB of the lungs often starts slowly. Symptoms include a cough lasting three weeks or longer, chest pain, coughing up blood or sputum, fever, night sweats, weight loss and fatigue.

TB is curable. Treatment for active TB takes 4, 6 or 9 months of medicine, depending on the regimen, and it must be finished completely. Without treatment, active TB can be fatal. Crowded living conditions, poverty and smoking raise the risk of spread and illness.

TRANSMISSION OF TUBERCULOSIS

According to the CDC, TB spreads through the air when a person with active TB of the lungs or throat coughs, speaks or sings. The germs can stay in the air for hours, especially in poorly ventilated indoor spaces.

People are most likely to catch TB from someone they spend a lot of time with. It does not spread by shaking hands, sharing food or touching surfaces.

A related bacterium, Mycobacterium bovis, can spread to people through unpasteurized milk from infected cattle.

TYPES OF MYCOBACTERIUM TUBERCULOSIS

There are two types of tuberculosis that are:

1. Primary tuberculosis

The illness that follows a first infection in a person who has never been infected before.

2. Secondary tuberculosis

Reactivation of an old, latent infection years later, or less often a new infection in someone who was infected before.

PRIMARY TUBERCULOSIS

Primary tuberculosis occurs when the bacteria enter the respiratory tract of a person who has never been infected. It is the very first phase, and it is often mild or causes no symptoms at all. When symptoms do appear, they can include:

• Fever
• Loss of appetite
• Weight loss
• Fatigue
• Restlessness
• Night Sweats
• Cough
• Upper respiratory tract infections

On a chest X-ray, primary TB usually shows patchy changes in the middle or lower parts of the lung, often with swollen lymph nodes. Cavities are more typical of reactivated TB in the upper lobes.

How Mycobacterium Tuberculosis Affects The Body?

Bacteria enter the lungs in tiny droplets breathed out by a person with active TB. In the lungs, immune cells called macrophages swallow the bacteria, but the bacteria can survive and multiply inside them.

The first area of infection in the lung is called the Ghon focus. The bacteria then travel to nearby lymph nodes. The Ghon focus and the affected lymph nodes together are called the Ghon complex.

The immune system walls off the infection with inflammatory cells, forming small lumps called granulomas. These areas can scar and harden, and the scars can sometimes be seen on X-rays.

In most people, the immune system keeps the bacteria in check. If it doesn’t, or if the infection is left untreated, TB can spread and affect almost any part of the body. It can affect:

1- Tb In Bones

If it infects the bones, known as Skeletal Tuberculosis. In primary infection, it spreads via blood or lymphatics to the bones or joints. It often affects the growing ends of long bones, which have a rich blood supply. The infection destroys bone tissue.

The spine is the most commonly affected site, a condition called Pott disease. The lower thoracic and upper lumbar vertebrae are most often involved. The infection usually destroys the front of the vertebral bodies and the disc between them, so X-rays often show a narrowed disc space and collapsed vertebrae.

Clinical Features

Patients having skeletal Tb presented with the complaints of

• Restlessness
• Fatigue
• Postural disturbance
• Abnormal gait
• Low-grade fever
• Pain

2- Tb In Lungs

Mycobacterium can spread to many parts of the body, but it most often affects the lungs, the site of the first infection. Reactivated TB most often affects the upper lobes, which have the most oxygen.

Many lung infections stay latent and cause no symptoms or X-ray changes. A tuberculin skin test or a TB blood test can show that a person has been infected.

Clinical Features

Children with active TB of the lungs often have:

• Fever
• Loss of appetite
• Weight loss
• Cough
• Upper respiratory tract infection
• Night Sweats
• Fatigue
• Irritability
• Malaise

On X-rays, the lymph nodes near the center of the chest (hilar nodes) are often swollen, and patches of infection may appear in the lung. If the lymph nodes press on an airway, part of a lung can collapse or become infected. The child may then have fever, cough, trouble breathing and crackling sounds in the lungs.

3- Tb In Systemic circulation

From the early lesion, the Ghon complex, the bacteria can multiply and spread through the bloodstream. This causes miliary tuberculosis, named for the tiny seed-like spots it leaves in the lungs and other organs.

Miliary TB can affect many organs at the same time. In children, it often appears within a few months of the first infection.

Clinical Features

Miliary Tuberculosis is manifested by the following symptoms:

• Low-grade fever
• Anorexia
• Weight loss
• Malaise
• Generalized lymphadenopathy
• Hepatosplenomegaly
• If it infects the brain, then headache is the complaint of meningitis.

If it infects the abdomen, abdominal pain and tenderness are the signs of peritoneal tuberculosis.

On chest X-rays, many tiny spots are spread through both lungs. Some patients also develop fluid around the lungs (pleural effusion) or trouble breathing.

4- Tb In Kidney

It is rare in children because it takes many years to develop. If the bacteria spread through the blood or lymph, they can cause renal Tuberculosis. The bacteria reach the kidney and multiply there. The infection may spread to the ureter, bladder, prostate and epididymis.

Clinical Features

Kidney TB may cause no symptoms for a long time. Signs can include:

• Sterile pyuria (white blood cells in the urine with a negative routine urine culture)
• Hematuria
• Flank pain
• Dysuria

It is diagnosed with special urine cultures that test for TB bacteria.

DIAGNOSIS OF MYCOBACTERIUM TUBERCULOSIS

Following tests are done to diagnose tuberculosis effectively

• Contact History
• Tuberculin skin test or TB blood test
• Chest X-ray and other imaging
• Sputum smear, culture and molecular tests for TB bacteria
• Biopsy

If you have a long-lasting cough, night sweats or unexplained weight loss, or you have been around someone with TB, see a clinician for testing.

HP Thoughts: Trouble breathing can also come from other lung conditions, such as asthma. A severe asthma attack is a medical emergency, so call 911 if breathing gets very hard. Learn more about asthma: Asthma Attack.

Sources

• About Tuberculosis, CDC
• Treating Tuberculosis, CDC
• Tuberculosis fact sheet, World Health Organization