Tuberculosis and HIV: Background Information

Tuberculosis (TB) is caused by bacteria called Mycobacterium tuberculosis complex and usually affects the lungs. However, among people co-infected with TB and HIV, parts of the body outside the lungs are often affected; this is called extra-pulmonary TB.

Summary

The germs that cause TB are spread when a person inhales tiny droplets produced by people who have infectious, or active, TB of the lungs. Infection with TB-causing germs can result in one of two forms of infection:

• latent TB infection: a person does not have symptoms and is not infectious
• active TB infection: a person has symptoms of illness and can spread TB-causing bacteria

If left untreated, TB can be fatal. However, timely treatment, consisting of combination therapy with antibiotics, can cure both latent and active TB. Having had TB in the past -- either the latent or active form of the disease -- does not protect a person from future infection with TB-causing bacteria.

Exposure

TB-causing bacteria are most commonly transmitted when someone inhales tiny droplets released into the air by an infected person during the following activities:

• coughing
• sneezing
• speaking

These tiny droplets are invisible and can remain floating in the air for several hours after they have been released.

Transmission of TB-causing bacteria from an infectious person depends on many factors, including the following:

• how close or for how long a person was exposed
• the degree of infectiousness of the person with TB
• the type of shared environment in which contact took place

Studies have found that people with TB whose sputum (the thick mixture that people with chest infections produce when they cough) contains TB-causing bacteria are likely to transmit these bacteria. Also, people with cavities in their lungs caused by TB are very infectious.

In high-income countries such as Canada, HIV-positive people exposed to TB-causing germs are prone to have TB-causing bacteria affect other parts of the body, including the following:

• the membranes surrounding the brain
• lymph nodes
• spleen
• liver
• kidneys
• bones

From Exposure to Infection

The risk of a person developing active disease from TB-causing bacteria after exposure depends on many factors, including the following:

• their overall health
• age: older people tend to have weaker immune systems
• among HIV-positive people, the lower the CD4+ count, the greater the risk of developing TB

In general, after exposure to TB-causing bacteria, disease does not immediately result. Instead what usually happens once the bacteria enter the lungs is that they are captured by cells of the immune system. Sometimes the immune system is able to destroy these bacteria. At other times the bacteria are able to subvert the immune system's defenses and go on to infect cells of the immune system. Once this happens, the infection can begin to spread slowly. For most people, at least initially, their immune system, if it does not kill the bacteria, is able to put the infection into a latent state. However, people whose immune systems are weakened by the following factors are at risk for ultimately developing active TB:

• HIV infection
• alcoholism
• diabetes
• injecting street drugs
• use of transplant drugs
• smoking tobacco

Two Forms of TB

TB infection can result in two different states -- latent infection (which is symptom free) or active infection.

Latent TB

People with latent TB do not have symptoms; tests are needed to help assess their condition. To help diagnose latent TB, nurses can inject purified protein from TB-causing bacteria just under the skin (this does not cause infection). People with latent TB are very likely to have a reaction -- usually a swelling or bump at the injection site within 48 to 72 hours.

The test is imperfect, and in people with severely weakened immune systems there may not be a reaction. In other cases, people infected with bacteria closely related to those that cause TB might develop a reaction. In some clinics, a blood test may also be used to help identify people with latent TB. These tests check for the presence of a chemical signal -- interferon gamma -- produced by the immune system.

Active TB

The symptoms of active TB can vary depending on the person and the organ-system affected. However, the following symptoms tend to be relatively common:

• fever
• night sweats
• unintentional weight loss
• swollen lymph nodes

In cases where TB has affected the central nervous system (CNS) -- the brain and spinal cord -- additional symptoms might include the following:

• severe headache
• confusion
• extremely stiff neck

In cases where TB-causing bacteria have spread throughout the body, the following symptoms may be present:

• night sweats
• loss of appetite
• weakness

In cases where TB has affected the lungs, the following symptoms can occur:

• persistent coughing
• coughing up blood
• chest pain

When examining a person with active TB, doctors may find that the following parts of the body are swollen:

• liver
• spleen
• lymph nodes

Doctors rely on several aids to help them diagnose active TB, including the following:

• X-rays
• examining fluid taken from an affected lymph node or organ under a microscope
• attempting to grow TB bacteria in the lab from fluid samples taken from a swollen lymph node or affected organ