My father-in-law was diagnosed with Mycobacterium avium-complex. His doctor didn't put him on any medication but recommended he quit smoking. I am confused because when they originally suspected he had TB, they put him on meds immediately. Is MAC that different from TB?

T.T.

Mycobacterium avium-complex (MAC) is a bacterium that is closely related to Mycobacterium tuberculosis (MTB), the cause tuberculosis. Unlike MTB, MAC is found nearly everywhere in the environment. It is not a particularly virulent (strong) microorganism, and generally only people at risk, such as those with AIDS or an underlying lung disease, become infected. Typically, the disease is not spread from person to person, and it is not a public-health threat.

MAC causes two main types of disease. The most common nowadays occurs in patients with AIDS. The bacterium infects the blood and internal organs, causing fever, weight loss and even death. Fortunately, this serious infection has become less common in the past few years as better therapies for HIV have become more widely used.

The second type of infection is a lung illness that is similar to but usually not as severe as tuberculosis. The infection causes a chronic pneumonia with fever, cough, weight loss and night sweats. Most commonly, this illness strikes people who already have chronic lung disease, usually emphysema and chronic bronchitis.

When someone has a lung infection and is tested for MTB, MAC is sometimes found. Both organisms look the same under the microscope, so additional lab tests are required to distinguish the two. Since tuberculosis is more dangerous both to the patient and to others, we will usually treat for that illness until further testing shows that the organism is MAC. This is why your father-in-law was initially treated for tuberculosis and why the medicines were stopped later.

Treatment for MAC is a tricky subject. First, a lot of people have MAC in their lungs but do not have any disease -- they have so-called MAC "colonization" rather than infection. Therefore, just because someone has a lung infection and has MAC in their lungs, it is not necessarily the case that the MAC is causing the lung infection. According to the American Thoracic Society, to make the diagnosis of MAC infection, the organism should be found in lung secretions more than once and in large amounts. Other illnesses must be also be ruled out. Once MAC infection is diagnosed in this manner, treatment usually consists of multiple drugs taken for a long time, usually several months. Most people respond well to treatment.

In your father's case, you need more information from his doctor. Does your father have MAC infection or MAC colonization? If it's an infection, then you should discuss treatment. But usually, the underlying lung disease is causing the symptoms, and MAC became an issue only because it looks like MTB under the microscope.