Mycobacterium avium complex



  • Mycobacterium avium complex (MAC) infection is caused by a group of environmental bacteria called Mycobacterium avium complex.
  • These bacteria live harmlessly in the bodies of people with healthy immune systems.
  • In persons with advanced HIV, MAC can spread throughout the body and damage tissue.
  • Symptoms vary, according to the parts of the body that are affected.
  • This disease can be effectively treated by combined drug therapy.
  • Prophylactic medication can also be taken to reduce the risk of disseminated MAC infection.

What is MAC infection?

MAC infection is caused by a related group of bacteria, together referred to as the Mycobacterium avium complex (MAC). These bacteria flourish in, for example, water, soil and the bodies of certain animals.

People with healthy immune systems can occasionally develop disease as a result of MAC infection, but this is rare. More usually, small numbers of these bacteria exist in the human body without causing any damage. This is known as “colonisation”.

However, when the immune system is severely compromised, as in advanced HIV, MAC can be an extremely dangerous infection. “Disseminated infection” is the term used for the serious condition that arises when MAC infection spreads widely through the body, causing tissue damage.

Although scientists are unsure of exactly how MAC is transmitted, it probably passes into the human body via contaminated food or drink, or by inhaling contaminated dust or soil particles. This is hard to avoid, as MAC bacteria are present throughout the environment.

MAC is unlikely to be passed directly from one person to another.

How can MAC affect people with HIV/Aids?

In the early stages of HIV infection, MAC colonisation is common. In rare cases, the disease might develop in one localised part of the body.

As HIV progresses, the risk of disseminated MAC infection increases. This severe condition is particularly common in people whose CD4 cell count has dropped to less than 50/mm3.

What are the symptoms of MAC infection?

Disseminated MAC infection can occur in the blood and in many other organs and tissues; therefore, symptoms vary greatly, according to which part of the body is affected.

Symptoms may include:

  • loss of appetite
  • diarrhoea
  • high fever
  • weight loss
  • night sweats
  • fatigue
  • abdominal pain
  • anaemia (low red blood cell count)
  • enlarged liver
  • enlarged spleen

How is MAC disease diagnosed?

The symptoms mentioned above are also characteristic of a number of non-MAC disorders. A doctor would therefore have to conduct certain tests to confirm a diagnosis of disseminated MAC infection:

  • Blood culture:s is the most common diagnostic procedure. Nutrients are added to a sample of blood in order to culture (grow) MAC, which can then be detected. This process can take several weeks to complete.

Sometimes, the doctor may need to examine or culture other kinds of tissue, depending on which parts of the body are affected. The following procedures may be performed:

  • Bone marrow aspiration: A needle is used to extract bone marrow from the hipbone.
  • Endoscopy (gastroscopy): A thin, flexible tube is inserted into the stomach or bowels to allow the doctor to view the internal organs.
  • Bronchoscopy: A similar procedure, in which a thin viewing tube is inserted into the lungs.

How is MAC treated?

MAC infection can be effectively treated, even in cases of advanced HIV.

The drugs clarithromycin and azithromycin have been proved particularly useful drugs for treating MAC infection. Your doctor will usually prescribe one of these, plus at least one other medicine, such as rifabutin, ethambutol or ciprofloxacin. Combining medicines in this way is the most effective approach. However, these drugs have side-effects, and may interact with other medications; these factors influence how they are prescribed in individual cases.

The side-effects of MAC medication vary, but may include:

  • nausea
  • abdominal pain
  • diarrhoea
  • fever
  • rash
  • liver disorders
  • eye problems

After a year of treatment, patients may stop taking medication for MAC if they are symptom-free, and their CD4 count stays at 100/mm3 or above while on antiretrovirals.

How can I help prevent MAC infection?

MAC bacteria are common in the environment, so it is extremely hard to avoid infection.

However, medication can be taken prophylactically (preventively) to reduce the chances of developing disseminated MAC infection. The drugs azithromycin or clarithromycin can be used as preventive measures by people whose CD4 cell count has dropped to below 50/mm3. These drugs also bolster the body against other non-MAC bacteria.

The good news is that, in countries where people have access to new drug treatment, the incidence of MAC infection in HIV-positive persons is decreasing. However, there is also evidence that the MAC organisms are developing increasing resistance to antimicrobial drugs.

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