Mycobacterium avium Complex (MAC)

November 2004     View PDF     En español

Prevention

It is difficult to avoid contact with MAC bacteria, but there are some ways to reduce the risk.

• Boil drinking water. MAC bacteria are found in most water systems, hospital water supplies and bottled water.
• Do not eat raw foods, especially salads, root vegetables and unpasteurized milk and cheese.
• MAC bacteria are killed at 176°F, so they are destroyed during normal cooking.
• Rinse and peel fruit and vegetables thoroughly.
• Avoid contact with animals, especially birds and bird droppings. Pigeons—common in most cities—can also transmit cryptococcosis, an OI affecting people with HIV.
• Avoid or reduce alcohol intake. Drinking alcohol regularly can help MAC infection to spread more quickly in people with HIV.

Drugs can be used to prevent or delay the onset of MAC in people with HIV. Three drugs are approved by the FDA for MAC prevention: azithromycin, clarithromycin and rifabutin. The decision to start MAC prevention should take into account possible drug side effects and interactions with other drugs. In addition, MAC bacteria may become resistant to a drug when it is used alone for preventing the disease.

The Public Health Service recommends that people with CD4+ cell counts below 50 should take either clarithromycin (500mg twice a day) or azithromycin (1,200mg once a week) for preventing MAC disease. Rifabutin (300mg/day) should only be used if clarithromycin or azithromycin cannot be taken. There is a risk of developing resistance to clarithromycin, so azithromycin might be the best first choice.

Studies have shown that clarithromycin can reduce the chance of developing MAC by 70%. Another study showed that outbreaks of MAC disease were reduced by 65% in people who took azithromycin. Both drugs also protect against bacterial infections in the lungs and airways. Rifabutin can reduce the risk of developing MAC by 30-50%. Combinations of these drugs have been tested for MAC prevention, but in all cases increased side effects and cost outweigh any added protection from disease.

One worrying outcome of these studies was that some people develop resistant MAC. MAC bacteria were three times as likely to become resistant to clarithromycin taken alone than to azithromycin. Taking clarithromycin with rifabutin did not decrease the risk of resistance to clarithromycin.

As clarithromycin is often the drug of choice to treat MAC, some doctors choose not to use it for MAC prevention. That way, it is still an option for treating the disease later if necessary. However, the chance of having an outbreak of MAC disease is very low when on preventive therapy with clarithromycin or azithromycin, so resistant bacteria are quite rare.

No studies have yet compared clarithromycin and azithromycin for MAC prevention. The two drugs are very similar, so developing resistance to one is likely to result in some level of resistance to the other (cross-resistance). Rifabutin belongs to a different class of drugs, so there should be no cross-resistance between rifabutin and either azithromycin or clarithromycin.

It’s important to make sure that you do not have disseminated MAC disease before you start preventive therapy, as this may lead to drug-resistant bacteria. It is also essential to have a chest X-ray and tuberculosis (TB) skin test. The bacteria that cause MAC and TB are related. Some drugs used to treat MAC are also used for TB. If you have active TB, using a single anti-MAC drug may lead to resistant TB. This is most important before starting MAC prevention with rifabutin. Rifabutin is closely related to rifampin, a drug used to treat TB, and cross-resistance (where resistance to one drugs results in resistance to another) can develop.