Mycobacterium kansasii is a type of bacterial infection that can cause life-threatening symptoms in people who have compromised immune systems. People who have healthy immune systems may also be infected with M. kansasii. However, the symptoms they experience are not usually life threatening and are often limited to lung problems. In people with advanced HIV disease, M. Kansasii usually involves the lungs and can spread to other organs, including the liver, the spleen, and the bone marrow.

M. kansasii can be found virtually anywhere in the environment. They live in water, soil, foods, and a variety of animals. HIV-positive people with compromised immune systems living in midwestern and southwestern parts of the United States are at a higher risk of developing disease caused by M. kansasii.

Fortunately, the same drugs used to prevent Mycobacterium avium complex (MAC)—which should be started by all HIV-positive people with T-cells below 75—may also help prevent M. kansasii infection from causing disease. In other words, an HIV-positive person who is taking medications to prevent MAC may also be protected against M. kansasii.


What are the symptoms, and how is it diagnosed?

Breathing problems and fever are two common symptoms of M. kansasii infection, along with night sweats, chills, weight loss, muscle wasting, abdominal pain, fatigue (often caused by anemia), and diarrhea. M. kansasii can also cause enlargement of the liver and spleen, as well as the lymph nodes.

To diagnose M. kansasii infection, x-rays usually show disease of the lungs cause by the bacteria. CT scans can also be performed to take a closer look at the lungs if the x-ray does not show any telltale signs of infection. If the x-ray or CT scan shows signs of infection, sputum samples (phlegm) are collected and analyzed by a laboratory. Blood samples may also be collected to determine if the bacteria has escaped into the bloodstream and possibly spread to other organs in the body. Another test that might be necessary is a bone-marrow biopsy, to see if the infection has spread to the bone marrow. To collect a sample of bone marrow, a doctor inserts a needle into the hip bone, usually near the top of the butt or the lower back.

How is it treated or prevented?

M. kansasii is treated using a combination of drugs called antibiotics. As with HIV, in which three drugs are used to help prevent resistance and keep viral load undetectable, M. kansasii must be treated with a combination of drugs to maintain control over the infection.

It can take between two to eight weeks for a patient with M. kansasii to start feeling better upon starting treatment. Because of this, the infection is often treated in a hospital, where resources are readily available to help manage symptoms, such as weight loss, fever, and dehydration.

Almost always, M. kansasii combination therapy includes at least two of the following drugs, which are taken for two months:

  • Rifampin (Rifamate) or rifabutin (Mycobutin): Rifampin is actually the preferred choice. However, this drug does not combine well with many protease inhibitors or non-nucleoside reverse transcriptase inhibitors used to treat HIV. If you are being treated for both kansasii and HIV, a better option is rifabutin. However, the dose of rifabutin may still need to be adjusted, depending on which antiretrovirals you are taking. 

  • Ethambutol (Myambutol): This antibiotic is active against kansasii, but not powerful enough to be used on its own. As a result, it is almost always combined with either rifampin or rifabutin. 

  • Clarithromycin (Biaxin) or azithromycin (Zithromax): Both of these drugs are considered to be alternatives if either of the above options are not possible. Test tube studies suggest that these two drugs are effective against kansasii. However, there has only been a limited amount of information from clinical trials to prove this.

After two months of therapy has been completed, the patient is usually switched to isoniazid (Nydrazid) combined with pyridoxine (vitamin B6). Isoniazid is an antibiotic that is most commonly used as a treatment for tuberculosis (pyridoxine is used in combination with isoniazid to help prevent peripheral neuropathy, a possible side effect of isoniazid). The isoniazid and pyridoxine are taken every day for at least 18 months. If an HIV-positive person is diagnosed with M. kansasii, he or she may be required to continue isoniazid/pyridoxine therapy for life. In some cases, antiretroviral therapy can help improve the health of the immune system. If the immune system improves significantly, stopping isoniazid/pyridoxine therapy is possible.


It is very difficult to prevent coming into contact with M. kansasii. However, the same medications used to prevent Mycobacterium avium complex (MAC)—once-daily Clarithromycin (Biaxin) or once-weekly azithromycin (Zithromax)—may help reduce the risk of M. kansasii infection in HIV-positive people with suppressed immune systems. The risk of developing disease from MAC and M. kansasii is greatest when a person’s T-cell count falls below 50. In turn, most experts recommend starting preventative therapy—called prophylaxis—when the T-cell count falls below 75.

Are there any experimental treatments?

If you would like to find out if you are eligible for any clinical trials that include new therapies for the treatment or prevention of M. kansasii, visit, a site run by the U.S. National Institutes of Health. The site has information about all HIV-related clinical studies in the United States. For more info, you can call their toll-free number at 1-800-HIV-0440 (1-800-448-0440) or email

Last Reviewed: January 18, 2016