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Asymptomatic lung disease caused by Mycobacterium kansasii as an opportunistic infection in a patient treated with natalizumab for relapsing–remitting multiple sclerosis

Natalizumab is approved for the treatment of highly active relapsing–remitting multiple sclerosis (RRMS). Although the efficacy of this agent is considered to be high, treatment may be associated with some potentially serious infections. Progressive multifocal leukoencephalopathy (PML) with a potentially fatal outcome is the best known. Some other opportunistic infections may also occur, although literature references are rather scarce. Italian authors describe a case of rickettsiosis in a natalizumab-treated MS patient.1 While a possible association between treatment with natalizumab and infections caused by nontuberculous mycobacteria have been reported, this was based on clinical trials performed with patients with haematological diseases.2 We did not identify any publication describing a lung disease caused by Mycobacterium kansasii in a MS patient treated with natalizumab. We discuss a case of asymptomatic Mycobacterium kansasii infection that occurred during natalizumab treatment for MS. After 43 doses of natalizumab, when the MS course was stable, a pathological lesion in the upper lobe of the left lung was found on a chest X-ray during a pre-operative exam, due to planned surgery for an inguinal hernia. A previous chest X-ray prior to natalizumab treatment was normal. The patient did not experience any respiratory or other symptoms that could lead to a diagnosis of a lung disease. Mycobacterium kansasii was found in the sputum specimen repeatedly, using the MGIT (Mycobacteria Growth Indicator Tube) technique.3 Natalizumab was discontinued and we initiated a long-term treatment with a combination of anti-tuberculosis drugs. The further course of pulmonary disease was favourable, with significant regression of the cavitation in the upper lobe of the left lung, which subsequently disappeared. Nontuberculous mycobacteria are not strictly human pathogens and are not transmissible between humans. The source of Mycobacterium kansasii is environmental.4 Treatment is long-term. Three possible risk factors for the development of the opportunistic infection could be judged in our subject: