Evaluation of an adult demonstrates chronic headaches accompanied by chronic mild nuchal rigidity. Cerebrospinal fluid sampling demonstrates a chronic inflammatory infiltrate with lymphocytes, plasma cells, macrophages, and fibroblasts. Which of the following is the most likely etiologic agent?

Nuchal rigidity suggests meningitis. It is convenient to classify meningitis based on the cerebrospinal fluid (CSF) findings: 1) acute pyogenic meningitis if neutrophils are markedly increased; 2) acute lymphocytic meningitis if lymphocytes (alone) are markedly increased, and 3) chronic meningitis if lymphocytes, plasma cells, macrophages, and fibroblasts are increased. This patient has chronic meningitis. The classical cause of chronic meningitis is tuberculosis, whose etiologic agent is Mycobacterium tuberculosis. Other causes include other indolent meningeal infections such as syphilis, brucellosis, and chronic fungal infections. The granulomas that are characteristic findings in other tissues may or may not be present in the meningeal tissue, and are usually not recognizable in CSF. Tubercular meningitis is now uncommon in this country. In immigrants from third world countries, a history of pulmonary tuberculosis may be distant, undiagnosed, or deliberately concealed. Herpes virus and mumps virus are causes of acute lymphocytic meningitis. Neisseria meningitidis and Streptococcus pneumoniae are causes of acute pyogenic meningitis. Ref: Ray C.G., Ryan K.J. (2010). Chapter 27. Mycobacteria. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.