Microsporidia are ubiquitous, obligate intracellular parasites that can infect all classes of animals. Five genera—Enterocytozoon, Encephalitozoon, Septata, Pleistophora, and Nosema—and a sixth taxon, which contains unclassified microsporidia, infect humans [1, 2]. The sources and modes of transmission remain unknown [1, 2]. Microsporidia species infect the central nervous system in animals, but brain involvement in humans has not been reported [2].

A 33-year-old man presented with transient right-sided hemiparesis and left temporal headache. The patient had a 10-year history of human immunodeficiency virus (HIV) infection (CD4 count, 4 cells/mm³) and cytomegalovirus retinitis. His medications included trimethoprim-sulfamethoxazole, fluconazole, and ganciclovir. A physical examination showed wasting and psychomotor retardation. His cerebrospinal fluid protein level was 100 mg/dL. Serum and cerebrospinal fluid cryptococcal antigen and rapid plasma reantigen results were negative. Magnetic resonance imaging showed 24 lesions (sizes ranged from 0.5 to 1.5 cm), with minimal mass effect. The patient's serum Toxoplasma gondii antibody titer was less than 1:16, and his cysticercosis titer was less than 1:10. He received 1 week of empirical antitoxoplasmosis therapy, which was discontinued because of intolerance. A neurosurgical consultant believed that a biopsy was not warranted. The patient's condition continued to deteriorate; he developed more seizures and died 2 months after his initial presentation.

The autopsy was limited to the brain and showed an organism that resembled both Pleistophora and Thelohania species. The exact identification of the infecting organism awaits further analysis [3].

Toxoplasma gondii is the most common opportunistic pathogen causing focal central nervous system lesions in patients with the acquired immunodeficiency syndrome (AIDS). Infection with this organism was unlikely in our patient because of trimethoprim-sulfamethoxazole prophylaxis, a negative serologic result for T. gondii [4], and the many central nervous system lesions.

The standard approach to central nervous system lesions in patients with AIDS is empirical therapy for toxoplasmosis; refractory cases are considered for brain biopsy. A more aggressive approach to central nervous system lesions may be needed in a select subset of patients with AIDS. Earlier diagnosis might lead to administration of appropriate therapy and could prevent potential toxicities from empirical therapy. Albendazole may be used to treat central nervous system microsporidiosis. This drug is also effective for microsporidial diarrhea [5] and in other parasitic infections of the brain.

Recognition of central nervous system microsporidiosis may contribute to better management of HIV-infected patients.