The patient went on to have a biopsy. 

Histology

Sections show cylindrical core biopsies displaying intense and diffuse cellular infiltrates that consist of lymphoid cells, macrophages and atypical astrocytes. There are two patterns of infiltration by the cells displaying lymphocytic characteristics: an angiocentric pattern, and a diffuse infiltrating pattern. The well-defined angiocentricity is highlighted by a prominent reticulin network, and the lymphoid cells that are involved in these structures are B cells. The diffusely infiltrating lymphocytes into brain parenchyma consist of CD3 immunoreactive lymphocytes (T cells). In addition, numerous foamy macrophages are noted, and several of these contain structures that are immunoreactive for toxoplasma antibody. Some small, well-defined structures that are consistent with toxocysts are identified. Organisms that are interpreted as representing toxoplasmosis also display staining, variably, for methenamine silver. Fungal organisms are not identified in the methenamine silver and mucicarmine-stained sections, neither are acid-fast bacilli in the ZN-stained sections.

Clonality studies showed a polyclonal population of lymphoid cells. The case was also seen in consultation [...]. The opinion was that the lymphoid infiltrates represent a very prominent reactive inflammatory infiltrate rather than represent a B-cell lymphoma. These features are consistent with immune reconstitution syndrome.

Final diagnosis:

1. Florid reactive lymphoid infiltrates, B and T cell, consistent with immune reconstitution syndrome.
2. Toxoplasma encephalitis.