Parasites

Morphology

Cytology:
Tachyzoites crescent-shaped, about 6 x 2 µm, pointed at one end, often intracellular in macrophages. Blue cytoplasm and red chromatin on Giemsa stain, often PAS negative; usually scarce and easy to miss. Tissue cysts with bradyzoites are uncommon in cytology preparations; the cyst wall and bradyzoites are typically PAS-positive.

Histology: 
CNS tissue may show vasculitis and necrotizing encephalitis with tachyzoite clusters and cysts (no pictures available), often at the viable-necrotic interface; in FFPE sections, tachyzoites may appear round to oval rather than distinctly crescentic; scattered spherical cysts, about 5 to 100 µm, are PAS positive and contain dozens of bradyzoites. 
Ocular toxoplasmosis is characterized by necrotizing retinochoroiditis with vitritis.
Lymph nodes typically show reactive follicles, epithelioid microgranulomas, sinus histiocytosis, and monocytoid B-cell hyperplasia; organisms are rarely identified on routine HE stain. T. gondii immunhistochemistry is useful.

Differential Diagnosis:

• CNS: primary CNS lymphoma, tuberculosis, Nocardia, Aspergillus spp.: bacterial or fungal abscess
• Eye: CMV retinitis, herpetic retinitis
• Lymph node: EBV-associated reactive hyperplasia, infectious mononucleosis, Hodgkin lymphoma, HIV-related lymphadenopathy, Bartonella henselae lymphadenitis (cat-scratch disease), sarcoidosis, Rosai-Dorfman disease
• Cysts: Sarcocystis spp. (larger, thicker cyst wall, often compartmentalized, cytoplasm of bradyzoites PAS negative), Trypanosoma cruzi (heart tissue, amastigotes with kinetoplast)

Clinical notes

Immunocompetent: Usually asymptomatic or presenting with mild lymphadenitis; ocular toxoplasmosis (usually posterior uveitis) may cause floaters and decreased vision.
Immunosuppressed (patients with HIV and CD4 counts below 100 cells/µL or transplant recipients): Encephalitis with ring-enhancing brain lesions, fever, headache, and focal neurological deficits.
Congenital: Hydrocephalus, intracranial calcifications, chorioretinitis.

Ancillary Testing

Serology IgG and IgM; IgG avidity to time infection in pregnancy; PCR (CSF, blood, amniotic fluid, tissue), in ocular disease PCR on aqueous or vitreous fluid; radiology (brain)

Additional Information

Lymph nodes may show a reactive pattern compatible with toxoplasmosis, with clinicopathologic correlation and, where appropriate, confirmation by serology or PCR.
In HIV, reactivation and prophylaxis are relevant considerations, particularly in Toxoplasma-IgG-positive patients with CD4 counts below 100 cells/µL.
During pregnancy, relevant exposures include undercooked meat, unwashed contaminated produce, soil, and cat litter. A recent infection is suggested by a positive IgM with low IgG avidity, with PCR used in selected settings, especially in amniotic fluid.