Page 109 - AIDSBK23C
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Page 109




               TOXOPLASMA GONDII INFECTIONS

                       Toxoplasmosis is an uncommon infection that, before the AIDS epidemic, was rarely
               seen in adults.  It is more common in warm humid climates, and this distribution may influence
               its appearance in AIDS.  Toxoplasmosis can occur perinatally as a congenital infection in the
               absence of HIV infection.[475]  Ingestion of poorly cooked meat (usually pork) is a principle
               form of transmission in adults, though ingestion of food or water contaminated with T gondii
               oocysts is also an important route of infection.[476]
                       T gondii can invade virtually all tissues of the body, but in AIDS patients, the organ
               system distribution of T gondii infection is generally not widespread.  The central nervous
               system is involved in most cases.  Extracerebral toxoplasmosis is more likely to occur later in the
               course of AIDS with a greater degree of immunosuppression when the CD4 lymphocyte count is
               low.  Extracerebral sites for T gondii in AIDS are most often eye and lung, with heart and
               gastrointestinal tract involved much less often.  Other organs are infrequently involved, with
               reticuloendothelial tissues occasionally affected (Table 5).[477]
                       The clinical appearance of toxoplasmosis is typically that of altered mental status from
               central nervous system involvement.  Headaches, fever, and focal neurologic deficits may occur.
               Diagnosis may be suggested by elevated serologic titers, but many persons have antibodies to T
               gondii because of subclinical infection.  Serologic titers give no indication of dissemination.  The
               presentation of cerebral toxoplasmosis may appear quite similar to that for non-Hodgkin
               lymphoma, and stereotaxic brain biopsy may be useful for diagnosis.  Extracerebral
               toxoplasmosis may sometimes be diagnosed by bronchoalveolar lavage or endoscopic
               biopsy.[477]
                       The gross appearance of toxoplasmosis is not distinctive.  In the brain, the diagnosis is
               suggested by finding multiple small areas of necrosis or cystic change, while in the heart, a
               patchy parenchymal myocarditis with tan to white irregular infiltrates may occur in severe cases.
               In other organs, there are no specific features and grossly visible lesions may not be apparent.
                       In biopsy material, diagnosis is best made by finding characteristic cysts filled with the
               organisms--dubbed bradyzoites in this location.  The cysts may be "true" cysts formed only by
               the T gondii, or they may be "pseudocysts" that form within an existing cell and use the cell wall
               as a cyst wall.  Cysts average 50 microns in size.  Free T gondii organisms, called tachyzoites,
               are 2 to 3 microns wide and are often difficult to distinguish, with hematoxylin-eosin staining,
               from background cellular debris.[401]  The sexual cycle of T gondii occurs in the definitive host,
               the cat, where oocysts form in the intestine and are excreted into the environment to be ingested
               by other animals or man.[476]
                       Encysted T gondii usually produce no or minimal inflammatory reaction, but serologic
               titers may increase.  However, rupture of the cysts with release of T gondii as free tachyzoites
               does produce a host response.  The tachyzoites are too small to be morphologically distinctive by
               hematoxylin-eosin staining in most tissue sections.  Immunohistochemical staining may aid in
               finding not only the cysts, but also in identifying free tachyzoites.[475]
                       The inflammation that accompanies the cysts and free tachyzoites is usually mixed, with
               neutrophils, lymphocytes, macrophages, and plasma cells in varying proportions.  These mixed
               inflammatory cell infiltrates occur in a patchy pattern within involved organs.  Even though
               inflammation may be extensive, finding cysts is still difficult, though the greater the degree of
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