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Serological tests

Antigenic makeup

The kinetoplastid T. cruzi has many antigens distributed on its surface (cytoplasmic membrane and flagellum), and from the different organelles at the cytoplasm and nucleus. Some of these antigens are exposed when the parasite is circulating, as a trypomastigote, and others will be presented to the immune system after it is dead. A different set of antigens may be excreted. This parasite may also shed antigens and after some time internalize antigens that were previously exposed.

The antigenic makeup of T. cruzi from different geographical areas is rather similar, as demonstrated in several publications that used reagents made from T. cruzi circulating in the Southern Cone to diagnose serum from patients of Mexico.61 As known, T. cruzi is heterogeneous, and two main T. cruzi are recognized: T. cruzi I and T. cruzi II. The former has been isolated from silvatic regions in the United States and also from humans infected above the Amazon River. The second, now split into five groups, is found to the south of the Amazon River. A recent classification62 expanded to six groups defined as discrete typing units (DTU), from TcI to TcVI. Antigenic differences among those six T. cruzi genotypes are currently under study.

There is another trypanosome, T. rangeli, which is not pathogenic for humans but may be present in the same geographical regions as T. cruzi and may share some antigens. Several studies show that infected humans with this parasite, diagnosed by hemoculture, do not have anti-T. cruzi antibodies, at least with the high titer seen in those with Chagas infection. The reasons are probably due to the short contact with humans in their brief life spans.63,64

The other genus that has human pathogenic parasites is Leishmania, and again, some antigens may be shared. Several leishmanias are recognized to be present at the same geographical areas as T. cruzi, such as the agents of mucocutaneous leishmaniasis (Leishmania mexicana and Leishmania braziliensis) and Leishmania donovani chagasi, the American agent of visceral leishmaniasis (kalazar). Both diseases are quite different: in the former, amastigotes remain inside macrophages mainly on the skin and mucosal surfaces. The immune response is mainly by cell-mediated immunity and antibody responses are poor, making the serological diagnosis of mucocutaneous leishmaniasis difficult. On the contrary, visceral leishmaniasis is a severe disease, with a poor prognosis if not diagnosed and treated. As known, L. donovani parasites are widely distributed in bone marrow, liver, spleen, and in circulation. It is also well known that this parasite stimulates B lymphocytes in a polyclonal activation, with a huge increase in antibodies of several classes and subclasses, leading to a polyclonal hypergammaglobulinemia. As a consequence, in advanced stages, antibodies to an array of antigens are present. In relation with the diagnosis of Chagas disease, nearly any reagent used with sera from kalazar patients will be positive. It is our experience that, in laboratory terms, it is not possible to differentiate a kalazar patient, serologically, from a chagasic one. All the difference is on the clinical setup, which must be evaluated by the clinician. The kalazar patient who shows a positive serology for Chagas disease is severely ill, with fever, liver and spleen enlargement, edema, hemorrhagic lesions, and with a complete blood count that shows pronounced anemia, leukopenia, and plaquetopenia. Furthermore an electrophoresis of plasmatic proteins will show a large increase of gammaglobulin. Not one of these alterations is present in the chronic phase of Chagas disease. The laboratorial diagnosis of kalazar is confirmed by bone marrow aspiration, which should be conducted if this diagnosis is suspected. So, in this case, diagnosis should be clinical, epidemiological, and supported by the laboratory.

 
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