Diagnostic Tests


In 1909, when Carlos Chagas took a sample of the child Rita’s blood and examined it under the microscope, he saw trypanosomes. He was able to find them because they were abundantly present at this time, although, generally speaking, it is more difficult to find them in a blood sample. Trypanosomes of T. cruzi can be found in patients during the acute phase of Chagas’ disease by direct examination of the blood and later by centrifugation of clotted blood, xenodiagnosis, and animal inoculation. One problem is the possible misidentification of parasites in examination of the bloodfor example, the spirochete of syphilis has been confused with T. cruzi. If T. cruzi is reported in blood samples, retesting is recommended. Tables 3 and 4 concisely provide a list of the different tests and their validity.

Much less popular, but still common, xenodiagnosis appears to be a technique from the Middle Ages akin with the use of leeches; but it is an effective available resource. Uninfected vinchucas are placed within a jar, tucked under the patient’s armpit, and allowed to consume blood for thirty minutes. Their feces are examined thirty and sixty days later for T. cruzi. Obviously, xenodiagnosis has its problems, primarily, obtaining uninfected bugs. This technique is rarely used on children for obvious reasons. Many adults also have phobias and would rather go untested. However, sometimes medical examinations are as painful as the illness. Xenodiagnosis can be an excellent examination for the determining of parasite populations and strains.

Indirect tests that look for T. cruzi antibodies have recently been devised, but they usually are not used during the acute phase because the immune system is in the process of producing chagasic antibodies. Similar to that used for AIDS, an ELISA test has been designed to detect the presence of T. cruzi antibodies; however, the chagasic ELISA sometimes fails to differentiate antibodies of T. cruzi from those of either T. rangelii (a harmless cousin) or Leishmania braziliensis (the causative agent of the mucocutaneous form of leishmaniasis, common in Andean regions where Chagas’ disease is also found). Chronic patients should be tested both ways, even when ELISA comes out negative, to determine the rate of infection. People without noticeable symptoms living in chagasic areas are encouraged to have an ELISA test. If it comes out positive, xenodiagnosis is encouraged to determine the nature of the infection and form of treatment. The Ministry of Public Health and IBBA in La Paz, Bolivia, provide these tests for Bolivians at a reasonable cost (five dollars in 1997).


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