Diagnosis of Trypanosoma cruzi chronic infections in humans: usefulness of the complement regulatory protein antigens and lytic antibodies in the control of cure.

Abstract

Chagas disease, highly prevalent in Central andSouth America, is caused by the hemoflagellate,protozoan, Trypanosoma cruzi transmitted mainlyby vector contamination and blood transfusion. Theparasite is found in the vertebrate host in two stages:trypomastigotes and amastigotes. The trypo-mastigotes are first inoculated as metacyclics,which invade the host mucosa or the broken skin,through contaminated feces and urine of the tri-atoma vector. They transform and divide as intrac-ellular amastigotes, the multiplicative forms, toemerge as trypomastigotes, which circulate in theblood and re-invade various cells, promiscuously.The amastigotes are intracellular but may main-tain the cycle without trypomastigotes, as demon-strated in vitro with stage specific monoclonal an-tibodies by Andrews et al. (1987).The infection has an acute parasitemic phase,which lasts 1-2 months, often with no symptoms,and a chronic pauciparasitemic phase. Parasitemultiplication at the acute phase is followed byinflammatory reactions at the site of inoculation(delay-type hypersensitvity at the eye mucosa orskin), satelity lymphadenitis, and, other acute in-flammatory reactions (fever, splenomegaly, loss ofweight, weakness, myalgia), more severe in infec-tions induced by high inocula, including in acci-dental, non-treated laboratory infections in humans.Serology is useful for diagnosis of the acute phase,based on high levels of IgM anti- T. cruzi measuredby tests with fixed epimastigotes from axenic cul-tures or with recombinant antigens specific oftrypomastigote or amastigote.Diagnosis of chronic Chagas disease (CCD)relays chiefly on conventional serology (CS) i.e.indirect immunofluorescence (IIF), ELISA, hemag-glutination and complement fixation reaction. Al-though infections are thought to be life-long, de-tection of parasites is difficult in CCD. It requiresindirect methods of parasite amplification(hemocultures and xenodiagnosis, reviewed inChiari et al. 1989, Chiari 1992), not used in clini-cal routine. Both are rather fastidious, slow, of lowefficiency and time consuming. Such techniquesare not yet replaced by modern techniques of mo-lecular biology, presently reviewed by Chiari.Blood sub-inoculations in mice (babies or adultsimmunosuppressed) is also helpful to uncoverparasites during CCD.The xenodiagnosis is the more used parasito-logical test and gives consistent results in the lit-erature, differently from hemocultures. In severaldifferent studies of xenodiagnosis and hemocul-tures in parallel, the former was almost always moreefficient (8/11 reports) as reviewed by Junqueira(1996). Its drawn back is the invasiveness repre-sented by the need of hundreds of bites when thevector feeds on the patient, and injects saliva, re-sulting in local allergies. Although such problemis easy to overcome by artificial feeding throughmembranes an additional problem is the use of se-rial xenodiagnosis, proven to increase the test posi-tivity from 30-40% in one test (Schenone et al.1974) to about 70% in three tests (Coura et al.1991).The sensibility of hemocultures varies from 0%to 98% in the literature according to the volume ofblood, the temperature for maintaining and pro-cessing it, anticoagulants and culture medium used,time of culture, i.e. 4-6 months of culture withmonthly exams are recommended before a nega-tive test is released (Galvao et al. 1989, Luz et al.1994). We selected hemocultures to follow uptreated patients. Among them one third have para-sitological tests positives, even though about 90%remained positive by CS (Cancado 1985, Ferreira1990). However, in spite of positive CS, 30-40%of such treated patients lack functional antibodies