APPENDIX 2

Strains of T. cruzi

T. cruzi strains are classified into schizodemes and zymodemes: schizodeme classification is based on electrophoretic mobility of kinetoplast DNA (see Gonzalez et al. 1995 for description of methodology). Electrophoresis indicates characteristic mobility in an electric field by the DNA of various strains. Zymodeme classification is based upon enzymatic profiles of parasite strains (see Miles et al. 1977, Ready and Miles 1980). Metacyclic trypomastigotes are examined in vitro for genetic variation indicated by isoenzyme analysis (Breniere et al. 1991).

Zymodeme (schizodeme) classification is useful for identifying strains of differing pathogenic potential and for distinguishing between organisms which cause human diseases and those that cause animal diseases (Breniere 1989). Genetic interpretation of zymograms of T. cruzi from various hosts over a broad geographical range (from Argentina to the United States) has revealed great genetic variability (WHO 1991:13). Supposedly, these characteristics are fairly stable in a strain, but this is not always the case.

Schizodemes or zymodemes are important classifications to determine in infected patients because different strains show affinity for different tissues and cause varied pathologies. The fact that patients in Sucre, Bolivia, have a high incidence of colonopathy may be explained by zymodemes distinct from those found in La Paz, which favor cardiopathy (Brénière et al. 1989). In highly endemic areas of Santa Cruz, patients are often infected with more than one strain. These strains may cause differing pathologies in the same individual. This implies that humans may be challenged by different strains even after developing so‑called partial immunity to one strain.

In the Amazonian basin of Brazil, zymodemes Z1 and Z3 have been isolated from sylvatic sources, principally with armadillos (Dasypus) and opossums (Didelphis) as hosts and reduviids (Panstronglyus megistus) as vectors that are associated with these mammals (Miles, Provoa, Prata, et al. 1981 and Miles, Provoa, de Sovza, et al. 1981). Z1 is also found in some domestic environments of Venezuela, where megasyndromes infrequently occur. These sylvatic zymodemes are more frequently associated with acute Chagas’ disease than are Z2 zymodemes. Zymodeme Z2 has not been identified with a sylvatic source and is the primary source of the domestic transmission cycle; it has been found in domestic situations in acute and chronic cases with cardiac and digestive symptoms being associated with megasyndromes.

Significant research on zymodemes has been done in Bolivia by investigators from a branch of Pasteur Institute in La Paz, Instituto Boliviano de Biologia de Altura (IBBA) (see Brénière et al. 1989, Tibayrenc et al. 1984). In earlier studies, Tibayrenc and colleagues collected 132 Trypanosoma cruzi stocks in southern Boliviain Tupiza and Tarijathat were characterized using enzymes. Five different isoenzymatic strains (IS) were found that are distinguishable from those found in Brazil. The incidence of IS 2 is higher (60 percent) in Tarija (altitude 6,400 feet) than it is in Tupiza (31 percent; altitude 8,528 feet), which suggests that IS 1 seems to be more frequent at high altitude and that IS 2 seems to be more frequent at lower altitudes (Tibayrenc and Desjeux 1983). Genotype frequencies demonstrated the lack of Mendelian sexuality among stocks of T. cruzi from southern Bolivia, which supports a clonal theory of propagation for trypanosomatids (Tibayrenc, Hoffman, et al. 1986, Tibayrenc, Ward, et al. 1986).

All strains were transmitted by Triatoma infestans, in contrast to Brazil, where strains were transmitted by different vectors, perhaps suggesting that different T. cruzi strains were adapting to particular vector species (Miles, Provoa, de Sovza, et al. 1981). Different strains were found in the same suburb and house in Tupiza. Forty‑four houses were examined: nine had two different isoenzymic strains, two had three different strains, and one had four different strains. The migration of triatomine bugs from house to house is important; flights of several kilometers are possible (Lehane and Schofield 1981).

In a later Bolivian study (Brénière et al. 1989), researchers at IBBA performed serological and pathological studies on 495 patients with Chagas’ disease from different areas of Bolivia. Eighty‑nine Trypanosoma cruzi strains were isolated by xenodiagnosis and characterized by twelve isoenzyme loci; they were related to the presence of cardiac changes and enteric disease with megacolon. There was high heterogeneity of human zymodemes, presenting evidence of two predominant zymodemes genetically dissimilar from each other and ubiquitous in Bolivia. Researchers observed mixtures of different zymodemes within the same patients, and there was no apparent difference of pathogenicity between the two more frequent zymodemes isolated from humans.

In the Andean northern region of Chile, a recent study was completed concerning the biochemical, immunological and biological characterization of T. cruzi (González et al. 1995). Within this region (at Quebrada de Tarapaca, discussed in Chapter 2), autopsies performed on mummies dated around A.D. 500 revealed the presence of clinical manifestations of Chagas’ disease (Rothhammer et al. 1985), which indicates a very early adaptation of T. cruzi to human habitats.

Clinical surveys indicated that in the lower Andean region of northern Chile the infection rate was low and great evidence of cardiac involvement was detected by alteration of electrocardiograms (Arribada et al. 1990). In the higher Andean region a very high infection rate was detected, but cardiac involvement was lower than that of the lower region (Apt et al. 1987). This indicates the importance of altitude factors in the T. cruzi infection causing cardiac involvement (Villarroel et al. 1991). The more benign character of Chagas’ disease detected in Chile compared to other endemic areas (Neghme 1982) is significant, either because of the T. cruzi strain circulating in each area and/or because of the ancient adaptation of the parasite to the human host in this country and particularly on the Andean highlands of Quebrada de Tarapaca (González et al. 1995:126).

Researchers found more than one type of T. cruzi parasite strain in the Chilean highlands; they also found that the different strains had higher or lower parasitemia levels. Each T. cruzi strain displayed a unique characteristic. González and colleagues (1995:131) hypothesize that tissue tropism of individual T. cruzi strains and geographic distribution of different strains and their source (sylvatic or domestic) may play a role in the wide variety of clinical signs encountered in Chagas’ disease (Rassi 1977). Conclusions from the Andean northern of Chile study are as follows:

Finally, the parasite sample studied here from humans resembles the main ones circulating in humans of other endemic areas of Chile as described before (Solari et al. 1992; Hendriksson et al. 1993; Mufioz et al. 1994), in spite of the fact that other T. cruzi populations are also transmitted by the insect vectors and their poor infective capacity in the murine experimental models. This observation probably is explained by a special adaptation of the Zymodeme 30 parasite type in human hosts from the presumably mixed infective T. cruzi populations circulating in nature. This observation and the early adaptation of T. cruzi to humans in the [Andean] highlands and other endemic areas of Chile as well, compared to other countries, could explain the more benign character of Chagas’ disease in this geographic area. The biochemical characterization shows that several T. cruzi subpopulations exist in the endemic area of Chile, but it remains to be demonstrated whether the clinical evolution of this parasitism on humans varies depending upon the infective strain involved (Gonzalez et al. 1995:132‑33)