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On-line version ISSN 1678-9849
Rev. Soc. Bras. Med. Trop. vol.30 n.1 Uberaba Jan./Feb. 1997
BIODEMES AND ZYMODEMES OF TRYPANOSOMA CRUZI STRAINS: CORRELATIONS WITH CLINICAL DATA AND EXPERIMENTAL PATHOLOGY
Sonia G. Andrade and Juracy B. Magalhães
With the objective of establishing biological and biochemical characteristics of a significant number of Trypanosoma cruzi strains from different geographical areas, 138 strains isolated from naturally infected humans, triatomine or vertebrate hosts were studied; 120 were isolated from different areas of Brazil and 18 from other South and Central American countries. Inocula from triatomine or culture forms were injected into suckling Swiss mice, followed by passages into mice 10 to 12 g. Biological characters and histopathological study permitted the inclusion of the strains into three Types or biodemes: I, II, III. Isoenzymic analysis confirmed a correspondence between the biodemes and zymodemes : Type I and Z2b, Type II and Z2, Type III and Z1. Results showed the ubiquitary distribution of the several types of strains. The predominance of the same Type and zymodeme in one geographical area was confirmed : Type II strains among the human cases from eastern Bahia and east of Goiás; Type III strains from humans of north Brazil and Central America and from silvatic vectors or vertebrates from other geographical areas. The biological types of strains correlate with different histopathological lesions considering cardiac involvement and neuronal lesions. These findings suggest that the biological behavior together with isoenzymes patterns and pathological pictures in the vertebrate host can be an important tool for establishing correlations between strains behavior and clinico-pathological manifestations of Chagas' disease in different geographical areas.
Key-words: Trypanosoma cruzi strains. Geographical distribution. Biodemes. Zymodemes. Pathology of Chagas'disease.
Trypanosoma cruzi strains are complex multiclonal populations that differs in their genetic and biological characteristics and in their behavior in the vertebrate host. According to Thompson and Lymbery36 if one considers the extensive genetic heterogeneity within species of protozoan and metazoan parasites, a "strain" is not only genetically differentiated from another populations but also differs in one or more characters of epidemiological significance. The concept of T. cruzi strains fits well with this general view. Although genetical studies are important to clarify the intraspecific heterogeneity of the parasite, only the study of the biological behavior and the host-parasite relationships could clarify the importance of different strains, in the determination of clinico-pathological manifestations of Chagas' disease. T. cruzi strains represent subspecies based on intrinsic characteristics such as antigenic composition4, morphology14, susceptibility to chemotherapy7 15, isoenzyme patterns12 16 and the genomic profiles of DNA kinetoplast26 as well as in the host-parasite relationship1. An extensive study of the biological characteristics of the natural strains and the histopathological profile in experimental animals has disclosed the possibility of grouping them into a few well defined Types or biodemes1 2. Different patterns of behavior classified the strains into the Types: I, II and III that correspond to specific zymodemes9. The distribution of the diverse biodemes in different endemic areas is important to clarify their influence on the local manifestations of Chagas' disease.
In the present study 138 strains of T. cruzi are analysed in an attempt to: 1) correlate biological characteristics with zymodemes patterns; 2) evaluate pathogenicity and histopathological patterns of lesions in mice ; 3) identify the distribution of the Types of strains and zymodemes in South and Central America.
MATERIAL AND METHODS
Isolates of T. cruzi were received from different areas of Brazil and from other South and Central American countries either in triatomines used for xenodiagnosis or in acellular culture medium. Before inoculation, they were washed three times by centrifugation in PBS, pH 7.2. Suckling Swiss mice were initially inoculated and strains then cultivated in Warren medium. Some strains failed to infect mice, hampering their biological characterization. Seven of them were characterized only by isoenzymic profiles. The present survey refers to 138 strains from different geographical areas, as follows: 120 from different regions of Brazil: north (5), northeast (41), central west (64), central south (3) south (7); l5 strains were from other South American countries: Argentina (6), Bolivia (3), Chile (4), Colombia (1), Perú (l); 3 strains from Central America, Honduras (2) and Guatemala (1). Details about the origin of the strains are shown in Tables 1 and 2.
Biological characterization. Inocula (105 blood forms) were injected intraperitoneally into mice weighing 10 to 12g. Parasitemia were evaluated daily by microscopic examination of peripheral blood. Morphology: evaluation of the percentage of broad and slender forms were performed on the 7th, 10th, 14th day post infection. Histopathological study during the acute phase was performed on the 7th, 10th, 14th, 20th and 30th day after inoculation (three mice on each day) and in the chronic phase, with 150 and 180 days of infection. Paraffin sections (5m thick) of heart, skeletal muscle, liver and spleen, stained with hematoxylin and eosin were examined by optical microscopy. For each strain this methodology was repeated in three different passages into mice. The strains were classified into three biological Types or biodemes (I, II, III) as previously described1.
Isoenzymic characterization. The parasites were cultured in Warren medium, for obtention of enzymic extracts, according to Miles et al23. The following enzymes were investigated: aspartate aminotransferase (E.C.220.127.116.11. ASAT); alanine aminotransferase (E.C 18.104.22.168. ALAT); phosphoglucomutase (E.C.22.214.171.124. PGM); and glucosephosphate isomerase (E.C.126.96.36.199. GPI). Thin-layer stach-gel electrophoresis was performed by application of 30V/cm, during 90 minutes for ALAT and 60m for ASAT and of 20V/cm during 150m for GPI and 120m for PGM. The enzymes ALAT and ASAT were developed with phosphate buffer solution 0.1M and ßNAD, examined by ultra-violet light; for the enzymes GPI and PGM, TRIS/HCl buffer solution 0.3M and NADP were used besides the MTT (dimethilthiazole 2-yl 2-5 diphenil tetrazolium bromide) 0.36mM, agar-gel 0.06% and phenazine metasulfate, 0.03mM. As control of the isoenzyme characterization the prototypes of each of the three morphobiological patterns were included on each electrophoretic run: Peruvian (Type I), 12 SF (Type II) and Colombian (Type III). The nomenclature here used for the zymodeme patterns is based on that stablished by Miles et al23 and the genetic variant Z2b described for the Chilean strains by Miles et al24 with a three banded GPI pattern that corresponds to Bolivian Z2 described by Tibayrenc et al33.
In the present study the basic characteristics of the three biodemes were taken to classify all the studied strains, as previously published2. Briefly: Type I - macrophagotropism in the initial phase of infection, high virulence with 100% of mortality within 12 days, maximum parasitemia on the 7th to 12th days and predominance of slender blood forms in the initial phase of infection; Type II - myotropic, especially involving the heart, predominance of broad forms but with a percentage of slender forms in peripheral blood, parasitemic peaks from 12 to 20 days when mortality reaches a maximum; this type of strain can presents low, medium or high virulence; Type III, myotropic strains, mainly parasitism of skeletal muscle, predominance of broad forms, parasitemic peaks from 25 to 30 days or later, low mortality within 30 days.
Zymodeme patterns for all strains were identified as Z1, Z2 and Z2b based on the profiles of GPI, PGM, ASAT and ALAT (Figures 1 a, b and 2 a, b for the prototypes). A concordance was detected between the biological type and the zymodeme: Type I corresponded to zymodeme Z2b, a variant of Z2. Some strains with this zymodeme (Z2b) did not show the high virulence characteristic of the Type I strains (SC-44 from Santa Catarina and another from Argentina). Type II strains corresponded to Z2. Type III strains to Z1. For each biodeme a characteristic histopathological picture in acutely infected mice was disclosed as previously described1. Macrophagotropism was evident for the Type I strains (Figure 3), together with intense cardiac lesions. The Type II strains determined intense cardiac parasitism and myocarditis. Type III strains determined predominant skeletal muscle lesions and also cardiac parasitism and myocarditis. In chronically infected mice, with the three types of strains a variable degree of inflammatory lesions in the myocardium were present, from mild to intense. Cardiac lesions were more frequent and more intense in those infected with Type III strains (Figures 4 a, b). Involvement of the myoenteric plexus was more evident in mice infected with the Type I and II strains (Z2b, Z2), with the presence of amastigotes, inflammatory infiltration and neuronal cells destruction (Figures 5 a, b).
Results of the biological and isozymic characterization of the 138 strains are summarized in Tables 1 and 2. Table 1 shows the exclusive presence of Type III, Z1 strains in the north and the predominance of Type II, Z2 in the east of Bahia State for the strains from patients of São Felipe (Table 3). In the central west region of Brasil (Table 1) two areas deserve consideration: east of Goiás State (Mambaí) where 28 strains from human cases (Table 4) disclosed Type II, Z2 and north of Minas Gerais State (Montalvania) and west of Bahia from where 32 strains isolated from human cases (Table 5) were either Type II, Z2 (17 strains) or Type III, Z1 (15 strains). From the south of Brazil (Table 1) of the four strains from Santa Catarina State, three were classified as Type III, Z1 - P. megistus (2) and opossum (1) and one strain from an opossum was classified as Type I, Z2b, although disclosing a low virulence for mice. Table 2 shows the distribution of the three types of strains for various countries of South America. Of the six strains from Argentina, the CA-I and RA strains described by Gonzalez Cappa17 18 were classified as Type III (Z1) and Type II (Z2) respectively; other strains from Corrientes did not infect mice and showed the zymodeme Z2b. Strains from Bolivia, Chile, Colombia, Peru, were classified into the three types (I, II, III) and zymodemes (Z2b, Z2, Z1). From Central America (Table 2) the 3 strains were included into Type III, Z1.
As postulated by Thompson and Lymbery36 parasite strains must be described by a combination of genetical and biological characteristics since reliance on the genotype alone may confer significance on a feature of little biological relevance. Concerning T. cruzi strains, genetical studies based on isoenzyme patterns have demonstrated the multiclonal structure of natural parasite strains33 35. Intra-zymodeme variability has been demonstrated by the genetic distances or expressed by different alleles in the same zymodeme12 23 34. However, we admit that stable populations may represent the equilibrium of multiple clones with predominance of a characteristic biological behavior. Experimental evidences of the stability of strain behavior has been obtained by biochemical and biological characterization after parasites were passed through different conditions of maintainance and cultivation21; strains obtained from mice submitted to treatment with different drugs and not cured, which supposedly submitted them to pression by clonal selection, also maintained their biological behavior and isoenzyme profiles22.
Studies with cloned populations of natural strains have demonstrated either homogeneity or heterogeneity of several clones13 including differences in virulence and pathogenicity27. However, in an epidemiological study, cloned populations can not be taken as representing the strains isolated from different geographical areas. Only "natural" strains can be taken as representatives of the epidemiological profiles of those areas. The present study confirms previous observations indicating the predominance of one type of strain in the same geographical area1, showing a large distribution of Type II, Z2 strains in Brazil. The zymodeme corresponding to Type I strains has been first described by Andrade et al9 in 1983 and after that in Bolivia34 and Chile24 being designated as a variant of the Z2 (Z2b); it is rarely found in Brazil being represented by the Y strain and the strain SC-44 isolated from an opossum from Santa Catarina; this same zymodeme has been identified in one case of congenital transmission of Chagas' disease in Bahia11. The type III strains (Z1), associated with the silvatic cycle23, has also occurred in human patients in north and northeast states of Brazil. In Montalvania, MG and neighboring localities of west central Brazil20 an overlap between the silvatic and domiciliary transmission cycles determined the concomitance of Types II and III in the same geographical area. Schlemper Jr.30 studying 23 strains from Minas Gerais (south central region of Brazil), detected in all of them the same zymodeme A as described by Romanha28, corresponding to Z223. In southern Brazil especially in Santa Catarina, where the parasite was isolated from silvatic vectors or vertebrate hosts with no human cases being registered, Type I (Z2b) and III (Z1) strains were identified, confirming data of Steindel et al32. The small number of strains from other countries of South and Central America was not sufficient to evaluate the distribution of the strain Types and zymodemes, but confirmed findings from other authors, such as Apt et al10 in Chile and Tibayrenc et al34 in Bolivia, who registered the presence of Z2 and genetic variants in the domestic cycle and Z1 in the silvatic reservoirs; in the present study the three biodemes (I, II, III) have been detected in the several countries. In Central America as well as in northern Brazil, Type III, Z1 strains were predominant.
The biological and biochemical characteristics of strains are correlated with different tissue lesions, as first observed in acute infection of mice with strains of different Types1 2. In the chronic phase, a clearcut influence of the biological type of strain on the histopathological lesions has also been detected. In a previous study of 200 chronically infected mice3, it was demonstrated that the Type III strains (Z1), were the most pathogenic, determining intense cardiac and skeletal muscle lesions with patent parasitism of tissues even in a late stage of infection; electrocardiographic alterations were more frequent and intense in mice chronically infected with Type III strains29. Cardiac lesions in the chronic phase occurred also in mice infected with Type I and II strains. Besides cardiac lesions, these two types of strains determined significant involvement of neuronal cells of the myoenteric plexus as previously described for the strains of S. Felipe - Bahia, Type II1 and for the Y strain Type I5. A quantitative study of neuronal cells in myoenteric plexus during the chronic phase of infection with the Y strain has shown a significant decrease, as compared with the Colombian strain (Type III)6. Recently31, segmentar inflammatory alterations in the ganglionic cells of the autonomic nervous system has been detected with the three types of strains showing that the inflammatory lesions caused by the Y strain (Type I) are more destructive to the neuronal tissue than that caused by the other two strain types. Taking into account that the zymodeme Z2b (Type I) is genetically closely related to Brazilian Z2 (Type II), probably the two Types are related to identical pathologies in chronic disease, particularly considering the neuronal destructions in myoenteric plexus. The Zymodeme 2 has been identified by Lauria-Pires19 in one stock and several clones, from a patient with the digestive form of Chagas' disease, a direct demonstration of the participation of this zymodeme, corresponding to Type II strain in the pathogenesis of megasyndromes in Chagas' disease.
The ubiquitary distribution of T. cruzi strains cause difficulties to interpret the different manifestations of Chagas' disease and their correlation with strains variability. It is conceivable that the predominance of the same biodeme and zymodeme in well defined endemic areas can be related with the main manifestations of the disease in these areas. The histopathological evidence that cardiac lesions occurred with the three types of strains, correlates well with the occurrence of cardiopathy as the main manifestation of human Chagas' disease anywhere this parasitic disease is endemic. A similar correlation could also be made with the megasyndromes in areas in which the types I and II strains (Z2b and Z2 zymodemes) are predominant, and their absence in areas without these biodemes. The transfer of experimental data to the interpretation of human disease is always difficult. However, a direct correlation betwen the strains behavior in humans and in experimental mice was observed in the response to chemotherapy in strains isolated from patients from Montalvania, classified into Types II and III, with a concordance in 81 per cent of the cases8. This is an evidence that experimental data can contribute to the understanding of the human disease.
Therefore, experimental data correlating the biological behavior, histopathological pictures and zymodeme patterns confirms epidemiological evidences indicating an influence of parasite strains on the histopathological lesions2 and clinical presentations25 of Chagas'disease in different geographical areas.
Foram estudados os caracteres biológicos e isoenzimáticos de 138 cepas do Trypanosoma cruzi de diferentes áreas geográficas, sendo 120 do Brasil e 18 de outros paises da América do Sul e Central. Camundongos recém-nascidos foram inoculados com formas metacíclicas de triatomineos ou de culturas axenicas, seguindo-se passagem em camundongo de 10 a 12g. Os caracteres biológicos e o estudo histopatológico permitiram incluir todas a cepas em três Tipos ou biodemas: I, II e III . A análise isoenzimática para PGM, GPI, ASAT e ALAT confirmou a correspondência entre os biodemas e os zimodemas: Tipo I e Z2b, Tipo II e Z2, Tipo III e Z1. Os resultados mostraram a distribuição ubíqua dos diversos Tipos de cepas, observando-se a predominância do mesmo biodema em uma mesma áreas geográfica: cepas de Tipo II de casos humanos do Leste da Bahia e Leste de Goiás; cepas de Tipo III do Norte do Brazil e da América Central e de vetores ou vertebrados silvestres em várias áreas geográficas. Os biodemas correlacionam com diferentes lesões histopatológicas na fase aguda e crônica da infecção, considerando-se o envolvimento cardíaco e as lesões neuronais. Estes achados sugerem que o comportamento biológico, os padrões isoenzimáticos e o quadro patológico podem se constituir em importantes elementos para o estabelecimento de correlações entre as cepas do parasito e as manifestações clinico-patológicas da doença de Chagas em diferentes áreas geográficas.
Palavras-chaves: Cepas do Trypanosoma cruzi. Distribuição geográfica. Biodemas. Zimodemas. Patologia da doença de Chagas.
Many thanks are due to the colleagues that provided the T. cruzi strains from different geographical areas: A Prata, V Macedo and C Castro (strains from S Felipe and Mambai); A Rassi (Montalvania and west of Bahia); J Alencar (Ceará); CB Marcondes (Paraiba); SA Valente (Pará); H Bringel (Goias); E Chiari (Minas Gerais); SM Gonzalez Cappa, EL Segura and CE Borda (Argentina); W Apt and H Sagua (Chile), M Tibayrenc (Bolivia); H Cosenza (Honduras); H Barbosa (strain from Guatemala); to the technicians who developed the electrophoresis of isoenzymes: R Castro-Silva, CMG Santiago and RCP Cunha; to A Pimentel for improving strain isolation, to Dinalva França, in charge of the parasite cultivation and M. Fatima Magalhães for maintainance and cryopreservation of the strains.
1. Andrade SG. Caracterização de cepas do Trypanosoma cruzi isoladas no Reconcavo Bahiano. Revista de Patologia Tropical 3:65-121, 1974. [ Links ]
2. Andrade SG. Morphological and behavioural characterization of Trypanosoma cruzi strains. Revista da Sociedade Brasileira de Medicina Tropical 18: 39-46, 1985. [ Links ]
3. Andrade SG. Influence of Trypanosoma cruzi strain on the pathogenesis of chronic myocardiopathy in mice. Memórias do Instituto Oswaldo Cruz 85:17-27, 1990. [ Links ]
4. Andrade SG, Andrade V, Rocha Filho FD, Barral Netto M. Análise antigênica de diferentes cepas do Trypanosoma cruzi. Revista do Instituto de Medicina Tropical de São Paulo 23:245-250, 1981. [ Links ]
5. Andrade SG, Andrade ZA. Doença de Chagas e alterações neuronais do plexo de Auerbach. (Estudo experimental em camundongos). Revista do Instituto de Medicina Tropical de São Paulo 8:219-224, 1966. [ Links ]
6. Andrade SG, Andrade ZA. Patologia da doença de Chagas experimental de longa duração. Revista do Instituto de Medicina Tropical de São Paulo 10:180-187, 1968. [ Links ]
7. Andrade SG, Magalhães JB, Pontes AL. Evaluation of chemotherapy with benznidazole and nifurtimox in mice infected with Trypanosoma cruzi of different types. Bulletin of the World Health Organization 63:721-726, 1985. [ Links ]
8. Andrade SG, Rassi A, Magalhães JB, Ferriolli Filho F, Luquetti AO. Specific chemotherapy of Chagas disease: a comparison between the response in patients and experimental animals inoculated with the same strain. Transaction of the Royal Society of Tropical Medicine and Hygiene 86:624-626, 1992. [ Links ]
9. Andrade V, Brodskyn C, Andrade SG. Correlation between isoenzyme patterns and biological behavior of different strains of Trypanosoma cruzi. Transaction of the Royal Society of Tropical Medicine and Hygiene 77:796-799, 1983. [ Links ]
10. Apt WB, Aguilera X, Arribada A, Gomes L, Miles MA, Widmer G. Epidemiology of Chagas disease in northern Chile: isozyme profils of Trypanosoma cruzi from domestic and sylvatic transmission cycles and their association with cardiopathy. American Journal of Tropical Medicine and Hygiene 37:302-307, 1987. [ Links ]
11. Bittencourt AL, Mota E, Povoa M. Isoenzyme characterization of Trypanosoma cruzi from congenital cases of Chagas' disease. Annals of Tropical Medicine and Parasitology 79:393-396, 1985. [ Links ]
12. Bogliolo AR, Chiari E, Silva-Pereira RO, Silva-Pereira AA. A comparative study of Trypanosoma cruzi enzyme polymorphism in South America. Brazilian Journal of Medical and Biological Research 19:673-683, 1986. [ Links ]
13. Bongertz V, Dvorak JA. Trypanosoma cruzi: Antigenic analysis of cloned stocks. American Journal of Tropical Medicine and Hygiene 32:716-722, 1983. [ Links ]
14. Brener Z. Comparative studies of different strains of Trypanosoma cruzi. Annals of Tropical Medicine and Parasitology 59:19-26, 1965. [ Links ]
15. Brener Z, Costa CAG, Chiari C. Differences in the susceptibility of Trypanosoma cruzi strains to active chemotherapeutic agents. Revista do Instituto de Medicina Tropical de São Paulo 18:450-455, 1976. [ Links ]
16. Ebert F. The identification of two main-groups of Trypanosoma cruzi stocks from Brazil by their isoenzyme patterns of isoelectrofocusing. Tropenmedizin und Parasitologie 33:140-146, 1982. [ Links ]
17. Gonzalez Cappa SM, Chiale P, Del Prado GE, Katzin AM, Martini GW, Isola ED, Abramo-Orrego L, Segura EL. Aislamiento de una cepa de Trypanosoma cruzi de un paciente con miocardiopatia chagasica cronica y su caracterization biologica. Medicina (Buenos Aires) 40 (supl 1):63-68, 1980.
18. Gonzalez Cappa SM, Bijovsky AT, Freilij H, Muller L, Katzin AM. Aislamiento de una cepa de Trypanosoma cruzi a predominio de formas delgadas en la Argentina. Medicina (Buenos Aires) 41:119-120, 1981.
19. Lauria-Pires L. Efeito de superinfecções na evolução da doença de Chagas' experimental com estoques e clones de Trypanosoma cruzi geneticamente caracterizados. Revista da Sociedade Brasileira de Medicina Tropical 28:295-300, 1995. [ Links ]
20. Luquetti AO, Miles MA, Rassi A, Rezende JM, Souza AA, Povoa MM, Rodrigues I. Trypanosoma cruzi:zymodemes associated with acute and chronic Chagas' disease in Central Brazil. Transactions of the Royal Society of Tropical Medicine and Hygiene 80:462-470, 1986. [ Links ]
21. Magalhães JB, Pontes AL, Andrade SG. Comportamento das cepas Y e Peruana do Trypanosoma cruzi no camundongo, após passagem em diferentes meios. Memórias do Instituto Oswaldo Cruz 80:41-50, 1985. [ Links ]
22. Marretto JPM, Andrade SG. Biochemical behavior of Trypanosoma cruzi strains isolated from mice submitted to specific chemotherapy. Revista da Sociedade Brasileira de Medicina Tropical 27:209-215, 1994. [ Links ]
23. Miles MA, Lanham SM, Souza AA, Povoa M. Further enzymic characters of Trypanosoma cruzi and their evaluation for strain identification. Transaction of the Royal Society of Tropical Medicine and Hygiene 74:221-237, 1980. [ Links ]
24. Miles MA, Apt WB, Widmer G, Povoa MM, Schofield CJ. Isozyme heterogeneity and numerical taxonomy of Trypanosoma cruzi stocks from Chile. Transactions of the Royal Society of Tropical Medicine and Hygiene 78:526-535, 1984. [ Links ]
25. Miles MA, Cedillos RA, Povoa MM, Souza AA, Prata A, Macedo V. Do radically dissimilar Trypanosoma cruzi strains (zymodemes) cause Venezuelan and Brazilian forms of Chagas' disease? Lancet, I:1338-1340, 1981. [ Links ]
26. Morel CM, Deane MP, Gonçalves AM. The complexity of Trypanosoma cruzi populations revealed by schizodeme analysis. Parasitology Today 2:97-101, 1980. [ Links ]
27. Postan M, Dvorak J, McDaniel JP. Studies of Trypanosoma cruzi clones in inbred mice. I. A comparison of the course of infection of C3H/HEN mice with two clones isolated from a commom source. American Journal of Tropical Medicine and Hygiene 32:497-506, 1983.
28. Romanha AJ, Silva-Pereira AA, Chiari E, Kilgour V. Isoenzyme patterns of cultured Trypanosoma cruzi:changes after prolonged subculture. Comp. Biochemical Physiology 62:139-142, 1979. [ Links ]
29. Sadigursky M, Andrade SG. Electrocardiographic changes in experimental chronic murine Chagas' disease. Brazilian Journal of Medical and Biological Research 19:379-388, 1986. [ Links ]
30. Schlemper Jr BR, Avila CM, Coura JR, Brener Z. Course of infection and histopathological lesions in mice infected with seventeen Trypanosoma cruzi strains isolated from chronic patients. Revista da Sociedade Brasileira de Medicina Tropical 16:23 - 30, 1983 [ Links ]
31. Souza MM, Andrade SG, Barbosa Jr AA, Santos RTM, Ferreira VA, Andrade ZA. Influence of Trypanosoma cruzi strains on autonomic nervous system pathology (An experimental study in mice). Memórias do Instituto Oswaldo Cruz 92:217-224,1996. [ Links ]
32. Steindel M, Toma HK, Carvalho Pinto CJ, Grisard EC, Schlemper Jr BR, Ribeiro-Rodrigues R, Romanha AJ. Isoenzymatic characterization of Trypanosoma cruzi strains isolated from silvatic reservoirs and vectors from Santa Catarina island, Santa Catarina State. Memórias do Instituto Oswaldo Cruz, 86(suppl 1):141, 1991. [ Links ]
33. Tibayrenc M, Ayala FJ. Isozyme variability in Trypanosoma cruzi the agent of Chagas' disease: genetical, taxonomical and epidemiological significance. Evolution 42:277-292, 1988. [ Links ]
34. Tibayrenc M, Echalar L, Dujardin JP, Poch O, Desjeux P. The microdistribution of isoenzymic strains of Trypanosoma cruzi in southern Bolivia; new isoenzyme profiles and further arguments against Mendelian sexuality. Transaction of the Royal Society of Tropical Medicine and Hygiene 78:519-525, 1984. [ Links ]
35. Tibayrenc M, Ward P, Moya A, Ayala F. Natural populations of Trypanosoma cruzi, the agent of Chagas' disease have a complex multiclonal structure. Proceedings of the National Academy of Science, USA 83:115-119, 1986. [ Links ]
36. Thompson RCA, Lymbert AJ. Intraspecific variation in parasites - what is a strain? Parasitology Today 6:345-348, 1990. [ Links ]
Centro de Pesquisas Gonçalo Moniz. Fundação Oswaldo Cruz, Universidade Federal da Bahia, Salvador,BA.
Address to: Drª Sonia G. Andrade. Centro de Pesquisas Gonçalo Moniz/FIOCRUZ. Rua Valdemar Falcão 121, Brotas, 40295-001 Salvador, BA,Brasil.
Recebido para publicação em 02/01/96.