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Revista do Instituto de Medicina Tropical de São Paulo

Print version ISSN 0036-4665

Rev. Inst. Med. trop. S. Paulo vol. 39 no. 2 São Paulo Mar./Apr. 1997

http://dx.doi.org/10.1590/S0036-46651997000200008 

Human infection with Trypanosoma cruzi in Nasca, Peru: A seroepidemiological survey (1)

Hilda Maria SOLIS ACOSTA (2), Cláudio Santos FERREIRA (3) & Maria Esther de CARVALHO (4).

 


Summary

We estimated the proportion of seropositivity for infection with Trypanosoma cruzi (Chagas’ disease) in a sample of the rural population of the Province of Nasca, Department of Ica, southwestern Peru. Although Triatoma infestans, the only vector species identified in the Department of Ica, is often found in domestic environments, data of the extent of human infection with T. cruzi are scant. This study comprised 446 houses, known to be infested with triatomines, distributed in 19 rural localities. While visiting those houses we collected filter paper bloodspots from 864 occupants (of both sexes, aged one year or over). By means of the indirect fluorescent antibody test (IFAT), we detected anti-T. cruzi IgG antibodies in samples from 178 individuals (20.6%). Seropositivity was significantly more frequent in females (23.8%) than in males (17.5%). Among the 410 individuals in the 1- to 10-year-old age group (47.5% of the population sample), 85 (20.7%) were found seropositive, which is indicative of an early acquisition of the infection. Within this group no significant differences in seropositivity were associated with sex.

Keywords: Chagas’ disease; Seroepidemiology; Nasca, Peru.


 

Introduction

Chagas’ disease is endemic in both the coastal desert and the forested eastern regions of Peru 7, 9, 16, 17, 19, 20, 21. ESCOMEL reported for the first time, in 1917, the occurrence of Chagas’ disease in Peru, as quoted by CUBAS9, LUMBRERAS16 and VILLANUEVA26. According to HERRER14, the vector species Triatoma infestans must have penetrated into the Peruvian territory around 1879, during the war between Peru and Chile. Since then, it has been found exclusively in domestic environments26. The Province of Nasca, where this study was conducted, is part of the Department of Ica (southwestern Peru), which also comprises the provinces of Chincha, Pisco, Ica, and Palpa.

Triatomine bugs, locally known as chirimachas, were observed to occur in the Province of Nasca by AREBALO1 in 1946. During the same year, an investigation by AYULO 2 in the Department of Ica failed to demonstrate infection of humans, other mammals and triatomines with Trypanosoma cruzi; but TEJADA 25 in 1962 and VILLANUEVA 26 in 1973 detected natural infection of T. infestans with T. cruzi, in the provinces of Nasca and Ica respectively. While conducting a seroepidemiological study in 14 localities of the Department of Ica, CORNEJO et al.8 (1986) found specimens of T. infestans naturally infected with T. cruzi and, by the complement fixation (CFT) and the indirect hemagglutination (IHAT) tests, seropositivity in humans. More recently, CARRANZA & MARQUEZ (1992)5 used the indirect fluorescent antibody test (IFAT) and IHAT to identify anti-T. cruzi antibodies in serum samples from blood donors at two hospitals of Ica. PEREZ & TORRES (1994)21 also used IFAT and IHAT to detect anti-T. cruzi antibodies in prospective blood donors from the provinces of Palpa and Nasca.

Currently available seroepidemiological data are insufficient to convey an acceptable picture of the prevalence of American trypanosomiasis in Peru. We thus decided to design a first approach to a seroepidemiological investigation aimed at the human population of the Province of Nasca, where no comprehensive epidemiological study on Chagas’ disease has been done, serological or otherwise. By means of a questionnaire, we collected information about housing conditions and the habit of keeping domestic animals in the immediate vicinity of, or inside the houses.

This study was aimed at gathering seroepidemiological data necessary to guide further investigation in Nasca, rather than estimating the prevalence of Chagas’ disease in each one of the localities where bloodspots were collected.

 

Materials and methods

The present study was done during 1994 and 1995. It comprised the examination of serum samples from residents of 19 rural localities of the Province of Nasca, viz: Belén Bajo, Buena Fe, Cantayo, Coyungo, Chauchilla, Copara, Hacienda Belén, Huachuca, Las Cañas, La Ayapana, Los Jardines, Orcona, Pajonal Bajo, Pongo Grande, Santa Fe, Saysongo, San Luís Pajonal, Taruga and Unión Victoria (Fig. 1).

 

Fig. 1 - Spatial distribution of the 19 rural localities surveyed.

 

According to data supplied by the Ministry of Health of Peru, there are 6,775 inhabitants and 1,544 houses (some of them vacant) in the region where this study was conducted 22. No demographic information was available concerning Hacienda Belén. Nevertheless, we estimated that our sample included about 13% of the population of this region. The 446 dwellings surveyed had been previously found to be colonized by T. infestans, according to information supplied by personell in charge of field work on malaria.

We collected bloodspots, absorbed into Watman nº 3 filter paper, from 864 residents, of both sexes, aged one year or over. Estimated as recommended by FERREIRA & CARVALHO 11, each bloodspot contained approximately 15.18 µl of blood per cm2 of filter paper.

The Province of Nasca (4,828 km2) is part of the Department of Ica, to the southwest of the Peruvian coastal desert. It is bordered by the provinces of Ica and Palpa on the north, the Department of Arequipa on the south, the Department of Ayacucho on the east and the Pacific Ocean on the west (Fig. 1). The localities surveyed stand at an altitude of 588 meters.

Iron ore extraction, a wide range of handicrafts and tourism are among the most economically important activities in the Province of Nasca. Agriculture and cattle raising are sparse, largely due to local unfavorable climatic conditions 15. A major tourist attraction in this province are the Lines of Nasca, huge figures traced on the dark surface of the desert soil, exposing the bright silica layer underneath. They represent human, animal, plant and geometrical shapes. Dated from the 3rd to the 6th century AD, the Lines of Nasca are, admitedly, astrological calendars. Such lines are best observed from the air15.

Most local houses have adobe walls in which crevices left by ill-fitting bricks are massively colonized by triatomine bugs. Although some of such walls are plastered, they often develop cracks for lack of maintenance. Earthen floors and roofs thatched with straw or cane are the rule. On the average, a house has 5 rooms. Traditionally, poultry and guinea-pigs are kept indoors. The guinea-pig (cuy, as it is called by locals), a notorious reservoir of T. cruzi 6, was domesticated by the Incas in the pre-Columbian times.

The collection of blood samples in this region of the Province of Nasca was full of difficulties: no electricity, water in very short supply and most of the occupants of the houses out for work from dawn till dusk, only children remaining at home in the daytime.

We performed IFAT according to the technique described by CAMARGO3 with the modifications suggested by FERREIRA & CARVALHO10, using T. cruzi antigen fixed on microscope slides (Salck, São Paulo, Brazil), and fluorescein isothiocyanate conjugated caprine anti-human IgG ( Salck, São Paulo, Brazil), previously standardized in terms of maximum reactivity (titer = 200). Positive (low and high titer) and negative control sera were used to check the intensity of fluorescence, as suggested by CAMARGO et al. 4 and GUIMARÃES et al.13. In accordance with the recommendation by SADUN et al.23, we chose a cut-off dilution of 1:16, not 1:32 (or its near equivalent 1:40) which is used frequently4, 12. The responsiveness of this population being unknown to us, we chose to improve sensitivity, even at the risk of some loss of specificity. Dilution series were conveniently expressed as powers of 2, as their logarithms (to the base 2) are integers27. Doubtful results at the screening dilution were not considered, as we were not counting on the use of an alternative technique.

Statistical analysis of epidemiological data: G-test for independence24. Level of significance 5% (a = 0.05). Confidence limits (95%) for percentages were calculated by normal approximation to binomial 28.

 

Results

IFAT seropositivity for anti-T. cruzi IgG antibodies was found in 178 (20.6%) of the 864 subjects under investigation, implying 95% confidence limits from 156 (18.0%) to 202 (23.4%). The numbers of samples from Buena Fe, Los Jardines, Copara and Pajonal Bajo (Table 1) added up to 549 (63.5% of the total). This group included 103 (18.8%) seropositives. The remaining 315 samples (36.5% of the total) included 75 seropositives (23.8%). The hypothesis was accepted that seropositivity was not significantly different in both groups (G = 3.074 p = 0.08 [X2 0.05(1) = 3.841]).

 

Table 1

Distribution by locality, and indirect fluorescent antibody test (IFAT) seropositivity for anti-Trypanosoma cruzi IgG antibodies, of 864 inhabitants of 19 localities of the Province of Nasca, Peru, 1994. Confidence limits for percentages calculated by normal approximation to binomial.

Locality

Population

Samples

Positive

Confidence limits (95%)

Examined

% Examined

Total

%

Lower %

Upper %

Buena Fe

289

177

61.2

26

14.7

10.2

20.7

Los Jardines

1,684

169

10.0

48

28.4

22.1

35.6

Copara

211

117

55.5

14

12.0

7.3

19.1

Pajonal Bajo

405

86

21.2

15

17.4

10.9

6.8

Cantalloc

232

46

19.8

13

28.3

17.3

42.5

Orcona

103

38

36.9

3

7.9

2.7

20.8

Taruga

187

34

18.2

8

23.5

12.4

40.0

Saysongo

155

34

21.9

9

26.5

14.6

43.1

Belén Bajo

45

33

73.3

16

48.5

32.5

64.8

Chauchilla

150

28

18.7

3

10.7

3.7

27.2

San Luís Pajonal

86

28

32.6

1

3.6

0.6

17.7

Las Canas

21

26

123.8

7

26.9

13.7

46.1

La Ayapana

56

20

35.7

2

10.0

2.8

30.1

Huachuca

6

6

100.0

3

50.0

18.8

81.2

Pongo Grande

22

2

9.1

0

0.0

0.0

65.8

Subtotal

3,652

844

23.1

168

19.9

17.4

22.7

Hacienda Belén

*

8

*

7

87.5

52.9

97.8

Coyungo

498

6

1.2

1

16.7

3.0

56.4

Santa Fe

2,367

5

0.2

1

2.0

3.0

62.4

Unión Victoria

258

1

0.4

1

100.0

20.7

100.0

Total

6,775

864

12.8

178

20.6

18.0

23.4

* Population data lacking

 

Table 2 shows the distribution of the 864 subjects according to seropositivity, sex and age group. The proportion of seropositives was significantly higher in females (23.8%) than in males. (17.5%). (G = 5.29 p = 0.022 [X2 0.05(1) = 3.841]).

 

Table 2

Distribution, by age group, sex and indirect fluorescent antibody test (IFAT) seropositivity for anti-Trypanosoma cruzi IgG antibodies, of 864 inhabitans of 19 localities of the Province of Nasca, Peru, 1994.

Age group
Females
(years)
Males Females Males +
Total Posit. % Posit. Total Posit. % Posit . Total Posit. %Posit.
0-5 25 7 28.0 22 3 13.6 47 10 21.3
5 –10 219 49 22.4 144 26 18.1 363 75 20.7
10-15 119 10 8.4 104 24 23.1 223 34 15.2
15-20 24 3 12.5 35 15 42.9 59 18 30.5
20-25 9 2 22.2 26 8 30.8 35 10 28.6
25-30 11 3 27.3 25 6 24.0 36 9 25.0
30-35 11 2 18.2 14 5 35.7 25 7 28.0
35-40 7 0 0.0 11 6 54.5 18 6 33.3
40-45 3 0 0.0 10 2 20.0 13 2 15.4
45-50 2 0 0.0 12 4 33.3 14 4 28.6
‘> 50 10 1 10.0 21 2 9.5 31 3 9.7
Total 440 77 17.5 424 101 23.8 864 178 20.6

 

The 1-to 10-year-old age group was composed of 244 individuals (28.2% of the total), 244 males and 166 females. Seropositivity was demonstrated in 56 males (23.0%) and 29 females (17.5%). This difference was not significant. (G = 1.83 p = 0.18 [X2 0.05(1) = 3.841]).

Fig. 2 shows variation in seropositivity by age group.

Table 3 summarizes epidemiologically relevant peculiarities of local dwellings. Data about keeping domestic animals in or near the houses are shown in Table 4.

 

Table 3

Physical characteristics of the 446 domiciles investigated. Province of Nasca, Peru, 1994.

Walls Number of houses Percentage
Adobe 432 96.8
Mud-stick 6 1.3
Reed 6 1.3
Straw matting 1 0.2
Brick 1 0.2
Roof
Reed 436 97.7
Thatch (straw) 10 2.2
Floor
Earthen 232 52.0
Cement 214 48.0
Plaster
Yes 345 77.4
No 101 22.6
Cracks
Yes 371 83.2
No 75 16.8
Total 446 100.0

 

Table 4

Domestic animals in 446 houses. Province of Nasca, Peru, 1994.

Animals Number of houses Percentage
Mammals and birds 319 71.5
Mammals only 54 12.1
Birds only 37 8.3
Subtotal 410 91.9
No animals 36 8.1
Total 446 100

 

Serological results and the use of keeping animals indoors (Table 5) were demonstrated to be statistically independent (G = 0.32 p = 0.57 [X2 0.05(1) = 3.841]).

 

Table 5

Seropositivity or seronegativity by the indirect fluorescent antibody test (IFAT) of the population sample investigated and the presence or absence of animals in the houses. Nasca, Peru, 1994.

Inhabitants Number of cases Percentage
Positives: animals inside the house 165 92.7
Positives: no animals inside the house 13 7.3
Subtotal 178 100.0
Negatives: animals inside the house 627 91.4
Negatives: no animals inside the house 59 8.6
Subtotal 686 100.0
Total 864 100.0

 

Discussion

A rather high prevalence of Chagas’ disease in the Province of Nasca would be expected in view of the ubiquitous presence of T. infestans in domestic environments. Indeed, the proportion of seropositivity we found in Nasca (20.6%) is closer to that recorded by NAQUIRA et al.18 elsewhere in southern Peru (32%) than to overall results for Peru (between 6% and 10%) 17. In spite of the fact that Chagas’ disease has for a long time been recognized as a serious public health problem in Peru, comprehensive epidemiological data concerning the human population are still lacking.

The results of this cross-sectional sampling study are indeed a cause for concern. The epidemiological factors involved in the natural transmission of Chagas’ disease demonstrably occur throughout the region under investigation. T. infestans is omnipresent and local human habitations provide the necessary conditions for its breeding: most houses have adobe walls, but loosely laid bricks leave spaces which are profusely colonized by triatomines. Such spaces are also abundant in mud-stick and wooden walls (Table 5). Earthen floors and roofs thatched with reeds or straw are the rule.

Domestic animals, chiefly guinea-pigs and poultry are often kept indoors. The latter have been, since pre-Columbian times, used by locals as a source of proteins. Thus, an ecological relationship has been established between these animals and humans (HERRER14, 1955; VILLANUEVA26, 1973). Under such circumstances, natural transmission of Chagas’ disease is a hazard, particularly to very young children who remain indoors for long periods and to women, who handle the carcasses of possibly infected guinea-pigs. To what extent the latter circumstance is responsible for a higher proportion of infected women is a subject for investigation. As shown in Table 2, among 410 individuals in the 1- to 10-year old age group (47.5% of the population sample), 85 (20.7%) were found seropositive, which is indicative of an early acquisition of the infection.

A program intended to control the transmission of Chagas’ disease has been recently started in Peru (CARRANZA & MARQUEZ5, 1992), but evaluation of its results imply a nationwide epidemiological investigation. Since the first observation by ESCOMEL, in 1917, only scattered data have been collected about the distribution of tritomines in Peru. The same applies to data about human infection, which has been serologically investigated by CORNEJO & LUMBRERAS (1951)7; CUBAS (1965)9; NAQUIRA (1967)17; LUMBRERAS (1972)16; HERRER (1955)14; VILLANUEVA (1973)26; CORNEJO et al. (1986)8; CARRANZA & MARQUEZ (1992)5; PEREZ & TORRES (1994)21.

In Peru, it is surmised that a considerable proportion of cases of cardiopathy and megaviscera, ascribed to other causes for lack of etiologic diagnosis, were indeed complications of Chagas’ disease. As pointed out by CORNEJO et al.8, investigation is not often directed towards tracing sudden death cases back to Chagas’ disease.

There is no question about the importance of Chagas’ disease as a cause of disability among those who would otherwise compose the work force of Peru. As a chronic disease which can cause irreversible tissue damage and for which no satisfactory specific treatment is available, it involves heavy economical losses and human suffering.

A thorough understanding of the epidemiological conditions prevailing in the Province of Nasca, obviously invaluable in the control of natural transmission of Chagas’ disease, has not been attained until now; but the experience acquired in other regions of South America, where natural transmission of American trypanosomiasis has eventually been controlled, can provide investigators and the community with relevant information and a great deal of stimulus to achieve this goal.

Resumo

Infecção humana por Trypanosoma cruzi em Nasca, Peru: um inquérito soroepidemiológico

Estimamos a proporção de positividade sorológica para infecção chagásica em amostra da população rural da Província de Nasca, Departamento de Ica, sudoeste do Peru. Embora Triatoma infestans, o único vetor identificado no Departamento de Ica, seja encontrado freqüentemente em ambientes domésticos, são escassos os dados sobre a infecção de indivíduos humanos por Trypanosoma cruzi. Este estudo compreendeu 446 unidades domiciliares sabidamente infestadas por triatomíneos, distribuídas em 19 localidades. Ao visitá-las colhemos, em papel-filtro, amostras de sangue de 864 pessoas de um ano ou mais de idade, sem seleção por sexo. Por meio da reação de imunofluorescência indireta (RIFI), detectamos anticorpos IgG anti-T. cruzi em amostras de 178 (20,6%) pessoas. A proporção de positivos foi significantemente maior nos indivíduos do sexo feminino (23,8%) do que nos do masculino (17,5%). Dentre os 410 indivíduos pertencentes ao grupo de idades entre 1 e 10 anos (47% da amostra da população), 85 (20,7%) resultaram sorologicamente positivos, o que indica aquisição precoce da infecção. Neste grupo, a positividade sorológica se mostrou independente de sexo.

 

Acknowledgements

We are deeply indebted to Scientific Researchers Vera Lucia Pereira Chiaccola and Antonio Marcos de Aparecida Levy from the Instituto "Dante Pazzanese" de Cardiologia, São Paulo, for providing standard negative and positive sera; to Marcia Regina Delgado, Regina Yoshika Yazawa, Guiomar Fonseca Matos and Andrea Zanardi, of the Laboratório de Imunoepidemiologia, SUCEN, for technical support and to Maria Eduarda Pereira Farinha, of SUCEN, for bibliograhical support.

 

References

1. AREVALO, G. - Se ha presentado en Nasca de propagarse un peligroso insecto parásito del ser humano que transmite grave enfermedad., Ica, Peru, La voz de Ica., 1946.        [ Links ]

2. AYULO, V. - Estudio sobre tripanosomiasis americana en el Perú. Observaciones en el Departamento de Ica. Rev. Med. exp. (Lima), 5: 21-35, 1946.        [ Links ]

3. CAMARGO, M.E. - Fluorescent antibody test for the serodiagnosis of American trypanosomiasis. Technical modification employing preserved culture forms of Trypanosoma cruzi in a slide test. Rev. Inst. Med. trop. S. Paulo, 8: 227-234, 1966.        [ Links ]

4. CAMARGO, M.E.; SILVA, G.R.; CASTILHO, E.A. & SILVEIRA, A.C. - Inquérito sorológico da prevalência de infecção chagásica no Brasil, 1975/1980. Rev. Inst. Med. trop. S. Paulo, 26: 192-204, 1984.        [ Links ]

5. CARRANZA, J. & MARQUES, R. - Riesgo de contraer la enfermedad de Chagas por transfusión sanguínea. Ica, Peru, 1992. (Tesis de Título de Químico Farmacêutico - Univ. San Luis Gonzaga de Ica, Facultad de Farmacia y Bioquímica).        [ Links ]

6. CORDOVA, B.E. - Investigación sobre la enfermedad de Chagas en el Perú. Estudio epidemiológico en el Valle de Tambo (Distrito de Matalaque, Departamento de Moquegua). I. Observaciones preliminares. Bol. chil. Parasit., 3: 54-59, 1961.        [ Links ]

7. CORNEJO, D.A & LUMBRERAS, C. - Encuesta epidemiológica de la enfermedad de Chagas en el Valle Majes. Presentación de diez nuevos casos. Arch. peru. Pat. clín., 5: 79-106, 1951.        [ Links ]

8. CORNEJO, S.J.; CORNEJO, D.A.; CUBAS, E. et al. - Epidemiología de la Enfermedad de Chagas en el Departamento de Ica. I. Estudio del vector y reservorio humano. Bol. Med. trop. Lima, 5: 10-13, 1986.        [ Links ]

9. CUBAS, E. - Reacción de Guerreiro-Machado en 1010 personas, procedentes de diversos Departamentos del Perú. An. Fac. Med (Lima), 48: 165-175, 1965.        [ Links ]

10. FERREIRA, C.S. & CARVALHO, M.E. - Reações de imunofluorescência indireta: algumas simplificações de sua técnica. Rev. Saúde públ. (S. Paulo), 7: 303-306, 1973.        [ Links ]

11. FERREIRA, C.S. & CARVALHO, M.E. - Padronização de uso de papel-filtro como suporte de material para reações sorológicas. Rev. bras. Malar., 34: 82-86, 1982.        [ Links ]

12. FUCHS, A.P.; FIORATTI, V.L.; MELLO, V.A. & BOAINAIN, E. - Diagnóstico sorológico na doença de Chagas: estudo comparativo de diferentes técnicas. Rev. Inst. Med. trop. S. Paulo, 22: 242-245, 1980.        [ Links ]

13. GUIMARÃES, M.C.S.; SUGGS, M.T. & KAGAN, I.G. - Chagas’ disease serology: specifications and evaluation methods for immunological reagents. Washington, D.C., OPS/OMS, 1984. (PNSP/84.08).        [ Links ]

14. HERRER, A. - Tripanosomiasis americana en el Perú. VI. Verificación de la enfermedad de Chagas en la Cuenca del Marañon. Rev. Med. exp. (Lima), 9: 83-91, 1955.        [ Links ]

15.LANCHO, R.J.S. - Nasca: datos geográficos-históricos. 2. ed. Lima, Calper, 1987.        [ Links ]

16. LUMBRERAS, H. - El problema de la enfermedad de Chagas en los diferentes Departamentos del Perú. Rev. Viernes méd., 1: 43-77, 1972.        [ Links ]

17. NAQUIRA, F. - Investigación de la enfermedad de Chagas en los bancos de sangre de Lima. Rev. méd. Hosp. Obrero (Lima), 16: 126-131, 1967.        [ Links ]

18. NAQUIRA, F.; CORDOVA, C.; NAQUIRA, C.& RUBIN DE CELIS, E. - Investigación sobre la enfermedad de Chagas en el Perú. Estudio epidemiológico en el Valle de Tambo (Distrito de Matalque, Departamento de Moquegua). II. Aplicación del xenodiagnóstico y Fijación del Complemento. 1959. Bol. chil. Parasit., 3: 57-59, 1961.        [ Links ]

19. OFICINA SANITARIA PANAMERICANA/ORGANIZACIÓN MUNDIAL DE LA SALUD - Enfermedad de Chagas. Bol. Epidem. Org. panamer. Salud, 3 (3): 1-6, 1982.        [ Links ]

20. PAN AMERICAN HEALTH ORGANIZATION/WORLD HEALTH ORGANIZATION - Status of Chagas’ disease in the region of the Americas. Epidem. Bull., 5: 5-9, 1984.        [ Links ]

21. PEREZ, R. & TORRES, M. - Detección de anticuerpos anti-T. cruzi en potenciales donantes de sangre de las Provincias de Palpa y Nasca (Región Libertadores-Wari). Ica-Perú, 1994. (Tesis de Biólogo - Facultad de Ciencias, Escuela Académico profesional de Biología, Univ. San Luís Gonzaga).        [ Links ]

22. PERU, Ministerio de Salud. - Subregión de Salud de Ica. Ica, 1994. p. 664-669.        [ Links ]

23. SADUN, E.H.; DUXBURY, R.E.; WILLIAMS, J.S. & ANDERSON, R.I. - Fluorescent antibody test for the serodiagnosis of African and American trypanosomiasis in man. J. Parasit., 49: 385-388, 1963.        [ Links ]

24. SOKAL, R.R. & ROHLF, F.J. - Biometry. 2. ed. New York, Freeman, 1981.        [ Links ]

25. TEJADA, A. - Encuesta clínico - epidemiológica acerca de la enfermedad de Chagas en Nasca. Lima, Perú, 1962. (Tesis de Bachiller - Facultad de Medicina de la Universidad Nacional Mayor de San Marcos).        [ Links ]

26. VILLANUEVA, C. - Estudio sobre la Enfermedad de Chagas en la Zona de Salud Sur Medio. I. Triatoma infestans en el Departamento de Ica. Rev. peru. Ent., 16: 3-6, 1973.        [ Links ]

27. WHITE, C. - Statistical methods in serum surveys. In: PAUL, J.R. & WHITE, C. Serological epidemiology. New York, Academic Press, 1973.        [ Links ]

28. WONNACOTT, T.H. & WONNACOTT, R. - Introductory statistics. 2. ed. New York, John Wiley & Sons, 1972.

 

Recebido para publicação em 26/11/1996
Aceito para publicação em 30/04/1997