Chagas disease in an area of recent occupation in Cochabamba, Bolivia
Doença de Chagas numa área de ocupação recente em Cochabamba, Bolívia
Hugo Albarracin-Veizaga, Maria Esther de Carvalho , Elvira M M do Nascimento, Vera Lúcia C C Rodrigues, Cláudio Casanova and José Maria S Barata
Ministerio de Desarrollo Humano. Secretaría Nacional de Salud. Escuela Técnica de Salud Boliviano-Japonesa de Cooperación Andina. Cochabamba, Bolivia (HAV); Superintendência de Controle de Endemias. São Paulo, SP - Brasil (MEC, EMMN, VLCCR, CC); Departamento de Epidemiologia da Faculdade de Saúde Pública da Universidade de São Paulo. São Paulo, SP - Brasil (JMSB)
Chagas disease, epidemiology.
Doença de Chagas, epidemiologia.
Áreas de pobreza.
Ecologia de vetores.
As is often the case in South America, Chagas disease remains a serious public health problem in Bolivia. It is one of the consequences of poor standards of housing, education and hygiene prevailing in the rural, peri-urban and urban places inhabited by more than half the population of the country. Unemployment is frequent and the distribution of land inequitable. On account of the lack of adequate quality control, blood transfusions contribute to the further spread of the infection13,16.
Triatoma infestans occurs in 7 of the 9 Departments into which Bolivia is divided, viz La Paz, Cochabamba, Tarija, Chuquisaca, Potosí, Santa Cruz and Beni. These departments add up to 84% of the Bolivian territory, and include valleys and lowlands (llanos), at altitudes between 300 and 3,500 meters23. No vector triatomines have been found in the Departments of Pando and Oruro, at altitudes of less than 300 and 4,000 meters respectively4,13. Local researchers place great emphasis on investigating such problems, posed by Chagas disease in peri-urban areas, as congenital transmission, the sylvatic cycle of T. infestans, the action of insecticides and specific treatment4.
The present work aims at gathering entomological and seroepidemiological information about Chagas disease in a peri-urban area of the city of Cochabamba: Villa Avaroa/Primer de Mayo, established 16 years ago. Its inhabitants are willing to do their part in programs intended to evaluate and eventually control the transmission of Chagas disease in their community. It has been understood that this implies an improvement in their housing and living conditions.
Study area - Avaroa/Primer de Mayo is a peri-urban locality of the city of Cochabamba (17º14'24"S and 66º5'24"W), Provincia Cercado, to the south of the Department of Cochabamba, Bolivia. Situated in a valley at an altitude of 2,400 meters, Avaroa/Primer de Mayo has a mild climate. Its population (3,000), besides having no guidance as to the prevention and control of naturally acquired Chagas disease, lives at a low socio-economic level.
By systematic random sampling 128 subjects aged one year or over were selected and the name, age, gender, birthplace, duration of residence in the locality, and occupation of each of them were recorded. The overall state of the houses and the characteristics of the walls (including the presence of cracks), flooring and roofing were also recorded, in view of the potential for colonization by triatomines. This survey was conducted during July 1996 (winter in the southern hemisphere).
Serology - Blood samples, collected by finger puncture, were absorbed into Whatman #3 filter paper strips (stapled together into booklets)8. By using a technique suggested by Ferreira & Carvalho9, the amount of blood per cm2 of bloodspot surface area was estimated as 11.2 µl. The quantitative immunofluorescent antibody test (IFAT) as described elsewere20 was undertaken, using total Trypanosoma cruzi antigens and anti-IgG and anti-IgM human conjugates (Salck, S. Paulo, Brazil) standardized in terms of maximum reactivity. On the basis of previous experience, a cut-off titer of 16 was adopted. Positive and negative controls were supplied by a hospital specialized in cardiology20.
Entomology - Triatomines in the egg, nymph and adult stages were captured, both inside and around 30 houses, for assessment of their density, house infestation indices and proportions of insects infected with T. cruzi19. Vector gut contents (stained according to the May Grünwald-Giemsa technique) were examined for metacyclical trypomastigotes, as described by Rodrigues et al.18, 1992. Wistar albino rats, aged 14 days on the average, were inoculated intraperitoneally with samples of gut contents from infected triatomines. A blood sample was transferred from each of the animals which developed parasitemia within 7 to 21 days of inoculation to 3 test tubes, each containing 5ml of the artificial Warren medium and incubated at 26.6ºC. Xenodiagnosis was performed on the inoculated animals by using 10 T. infestansnymphs (4th or 5th instar).
The precipitin reaction to identify feeds from human, bird, marsupial (opossum), dog, cat, pig and goat hosts was performed in capillary tubes18. All anti-sera had been previously demonstrated to react with homologous sera at 1:10,000 dilution and not to react at 1:10 dilution with heterologous sera.
Blood sample collection and search for triatomines had been authorized beforehand by the occupants of the respective houses. Dawson-Saunders & Trapp7 was referred to for statistical guindance.
Positivity Table 1 shows the distribution of the subjects by age, gender and seropositivity. Anti-T. cruzi IgG antibodies were detected in 12.5% (n = 128) of the subjects tested: 15.1% (n = 73) females and 9.1% (n = 55) males, proportions which did not differ significantly at the 5% confidence level (ts = 1.035, p = 0.300). No seropositives were detected among the 42 subjects (32.8% of the sample) aged under 10 years, although 9 of them (21.4%) were residents of houses in which triatomines infected with T. cruzi had been found (Table 5).
Among the 75 subjects aged from 10 to 49 years (58.6% of the sample) were found 87.5% of all seropositives (14 out of 16), 90.9% (10 out of 11) females and 80.0% (4 out of 5) males. Titers for IgG antibodies varied between 8 and 256 (geometric mean, 45.25). No cases positive for IgM antibodies were found, even at a low dilutions.
Time of residence in the region - The average time of residence was 6.1 years for the whole population sample and 7.4 years for the seropositives (median = 5.0 years, mode = 4.0 years). For the seronegatives the average time of residence was 5.8 years (median = 4.5 years, mode = 2.0 years).
Origin Among the seropositives, 8 (50.0%) had come from endemic (Cochabamba, Capinota and Sucre), 7 (43.8%) from non-endemic areas (Oruro and Potosí) and 1 (6.3%) was of unknown origin. Among the seronegatives, 55 (49.1%) were born in endemic (Cochabamba, La Paz and Santa Cruz) and 51 (45.5) in non-endemic areas (Llallagua, Catavi, Oruro and Potosí) and 6 (5.4%) were of unknown origin.
Occupation The population sample was chiefly composed of schoolchildren (27.3% , n =35), and housewives (17.2%, n = 22). There were also 2 farm-hands, 2 tailors, 6 traders, 1 dressmaker, 1 contractor, 1 seamstress, 1 electrician, 1 housemaid, 1 mechanic, 1motor vehicle driver, 5 bricklayers, 1 schoolteacher and 1 secretary. Among the seropositives, 43.8% (n = 7) were schoolchildren and 18.6% (n = 3) housewives, besides 1 farm-hand, 1bricklayer and 4 subjects with no occupational record.
Thirty houses were visited to investigate their potential for breeding triatomines, according to building materials and maintenance: adobe walls predominated (76.7%); brick, 6.7%; cement, 3.3; 13.3% of the walls were plastered. Corrugated galvanized iron was the commonest roofing material (86.7%); straw, 3.3%; other, 6.7%. Flooring: earthen, 53.4%; cement, 33.3%; wooden, 3.3%; other, 10.0%. Cracks were found in the walls of 80.0% of the houses.
Table 2 shows the distribution, according to developmental stage, habitat and infection with trypanosomes, of the triatomines captured: 147 specimens (nymphs included) of T. infestans and 1 of T. sordida, respectively 99.3% and 0.7% (n = 148). Triatomines were found infesting 53.3% (n = 30) of the houses (30% in domestic environments); 63 nymphs and 41 adults were captured inside, 23 nymphs and 20 adults around the houses. A significant difference between these proportions (5% confidence level) was found (ts = 7.34, p < 0.05).
Entomological indices3 (X 100)
Density per house visited (148/30) X 100 = 493.3
Confinement (per infested house) (148/19) X 100 = 779.0
Infestation (per house visited) (16/30) X 100 = 53.3
Intradomiciliary infestation (9/30) X 100 = 30.0
Trypanosome-triatomine infection (21/148) X 100 = 14.2
Rodent hosts (infected triatomines) (10/15) X 100 = 66.7
Table 3 shows precipitin bloodmeal identification of T. infestans. The triatomines which had their hosts identified (n = 126) had fed mainly on birds (67.5%), humans (27.8%) and rodents (11.9%). None had fed on marsupials, pigs or goats.
Table 4 shows precipitin bloodmeal identification of nymphs and adults of T. infestans, infected with trypanosomes, by site of capture (inside or around the house) and developmental stage. The triatomines in which host identification succeeded (n = 16) had fed mainly on rodents 10 (62.5%) and humans (31.3%). None had fed on marsupials, pigs, goats or dogs. The gut contents of most triatomines (80.3%) reacted with only one kind of antigen, 19.0% with 2 kinds.
No gut contents could be collected from 7 of the 147 specimens of T. infestans captured. None of the available antisera reacted with the gut contents of 14 specimens.
Between the 30th and the 45th days of feeding, 70.0% of the nymphs used in xenodiagnosis were positive for trypanosomes. Parasitemia was first observed within 15 days of the inoculation of rats with infected triatomine gut contents and lasted for 35 days. Growth and maintenance of trypanosomes in the Warren artificial medium were satisfactory.
Mummies of Wankarani Indians, found in northern Chile, provided evidence of their infection with T. cruzi and their migration, earlier than 500 years BC, from Bolivia1,12. The first cases of Chagas disease in Bolivia were reported by Arthur Neiva, in Potosí (1916), followed by Veintemillas, in La Paz (1931), Mazza & Chacon, in Cochabamba (1937) and Martins & Macedo, in Santa Cruz (1941)21.
It is now generally accepted that the center of dispersion of triatomines in South America was the region of the Bolivian Interandean Valleys where Cochabamba is located. The caves inhabited by pre-Columbian man presumably offered microclimatic conditions similar to those of rodent burrows, a habitat to which triatomines had already been adapted10. Further dispersion of the vectors resulted in the spread of Chagas disease among the human population of South America1.
Data from the 1990s show a prevalence of T. cruzi infection ranging from 9.0% to 45.0% (average 40.3%) in the Bolivian departments situated in endemic areas. The average prevalence in the Department of Cochabamba is 31%23. However, in such localities as Tabacal, prevalence values as high as from 70% to 100% have been observed in the segment of the population aged over 35 years15. Valencia (1990)21 estimates that, all age groups included, this proportion must be around 55.2%, varying between 41.3% and 59.0%.
Information about some peri-urban areas of Cochabamba is both scant and preliminary. About 17.5% of the general population and 7.2% of the segment aged between 1 and 4 years are seropositive4. In Avaroa/Primer de Mayo 12.5% were found to be seropositive, but none in the 1- 9 age group, despite the fact that 9 (21.4%) of the latter were residents of houses in which triatomines infected with T. cruzi had been found. An independent observation conducted by the authors in Avaroa/Primer de Mayo, using blood samples from children aged 5 to 9 years (n = 80), showed 2 (2.5%) to be seropositive. Although this locality was founded 16 years ago, the average time of residence of its inhabitants is 6.1 years (mode and median less than or equal to 5 years). The risk of human infection was demonstrated by the capture of triatomines, which had fed on human hosts, infected with trypanosomes. This risk can also be inferred, in regions of recent occupation, by the proportion of seropositive children rather than seropositive adult migrants (who might have acquired the infection elsewhere). Thus the children born in this locality should also be followed up and investigated to rule out the possibility of congenital transmission, as was done by Azogue & Darras2. Although the proportions of seropositive reactions showed no significant differences between the genders, the percentage of female seropositivity (15.1%) is a cause for concern. Azogue & Darras2 found 15.5% of the newborn children of seropositive mothers to be seropositive.
Migration from endemic areas of the country favors vector transmission of the infection23. About half the seropositive inhabitants of Avaroa/Primer de Mayo came from endemic areas of Bolivia, which indicates that they are possibly not autochthonous cases. On the other hand, the occurrence of seropositive subjects born in areas labeled non-endemic (as Potosí) is not necessarily a rare event, observed by Carrasco et al.6 (1990), who found high proportions of seropositivity among prospective blood donors from this region. It is therefore necessary to investigate more accurately the epidemiological situation of some "non-endemic" regions. The fact that no IgM-seropositive subjects were found does not preclude the possibility of transmission in this region; these antibodies are detectable only during the initial phases of the infection5,17. In Cochabamba, Chagas disease is frequently acquired through blood transfusion. Is has been estimated that, during 1988, blood transfusion contributed 5 new cases of Chagas disease per day4. As a great number of infected individuals live in the city, the danger of infection through blood transfusion should not be underrated. In Avaroa/Primer de Mayo T. infestans, infected with trypanosomes, is often found in domestic and peridomestic environments. They feed on humans and animals such as birds, rodents, and dogs, living in or around their dwellings. In a preliminary observation, carried out in the outskirts of Cochabamba, T. infestans was found more often inside than around the houses (29% and 16 % respectively)4. The season during which this investigation was undertaken was not recorded. In Avaroa/Primer de Mayo T. infestans was also found more frequently inside than around the houses (17.3% and 7.0%, respectively). The observations of this study were carried out in winter, when the activities of triatomines are expected to be reduced11.
People involved in massive migration from rural environments go to the outskirts of big cities, where they live in shacks which provide optimal conditions for triatomine breeding. Insects brought along with the belongings of migrating families can colonize dwellings still not infested. The guinea pig (Cavia cobaya), a notorious reservoir of T. cruzi14, is frequently kept inside the dwellings, a habit dating back to pre-Columbian times and still followed in the Andean regions4, 20. Thus the significance of the observation that the infected triatomines had fed predominantly on rodents should not be overlooked. Most of the houses visited were of recent occupation, which calls for prompt action against the vector, considering that Chagas disease has been diagnosed both in children and adults, and a relatively high proportion of triatomines feed on human or rodent hosts.
The occupations of the inhabitants of Avaroa/Primer de Mayo are typically urban, rural activities being exceptions. Students and housewives, the majority of this sample, deserve special attention, because they are in a position to cooperate with public health personnel in checking the spread of vector-transmitted Chagas disease.
To Scientific Researchers Vera Lucia Pereira Chiaccola and Antonio Marcos de Aparecida Levy of the "Instituto Dante Pazzanese de Cardiologia, S. Paulo", for providing standar negative and positive sera.
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José Maria Soares Barata
Av. Dr. Arnaldo, 715 01246-904 São Paulo, SP-Brasil
The publication of this article was supported by FAPESP (Process nº 98/13915-5).
Submited on 22.5.1998. Reviewed on 19.10.1998. Approved on 10.11.1998.