Revista da Sociedade Brasileira de Medicina Tropical
Journal of the Brazilian Society of Tropical Medicine
Vol.:53:(e20200411): 2020
https://doi.org/10.1590/0037-8682-0411-2020
Major Article
Schistosomiasis: an epidemiological update on Brazil's
southernmost low endemic area in Esteio
Angélica da Paz Ramírez[1]*, Vivian Favero[1], Catieli Gobetti Lindholz[1],
Carolina de Marco Veríssimo[1], Vanessa Fey Pascoal[1], Renata Russo Frasca Candido[1],[2],
Alessandra Loureiro Morassutti[1], Carlos Graeff-Teixeira[1],[3]
[1]. Pontifícia Universidade Católica do Rio Grande do Sul, Escola de Ciências,
Departamento de Ecologia e Biodiversidade, Porto Alegre, RS, Brasil.
[2]. University of Western Australia, Department of Physics, Perth, Australia.
[3]. Universidade Federal do Espírito Santo, Centro de Ciências da Saúde,
Departamento de Patologia e Núcleo de Doenças Infecciosas, Vitória, ES, Brasil.
Abstract
Introduction: Brazil’s southernmost state, Rio Grande do Sul (RGS), was considered schistosomiasis-free until 1998 when a low endemic
focus was identified in Esteio, a city located next to the capital of RGS. In the last two decades, the control interventions applied in the
region have been apparently successful, and the absence of new cases indicated the possibility of interrupted schistosomiasis transmission.
The objective of this study was to update the clinical and epidemiological data of schistosomiasis in Esteio. Methods: We reviewed all 28
individuals diagnosed with the infection since 1997 and a survey was applied to a group of 29 school-aged children residing in Vila Pedreira,
one of the most affected neighborhoods. Results: No eggs were detected in fecal samples using the Helmintex method, and all samples were
negative for serum antibodies on examination by the western blot technique using the Schistosoma mansoni microsomal antigen (MAMAWB). In contrast, 23 individuals (79%) tested positive for the cathodic circulating antigen with the point-of-care immunochromatographic
test (POC-CCA) on urine samples. Of the 28 formerly infected individuals, only eight were located, of which four tested positive, and
four tested negative for serum antibodies using the MAMA-WB technique. Conclusions: Current adverse conditions for S. mansoni
transmission in Esteio and the absence of a confirmed diagnosis suggests that there is (i) a lack of specificity of the POC-CCA test in low
endemic settings, and (ii) a high probability that interruption of schistosomiasis has been achieved in Esteio.
Keywords: Schistosomiasis. POC-CCA. Helmintex. MAMA. Esteio. Low endemicity.
INTRODUCTION
The areas in Brazil endemic for schistosomiasis infection include
six northeastern states, the southeastern state of Minas Gerais, and
12 additional states that were found to be the focus of transmission1,2.
Located next to the border with Uruguay and Argentina, the state
of Rio Grande do Sul (RGS) has been considered schistosomiasisfree; however, the autochthonous transmission of the parasite was
confirmed in 1998 after the identification of infected snails at two
sites next to the Sinos River in Esteio3. Biomphalaria glabrata snails
were first documented in Esteio in 1997 after schistosomiasis was
diagnosed in an adult male hospitalized with hepatitis. However,
the occurrence of autochthonous infection was not confirmed4.
*In memorian.
Corresponding author: Carlos Graeff-Teixeira.
e-mail: graeff.teixeira@gmail.com
https://orcid.org/0000-0003-2725-0061
Received 27 June 2020
Accepted 31 July 2020
www.scielo.br/rsbmt I www.rsbmt.org.br
The presence of active transmission was documented a year
later after the identification of snails infected with Schistosoma
mansoni3, concurrent with the detection of a second individual
diagnosed with schistosomiasis. In this study, we aimed to update
the epidemiological data for the transmission of schistosomiasis and
tested the hypothesis of transmission interruption in Esteio (RGS).
METHODS
Study areas and populations
The municipality of Esteio (29º51′ S, 51º10′ W) is located in
the metropolitan region of Porto Alegre, the capital of Brazil’s
southernmost state, RGS. The town of Esteio, which had an
estimated population of 84,114 in the year 2016, is located
alongside the Federal Road BR-116, in a north-south direction, to
the eastern side of the highway5. The Sinos River runs in a northsouth direction, where infected B. glabrata specimens were found
in the swampy plains along the riverside in the following sites: (1)
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Ramírez AP et al. - Schistosomiasis update in Esteio, Brazil
Banhado do Azeite (BA); (2) Valo da Três Portos (VTP); and (3)
Casa dos Trilhos (CT). The site CT was an isolated shallow pond
next to the Siderúrgica Riograndense railway, and snails were not
detected at this location after the initial findings in 19996. At the
site Fazenda Kroeff (FK), there were indications for transmission,
and snails were found, though the presence of S.mansoni in snails
was never documented. VTP was the only area where people living
next to the breeding sites of snails with active transmission last
documented in 2011. Infected individuals were residents of the VTP,
Novo Esteio (NE), and Vila Pedreira (VP) neighborhoods, where
most of the individuals that were initially infected lived (Table 1).
Collection of biological samples for the
epidemiological survey at VP
were stored at -80°C for further use. Strips containing S. mansoni
microsomal antigen (MAMA) and the protocol for western blot
were obtained from the Centers for Disease Control and Prevention,
USA7,8. Test sera were diluted 1:100 in PBS containing 0.3%
Tween 20 (Sigma-Aldrich, USA) and 5% nonfat milk powder
(Nestlé, Brazil) and incubated with the strips for 1 h at room
temperature (RT) with agitation. After 4 washes with 0.3%
Tween-PBS, the strips were incubated with anti-human IgG coupled
to peroxidase for 1 h at RT, given 4 washes with 0.3% Tween-PBS,
and were subsequently developed using 3,3’-Diaminobenzidine
(Sigma-Aldrich, USA). Positive and negative control sera were
included for each batch of strips.
Detection of antigen in urine
Fecal, urine, and serum samples were collected from 29
asymptomatic school-aged children (aged 6 to 9 years) from the
Centro Municipal de Educação Basica Trindade (Municipal Center
for Basic Education Trindade), a community school in VP, between
November to December 2015. The children examined had no history
of travel to endemic areas or exposure to transmission sites near
the Sinos River. The urine and blood samples were collected on a
single day at school, and students were given instructions to collect
their stools at home and bring it back to the school. Before the
collection of biological material from the students, several activities
were pursued in the classroom at the community school, including
playing games, watching videos, observing live snails, viewing
parasitic structures using the microscope, and having educational
discussions about health and other issues.
Follow-up investigation of
previously infected individuals
A search for previously infected individuals was performed by
visiting their homes at the addresses registered when the patient was
documented as being infected. Queries regarding the status of their
general health and risky behavior were put to them. For individuals
who had relocated, the neighbors/relatives were asked for the new
addresses. Additionally, we reviewed the files at the Laboratory of
Parasite Biology at The Pontifical Catholic University of RGS to
summarize and update the information related to the tests performed
for the infected individuals in Esteio.
Enzyme-linked immunoelectrotransfer blot
Approximately 5 mL of blood was collected via venipuncture
under aseptic conditions and transferred to 15 mL centrifuge
tubes for the separation of serum on the same day. Serum samples
Urine samples collected in VP were refrigerated and tested in
the laboratory on the same day. Point-of-care circulating cathodic
antigen test (POC-CCA; Rapid Medical Diagnostics, Pretoria, South
Africa; lot number: 50182, expiry date: 2017/10) was performed
as per the manufacturer’s instructions. Briefly, one drop of urine
was placed in the cassette, followed by a drop of the supplied
buffer. After 20 min, three observers independently checked for
the presence of the control and test lines. A “trace” was defined as
a faint line without clearly defined margins.
The Helmintex method (HTX) for the
detection of S. mansoni eggs
The HTX assay was performed according to the technique
described by Favero and collaborators9. The stool samples (30 g)
were fixed in a solution of 5% Tween-20/70% ethanol. After fixation
and homogenization, the fecal suspension was sieved through a
500 μm metal mesh, transferred to a conical flask, and washed
until a clear supernatant was obtained. The resulting sediment was
further sieved through metal meshes with openings of 150 μm and
45 μm sequentially, and the latter sieving process was performed to
retain the S. mansoni eggs. The fraction retained by the last sieve
(45 μm) was suspended in a 30% (v/v) ethyl acetate aqueous solution,
homogenized, and centrifuged for 10 min at 200 ×g. After discarding
the supernatant and the ring of debris, the pellet was transferred to
a microtube containing 19 µL of paramagnetic iron oxide particles
(Bangs Labs, USA). The microtubes were homogenized by orbital
rotation for 30 min and placed on a magnetic rack (Bangs Labs,
USA). After 3 min, the unbound material was discarded before
removing the tubes from the rack. The sediment was re-suspended
in 100 μL of 0.9% aqueous NaCl solution and stored at -4°C for
further analysis. For microscopy analysis, the sediments were
TABLE 1: Distribution of the dwellings of 28 infected individuals in three neighborhoods: Vila Pedreira (VP), Novo Esteio (NE), and Valo da Três Portos (VTP),
between 1997 to 2011, in Esteio, Rio Grande do Sul, Brazil.
Local/Year
97
VP
●●
2/7
98
99
00
01
●●
NE
●●●
●
VTP
●
●●
●
02
03
●
●●
●
●●
04
05
06
07
08
09
●
●●●
10
11
●
●●
●●●
Rev Soc Bras Med Trop | on line | Vol.:53:(e20200411): 2020
stained with 3% (w/v) ninhydrin (Sigma-Aldrich, USA) in 70%
ethanol and evenly spread over 24 μm pore filter paper (UNIFIL,
Brazil) of area 5 × 2.5 cm, provided an ID and kept for examination
by optical microscopy (100× magnification).
Ethical considerations
The research was conducted according to Brazilian regulations
and conformed to the principles outlined in the Declaration of
Helsinki of 1964 as revised in 2000. The study protocol was
approved by the Research Ethics Committee of PUCRS (CAAE
18944614.3.0000.5336).
RESULTS
Evaluation of formerly infected individuals
Between 1997 and 2000, 11 infected individuals from Esteio
were diagnosed as having schistosomiasis6, and an additional 17
individuals were found to be infected between 2001 and 2011
(Table 1). An updated summary of the clinical and epidemiological
data for the 28 infected individuals is presented in Table 2. The
patients were living in three neighborhoods: VP, NE, and VTP,
at the time of diagnosis. Table 3 summarizes the information on
the updated (2015) dwellings of the patients; information for 10
TABLE 2: Summary of the original and updated clinical and epidemiological data for the 28 infected individuals from Esteio, Rio Grande do Sul, Brazil, who were
infected between 1997 to 2011.
Follow-up in 2015
Id*
Date
Initial clinico-epidemiological situation
Neighborhoods
WB-MAMA
Clinical –
epidemiological
situation
1
January 1997
hepatitis, admitted to the local Hospital
Vila Pedreira
Positive
Asymptomatic
2
October 1997
positive in parasitological survey
Vila Pedreira
Negative
Asymptomatic
3
October 1998
urticaria, outpatient clinic
Novo Esteio
-
4
October 1998
sharing risk behavior with patient 3 (BA)
Novo Esteio
-
Not found
-
Moved, outbound
Esteio
5
October 1998
sharing risk behavior with patient 3 (BA)
Novo Esteio
6
October 1998
positive in parasitological survey
VTP**
-
7
May 2000
positive in parasitological survey
Vila Pedreira
Positive
8
May 2000
positive in parasitological survey
Vila Pedreira
-
9
July 2000
Abdominal pain
VTP
-
10
October 2000
positive in parasitological survey
VTP
Positive
11
October 2000
positive in parasitological survey
Novo Esteio
-
12
October 2002
positive in serological survey, confirmed parasitologically
Novo Esteio
-
13
October 2002
positive in serological survey, confirmed parasitologically
Vila Pedreira
-
14
January 2003
positive in serological survey, confirmed parasitologically
Vila Pedreira
15
June 2003
positive in serological survey, confirmed parasitologically
Novo Esteio
-
16
Jullho 2003
positive in serological survey, confirmed parasitologically
Novo Esteio
Negative
Moved, outbound
Esteio
Not found
Asymptomatic
Not found
Not found
Asymptomatic
Not found
Moved to Montenegro,
RS. Outbound Esteio
Death, not related to
schistosomiasis
Moved to Canoas, RS.
Outbound Esteio
Moved to Imbé, RS.
Outbound Esteio
Asymptomatic
17
Jullho 2003
positive in serological survey, confirmed parasitologically
Vila Pedreira
Positive
18
June 2001
positive in serological survey, confirmed parasitologically
Novo Esteio
-
Not found
19
July 2007
sharing risk behavior with patient 10 (VTP)
VTP
-
Not found
Asymptomatic
20
July 2007
sharing risk behavior with patient 10 (VTP)
VTP
-
Not found
21
November 2007
positive in parasitological survey
VTP
-
Not found
22
October 2008
positive in parasitological survey
Novo Esteio
-
Not found
23
June 2009
positive in parasitological survey
VTP
-
Not found
24
June 2009
positive in parasitological survey
VTP
-
Not found
25
July 2010
sharing risk behavior with patient 18 (BA)
26
August 2010
sharing risk behavior with patient 10 (VTP)
Novo Esteio
Canoas
Not found
Negative
27
November 2010
sharing risk behavior with patient 10 (VTP)
Canoas
Negative
28
November 2010
sharing risk behavior with patient 10 (VTP)
Unknown
-
Asymptomatic
Asymptomatic
Not found
*Id: identification code; ** VTP: Valo da Três Portos neighborhood.
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Ramírez AP et al. - Schistosomiasis update in Esteio, Brazil
TABLE 3: Updated (2015) information for the dwellings of the 28 individuals from Esteio, RS, Brazil who were infected with schistosomiasis between 1997 to 2011.
Information on residence
Number
Number
Known
18
Not moved
5
Moved, outbounds Esteio
12
Moved, inbounds Esteio
1
Unknown
10
Totals
18
individuals was not available, 5 were living in the same place, and
13 individuals had moved from their original place of residence
(1 had moved within, and 12 had relocated out of Esteio). Serum
samples for follow-up experiments were collected from 8 formerly
infected individuals, of which four each tested positive and negative
using the MAMA-WB assay. No other method was used for the
examination of these individuals.
School-aged children
Samples for urine, blood, and/or feces were collected from 29
children aged between 6 and 9 years (average 7.13 ± 0.97 yrs.), who
had a gender distribution of 55% male and 45% female children.
HTX and MAMA-WB assays were not performed on every sample
because a few children showed resistance to submitting blood or
feces, or both. Seven children provided only fecal and urine samples,
while 16 children provided blood and urine samples alone (Table 4).
No fecal or serum samples were found positive after examination by
HTX and/or MAMA-WB assays; however, the POC-CCA urine test
results were positive or a trace line was seen in 23 out of 29 samples
(79.3%). Trace results for the POC-CCA test were observed in 8
of the 23 positive samples (considered as positive CCA detection,
according to the manufacturer’s instructions), and 6 of the total 29
samples were CCA-negative (Table 4).
DISCUSSION
The focus of schistosomiasis transmission in Esteio has several
peculiar characteristics. First, B. glabrata snails have never been
detected in Brazil’s southernmost state, RGS4,10, and these have
probably been recently introduced in Esteio by fishermen to be
used as baits, as suggested by several residents. Endemic areas
in the northern state of Paraná, 1,100 km away from Esteio, have
been the southern limit for the occurrence of B. glabrata in Brazil1.
Second, swampy sites with infected snails were found to be sharply
delimited next to the Sinos River and were located at some distance
from the urbanized neighborhoods, especially in NE, west of BR116 road, which had ~5,000 inhabitants. In VTP, a small number
of families lived next to an old man-made channel (“valo”) to
pump water from the river to a nearby cellulose industry named
“Três Portos”. Between 1997 to 2011, the VTP area alone had
overlapping houses and transmission sites before environmental
changes and population reduction occurred, as described below.
The Vila Pedreira neighborhood located immediately to the east
4/7
28
TABLE 4: A comparison of the results from three diagnostic methods: point-of-care
immunochromatographic detection of circulating cathodic antigen (POC-CCA),
Western-blot with microsomal Schistosoma mansoni antigen (WB-MAMA) and
Helmintex for egg detection in feces. The data are from 29 school-aged children
from Vila Pedreira, Esteio, Brazil, 2015.
Sample
1
4
6
19
20
17
26
28
5
9
12
13
21
22
27
7
8
11
14
16
23
24
29
2
3
10
15
18
25
POC-CCA
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Positive
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Negative
Negative
Negative
Negative
Negative
Negative
WB-MAMA
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Helmintex
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
Negative
-
of the BR-116 road had ~1,500 inhabitants and also had infected
individuals. In Esteio, except for the inhabitants of VTP, exposure to
S. mansoni infection may have been hindered by the long distances
which require travel on foot, which may partially explain the
ultra-low endemicity observed for more than two decades. Positivity
rates were observed to be below 0.13% in surveys from 1997 to 20006.
As indicated in Table 1, most of the infected individuals initially resided
in VP, and in 2011, most of the infected individuals were from VTP.
Rev Soc Bras Med Trop | on line | Vol.:53:(e20200411): 2020
The main transmission site was found to be Banhado do Azeite
(BA), an approximately 48,000 m2 grassy shallow swamp on the
left margin of the Sinos River. It was a popular fishing location used
mainly by adults and young males, who commonly used snails as
bait. People from the NE and VP communities, and others, reported
that the areas next to the Sinos River were considered unsafe
because of violence and activities related to drugs, as the river is
used as an escape route to hide. Environmental modification has
also contributed to the reduced transmission in BA and VTP. Since
2003, successful drainage treatment has prevented the land in BA
from holding water, and consequently, it remains mostly dry. The
occurrence of transmission was documented in VTP until 2011, but
the construction of BR-448 road (between 2011 to 2013) affected the
area, reducing its water content. All residents relocated from VTP
after several conflicts, and a new owner started earthworks to build
a warehouse. Thus, a combination of the geographic characteristics,
environmental changes, and cultural-educational factors contributed
to the decline in the transmission of schistosomiasis in Esteio.
study of larger groups and the daily repetition of the HTX assay.
However, this was not possible in 2015 due to operational difficulties
resulting from the socially problematic settings in these areas. The
low specificity of 35.5% was estimated for the POC-CCA test in a
large population in the endemic area of southern Sergipe in Brazil24.
Specificities for the POC-CCA test were evaluated to be higher
in non-endemic areas in Tanzania (86.7%), Ethiopia (99%), and
Ecuador (100%)26-28. Coelho and collaborators have demonstrated
the inaccuracy of the “trace” results obtained using the POC-CCA
test (considered as positive per the manufacturer’s instructions):
after lyophilization, samples that previously showed ambiguous
results in the POC-CCA test were found to be either positives or
negatives29. The cost of such an improvement in the interpretation
of results is the loss of the intended “point-of-care” character of
the test23. The discrepancies in the accuracy of different batches of
POC-CCA kits require resolution before additional extensive
evaluations lead to the detection of antigen consistently and
reproducibly in urine, especially in areas with low endemicity22,23,30.
Further, based on the information provided by the children,
their parents, and relatives, a striking change in behavior was
observed concerning exposure to transmission sites next to the Sinos
River, although these anecdotes were not part of the systematized
data gathering procedure. The School Trindade in VP has made
continuous efforts to build awareness in children and the community
about behavioral risks and prejudices related to schistosomiasis.
This would partly explain the negative results obtained from the
serological and HTX assays in the 6 to 9 yr age group. Positivity
rates in school-aged children (SAC) are usually a good indicator
of the prevalence of schistosomiasis in the community at large.
Further, testing SAC can be used to infer active transmission, and
help avoid false-positive results from chronically infected, albeit
cured adult individuals11. Additionally, this group is a priority target
for control interventions12.
In this study, the mobility of the population living next to the
Sinos River schistosomiasis focus (Table 3) prevented the close
follow-up of infected individuals by public health agents or research
groups. For many individuals from the area, changing their address,
and concealing information is part of a strategy to cope with socioeconomic problems. However, although mobility can increase the
probability of schistosomiasis spread, it may involve distancing
from the transmission sites. Moreover, the active role played by
community health agents (agentes comunitários de saúde) has
considerably reduced and prevented the spread of schistosomiasis
through the evaluation of risk behavior, providing immediate
treatment, and close follow-ups.
The MAMA assay was employed for the detection of antibodies
and it has an estimated sensitivity and specificity of 99%13,14.
Microsomal antigens, including those for other Schistosoma species
such as S. japonicum (JAMA) and S. haematobium (HAMA), are
useful for monitoring the prevalence of the disease, especially in
areas of low endemicity15,16. Further, serum antibodies are longlasting and can give positive results in patients who no longer carry
the infection17.
Helmintex is a new egg-detection reference method for the
identification of true-positive schistosomiasis-infected individuals.
It has a 100% sensitivity for egg burdens higher than 1.3 eggs
per gram of feces18, and is based on the isolation of eggs from a
large volume of feces through their interaction with paramagnetic
particles that can be recovered using a magnetic field9,19.
Antigen detection in urine is a promising alternative as a
screening tool for the Kato-Katz method in areas with high
endemicity20,21. In contrast, the performance of POC-CCA may be
unreliable in low endemicity areas22-25. In our study, we obtained
positive results from the POC-CCA test in 79.3% samples, with
no corresponding positive results from the MAMA-WB and HTX
assays, suggest that the test has potential specificity limitations. The
confirmation of false-positives in the POC-CCA test requires the
In this study, there were difficulties in obtaining biological
material, especially feces. This was a major drawback in many
surveys undertaken in Esteio for two decades. In contrast, obtaining
urine samples is much easier; and these can be obtained immediately
after the first contact with households or at gatherings like the one
established at the Trindade School for this study. However, the
poor performance and reproducibility of the POC-CCA detection
method is an important concern for its implementation as a routine
screening procedure, especially in areas such as Esteio that have
low endemicity23,25,30.
To conclude, there are indications that schistosomiasis
transmission has been interrupted in Esteio, though the confirmation
of this observation requires long-term monitoring. However, it is
challenging to avoid a tendency to neglect epidemiological vigilance
for a health problem that does not translate into a detectable
disease. The current situation described in Esteio indicates a case of
successful schistosomiasis control, that contributes to the national
and global efforts of the Brazilian Ministry of Health, the PanAmerican Health Organization, and the World Health Organization
in containing the spread of schistosomiasis12,31.
ACKNOWLEDGMENTS
We acknowledge the support of the entire staff at the Vila
Pedreira School, Centro Municipal de Educação Básica Trindade,
and especially of Dilara Maria Acosta and the headmaster Marcelo
Ohlweiler. We thank Elsa Sinara Both, Daiane de Oliveira
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Ramírez AP et al. - Schistosomiasis update in Esteio, Brazil
Garcia (community health agents), Maria Ceci Salcedo Botelho
(schistosomiasis control manager), and the entire staff at the
Public Health Authority of Esteio. Special thanks are due to the
children, their families, and the communities at Vila Pedreira,
Novo Esteio, Valo da Três Portos (Seu Remy and family), and other
neighborhoods. John Noh and Sukwan Handali, from the Centers for
Disease Control and Prevention, USA kindly provided the MAMA
antigen and suggestions related to the study.
AUTHOR CONTRIBUTIONS
APR: study conception and design, and data acquisition,
analysis, and interpretation; VF: data acquisition, analysis, and
interpretation, and the critical revision and final approval of the
version of the manuscript to be submitted; CGL: data acquisition,
final approval of the version of the manuscript to be submitted;
CDMV: data acquisition, and drafting of the manuscript, it’s critical
revision, and final approval of the version to be submitted; VFP:
data acquisition, analysis, and interpretation, and the final approval
of the version of the manuscript to be submitted; RRFC: study
conception and design, analysis and interpretation of data, drafting
of manuscript; critical revision, and final approval of the version
to be submitted; ALM: study conception and design, analysis and
interpretation of data, and the critical revision and final approval
of the version of the manuscript to be submitted; CGT: study
conception and design, analysis and interpretation of data, drafting
of the manuscript, and the critical revision and final approval of the
version to be submitted.
FINANCIAL SUPPORT
C.Graeff-Teixeira is recipient of Conselho Nacional de
Desenvolvimeto Científico e Tecnológico PQ 1D fellowship and
funding: 307005/2014-3.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interest.
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