Am. J. Trop. Med. Hyg., 102(4), 2020, pp. 827–831
doi:10.4269/ajtmh.19-0904
Copyright © 2020 by The American Society of Tropical Medicine and Hygiene
Survey of Schistosomiasis in Saint Lucia: Evidence for Interruption of Transmission
Janice Gaspard,1 Madelaine M. Usey,2 Merlene Fredericks-James,1 Maria J. Sanchez-Martin,3 Lydia Atkins,1 Carl H. Campbell Jr.,4
Paul L. A. M. Corstjens,5 Govert J. van Dam,6 Daniel G. Colley,4,7 and W. Evan Secor8*
1
Ministry of Health and Wellness, Castries, Saint Lucia; 2Integrated Life Sciences Program, University of Georgia, Athens, Georgia; 3Neglected
Infectious Diseases, Pan American Health Organization, Washington, District of Columbia; 4Schistosomiasis Consortium for Operational Research
and Evaluation, Center for Tropical and Emerging Global Diseases, University of Georgia, Athens, Georgia; 5Department of Cell and Chemical
Biology, Leiden University Medical Center, Leiden, Netherlands; 6Department of Parasitology, Leiden University Medical Center, Leiden,
Netherlands; 7Department of Microbiology, University of Georgia, Athens, Georgia; 8Division of Parasitic Diseases and Malaria, Centers for Disease
Control and Prevention, Atlanta, Georgia
Abstract. Saint Lucia at one time had levels of schistosomiasis prevalence and morbidity as high as many countries in
Africa. However, as a result of control efforts and economic development, including more widespread access to sanitation
and safe water, schistosomiasis on the island has practically disappeared. To evaluate the current status of schistosomiasis in Saint Lucia, we conducted a nationally representative school-based survey of 8–11-year-old children for
prevalence of Schistosoma mansoni infections using circulating antigen and specific antibody detection methods. We
also conducted a questionnaire about available water sources, sanitation, and contact with fresh water. The total population of 8–11-year-old children on Saint Lucia was 8,985; of these, 1,487 (16.5%) provided urine for antigen testing,
1,455 (16.2%) provided fingerstick blood for antibody testing, and 1,536 (17.1%) answered the questionnaire. Although a
few children were initially low positives by antigen or antibody detection methods, none could be confirmed positive by
follow-up testing. Most children reported access to clean water and sanitary facilities in or near their homes and 48% of
the children reported contact with fresh water. Together, these data suggest that schistosomiasis transmission has been
interrupted on Saint Lucia. Additional surveys of adults, snails, and a repeat survey among school-age children will be
necessary to verify these findings. However, in the same way that research on Saint Lucia generated the data leading to
use of mass drug administration for schistosomiasis control, the island may also provide the information needed for
guidelines to verify interruption of schistosomiasis transmission.
Schistosomiasis, a parasitic disease caused by blood
flukes, affects more than 200 million people globally. The infection is acquired through contact with fresh water that is
inhabited by the parasite’s intermediate host, specific freshwater snails, and contaminated with human waste containing
schistosome eggs.1 Consequently, this neglected disease
primarily affects impoverished communities and is of major
importance to global public health.2 Of the schistosome
species that infect humans, Schistosoma mansoni is the only
species transmitted in Caribbean nations and in Latin America.3 Infection with S. mansoni results in intestinal schistosomiasis, which can cause diarrhea, bloody stool, anemia,
impaired cognitive development, and stunted growth when
left untreated. The most severe cases involve periportal hypertension, ascites, and hepatosplenomegaly.1,3,4
In the mid-20th century, inhabitants of Saint Lucia, an
island nation in the eastern Caribbean, demonstrated high
levels of infection with S. mansoni. In the 1970s, some hightransmission areas demonstrated greater than 50% infection
prevalence in all age groups, with children frequently exhibiting extremely high egg counts (> 1,000 eggs per gram feces)
and hepatosplenomegaly.5,6 In addition, the snail intermediate
host for S. mansoni, Biomphalaria glabrata, was widely distributed throughout water sources on the island.5
With the goal of reducing the rising infection levels, the Saint
Lucian government partnered with the Rockefeller Foundation
to enact a project to identify the optimal schistosomiasis
control strategy for the island.5 When this partnership began in
1965, available schistosomiasis control methods included
molluscicides, public health education, water and sanitation
improvements, and selective chemotherapy. However, because combinations of these methods had been used, it was
unclear which individual methods had contributed most to
schistosomiasis control and provided the most cost-effective
solution to reduce disease burden.5 The mountainous topography of Saint Lucia, which creates relatively secluded
valleys with distinct watersheds between which travel was
limited, offered a unique opportunity for the researchers to test
the individual effectiveness of these different schistosomiasis
control methods5 and subsequently to evaluate them in different combinations.
The cost-effectiveness findings from the Saint Lucia project
were pivotal in establishing the current global strategy of
preventive chemotherapy through mass drug administration
with praziquantel for schistosomiasis control. Following the
implementation of multiple control efforts, the prevalence of
infection had declined to 9% by 1981.6 However, once the
project with the Rockefeller Foundation concluded, few surveys to assess S. mansoni prevalence in Saint Lucia were
conducted. In 2006, an epidemiological survey was conducted
in 554 schoolchildren from the southern part of the island, and
four cases (0.6%) of S. mansoni were found using the Kato-Katz
stool microscopy assay.7 Between 2006 and 2015, primarily
through mandatory stool screening of those in the hospitality
industry, 36 individuals were diagnosed with schistosomiasis.
Most of these were adults who were screened by thick stool
smear. However, one 10-year-old boy was diagnosed with
S. mansoni infection in 2010,8 suggesting that some level of
transmission was still occurring later than 2000. Thus, although
the infection rates appeared very low, the status of schistosomiasis transmission on Saint Lucia remains unclear.
In 2012, through World Health Assembly Resolution 65.21,
the WHO called for nations endemic for schistosomiasis to
* Address correspondence to W. Evan Secor, Centers for Disease
Control and Prevention, 1600 Clifton Rd., Mailstop H23-10, Atlanta, GA
30329. E-mail: was4@cdc.gov
827
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GASPARD AND OTHERS
adopt intensive control programs to achieve elimination of
transmission where feasible.9 A limited number of studies in
Morocco, Japan, Puerto Rico, and Egypt10,11 suggest that
schistosomiasis transmission can be interrupted in endemic
areas, but criteria to verify elimination have not been defined.
The situation in Saint Lucia presents an opportunity to develop
and evaluate possible approaches for verifying elimination. A
first step is to assess the current state of S. mansoni infection
on Saint Lucia.
METHODS
Survey population. A school-based survey of children
aged 8–11 years (grades 3 to 6) across all public primary
schools was used to estimate schistosomiasis transmission.
Primary education is mandatory and enrollment across Saint
Lucia for the 2013/2014 school years approached 95%. Thus,
children sampled from schools were expected to be representative of the primary school-age population of the country.
Public schools were selected as they represented 94% of the
total enrolled school-age population. The Ministry of Education reported a total enrollment of 16,628 children, with 8,985
students enrolled in grades 3 to 6, across 63 primary schools
in the 2016/2017 school year. Primary schools are present
across the entire island with some aggregation in urban areas.
The survey was designed to detect a schistosomiasis prevalence of 1% with a 0.5% SD. The target sample size was
adjusted to include a design effect of 1.5 and a 15% buffer to
compensate for parents or children who declined study participation. Accordingly, it was estimated that 2,243 (1,950 total
samples, +15% refusal) children should be included in the
survey sample. This would represent approximately 25% of
children enrolled in school between the ages of 8 and 11.
Selection of children and sample collection in schools.
Before the survey, all parents and guardians were informed
about its purpose, the sample collection process and testing,
as well as the risks and benefits of participation via an official
letter and a parent–teacher conference. If parents did not
provide permission, their child was removed from the list of
eligible children. Children were also informed of the aim and
purpose of the survey and were provided with detailed information involving sample collection procedures. Children
provided assent to participate and had a right to withdraw at
any time. Participating students were given a flexi-pencil.
Sampling at individual schools was based on population
proportional to size. A list with the names of all children in
grades 3 to 6 was compiled. The first child was selected from
the class register using a random number generator, then
every fourth child (to represent 25% of total children) was
selected until the required sample was drawn from each
school. Children who participated were assigned a personal
study identification code to protect their personal health information. A bathroom monitor labeled a plastic container
with the child’s identification number, provided verbal instructions for fresh urine collection, and received the sample
to ensure that the urine corresponded with the correct identification number. Finger stick blood samples were obtained
by registered nurses using a safety lancet and collected on
TropBio filter paper collection cards (CeLLabs, Sydney, Australia). Finally, a questionnaire about water usage habits and
access to sanitation facilities was administered to each child
by a student nurse. All samples and questionnaires were
labeled with the child’s identification code and linked to the
master sample list, by date and time of sample collection.
Because it is not uncommon for children to attend school in an
area different from where they live, their community of residence was noted to provide more accurate mapping of areas
where transmission might have occurred.
Circulating antigen testing. Urine samples were transported to one of four nearby health posts in an insulated
container. Commercially available point-of-care circulating
cathodic antigen (POC-CCA) tests (Rapid Medical Diagnostics, Pretoria, South Africa batch #170316032 for the 2017
tests and batch #170622073 for the 2018 tests) were used to
detect CCA from adult schistosomes according to manufacturer’s directions. Samples were scored as 0 (negative), trace,
1+, 2+, or 3+. Trace results were considered positive. Urine
samples that yielded a positive result in the field along with a
subset of urines that tested negative were stored at 4°C and
subsequently sent to the University of Georgia (UGA) for
retesting. Urine samples with equivocal or consistently positive results were sent to Leiden University Medical Center
(LUMC) for up-converting particle-lateral flow circulating anodic antigen (UCP-LF CAA) assay as previously described.12
Schistosome-specific antibody testing. Filter papers
containing the blood spots were dried, sorted, placed in
plastic bags with desiccant, shipped to the CDC, and stored
at −20°C until testing. For serologic testing, enzyme-linked
immunosorbant assays were used to detect IgG antibodies
specific for S. mansoni–soluble egg antigen (SEA).13 Immulon
2HB plates (ThermoScientific, Rochester, NY) were coated
overnight at 4°C with 100 μL/well of 2 μg/mL SEA in 0.1 M
sodium bicarbonate buffer (pH 9.6). Each dried blood spot
(DBS) was eluted overnight in 250 μL of sample buffer (0.01 M
PBS, pH 7.2, containing 0.3% Tween-20% and 5% nonfat
dried milk) at 4°C on a shaker. Standards, controls, and
samples were diluted to a 1:50 concentration in sample buffer
and allowed to incubate on the plate at room temperature for
30 minutes. A standard curve of 0 to 500 arbitrary units (AUs)
using pooled positive sera was included on each plate. All
standards, controls, and samples were run in duplicate. Plates
were washed five times using 0.01 M PBS, pH 7.2, containing
0.3% Tween-20. Mouse antihuman IgG conjugated to
horseradish peroxidase (Southern Biotech, Birmingham, AL)
at a dilution of 1:50,000 in sample buffer was added and
allowed to incubate for 30 minutes. Following the incubation
and plate washing, SureBlue™ TMB substrate (SeraCare Life
Sciences, Gaithersburg, MD) that had been warmed to room
temperature was added and incubated for 5 minutes before
addition of 1 N sulfuric acid to stop any further reaction. The
standard curve cutoff for this assay has been established at 40
AU but to be sure that all possible positives were identified,
any samples with ³ 30 AU were also tested by immunoblot
against S. mansoni adult worm microsomal antigen for confirmation. Before Western blot testing, DBSs were eluted
overnight in 500 μL of sample buffer at 4°C on a shaker. Immunoblots were conducted as previously described.14
Ethical considerations. The survey protocol was approved
by the Saint Lucia Medical and Dental Council Ethics Committee, the Ministry of Education’s research approval committee, and the Pan American Health Organization’s Research
ethics committee. The protocol was also reviewed by CDC
and designated as a program activity. Local health and education authorities were also officially informed and agreed to
829
SCHISTOSOMIASIS TRANSMISSION ON SAINT LUCIA
participate. Any participant who tested positive would be
given a referral to the nearest health center for treatment
according to international standard treatment guidelines for
schistosomiasis: 40 mg/kg praziquantel.
RESULTS
Survey population. The survey took place in April and May
of 2017. A total of 1,536 children were enrolled in the survey
and provided responses to the questionnaire; 1,487 provided
urine samples for testing, 1,455 provided fingerstick blood for
immunologic analysis. The enrollment fell short of the targeted
sample size of 2,243, but it still represents more than one of
every six children in the eligible age range for every measure
across the entire country. The rate of participation in the survey was similar for every school and area of the island.
Circulating antigen testing. Initial urine testing using the
POC-CCA test was conducted in Saint Lucia by nurses and
laboratory workers who had recently been trained on how to
perform the POC-CCA. Of the 1,487 urine samples tested,
1,279 (86%) were scored negative, 59 (4%) were scored trace
or positive, and 150 (10%) were scored “suspicious,” meaning
that the reader was not comfortable scoring the test as a clear
positive or a clear negative. Urine samples that yielded positive or suspicious results, along with some negative samples,
were stored at 4°C until they could be shipped to UGA for
confirmation testing.
A total of 307 urine specimens were selected, shipped, and
received at UGA, but 116 of these specimens were deemed
not suitable for additional evaluation because they were received without proper temperature conservation. Of the 191
specimens considered suitable for evaluation and tested at
UGA, 94 (49.2%) were negative and 97 (50.8%) were positive
by POC-CCA. Of the 97 positive samples, 76 (78%) were read
as “trace.” If “trace” is considered a positive result, then 6.5%
(97/1,487 urine specimens) would be the percentage of children on Saint Lucia with positive results.
After testing, the 191 urines by POC-CCA, UGA then shipped 111 of those specimens, including both trace-positive
and negative specimens by POC-CCA, to LUMC. These
specimens were then tested using the highly sensitive and
specific UCP-LF CAA assay. Of the 111 specimens tested by
the UCP-LF CAA assay, nine gave non-negative results: two
were considered indecisive, six were very low positive, and
one was a low positive.
Schistosome-specific antibody testing. A total of 1,455
DBS samples were collected for serological testing. Of these,
10 (0.69%) had ELISA results > 30 AU, suggesting potential
exposure to the parasite at some point during their lives. A
second ELISA test was performed on these 10 samples, as
well as a confirmatory immunoblot (Table 1). None of the 10
samples retested by ELISA were immunoblot positive. In addition, none of the 10 children with ELISA results > 30 AU were
positive by POC-CCA (Table 1). Seven of the urine samples
from these 10 participants were clearly negative when tested
on Saint Lucia and were, therefore, not forwarded to UGA for
confirmation testing. The other samples (3/10) were scored
“suspicious” during POC-CCA testing on Saint Lucia and
subsequently tested negative by POC-CCA at UGA.
Follow-up testing. Of the nine children whose urine specimens tested indecisive, very low positive, or low positive by
UCP-LF CAA at LUMC, eight had provided DBS. When these
samples were tested for schistosome-specific antibodies by
immunoblot, one had a faint positive result. This was the same
participant who had tested low positive by UCP-LF CAA;
however, this individual had a negative ELISA result (6.2 AU).
In May 2018, the Schistosomiasis Program on Saint Lucia
collected follow-up urine and blood specimens from the nine
children whose initial urine specimens had provided nonnegative results by UCP-LF CAA. Because they were not
confirmed schistosomiasis positive in 2017, they had not been
treated with praziquantel. POC-CCA testing at UGA found that
six of the nine specimens still tested as positive with trace
readings. All nine repeat samples were negative by immunoblot
and UCP-LF CAA testing.
Questionnaire data. Of the 1,536 children who answered
the questionnaire, 1,384 (90.1%) reported that they had water
piped into their homes (1,147, 74.6%) or housing plot (237,
17.1%; 151 had both, 86 had water piped to their plot but not
into their homes). Of the 303 children who did not receive
piped water at their households, 105 (34.6%) had access to
water at a public standpipe, 46 (15.2%) reported using rainwater, and 41 (13.5%) reported using water from a spring. Ten
children overall, including one child that did not have access to
piped water at their house or plot, reported using surface water
from rivers or ponds. With respect to contact with rivers or
ponds, 40.9% (626/1,532) of the children reported swimming,
31.8% (419/1,316) reported wading, 18.9% (253/1,341) reported fishing, and 15.7% (239/1,526) reported collecting
water from rivers or ponds when service from the water supply
was interrupted. Overall, 48% (737/1,535) reported some
contact with fresh water.
With respect to sanitation, 98.7% (1,516/1,536) of survey
participants had access to some sort of improved toilet
TABLE 1
Immunoblot and POC-CCA results of children with borderline or positive ELISA results
School
ELISA 1 reading
ELISA 2 reading
Immunoblot
POC-CCA in Saint Lucia
POC-CCA in the University of Georgia
School A: urban
School B: urban
School C: urban
School D: urban
School D: urban
School D: urban
School E: rural
School F: rural
School G: rural
School H: rural
166.3
37.1
31.8
117.9
38.4
70.6
33.4
47.3
36.3
39.4
343.9
30.1
27.8
187.7
46.8
132.8
30.8
59.1
25.3
47.3
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
negative
suspicious
negative
negative
suspicious
negative
suspicious
–
–
–
–
negative
–
–
negative
–
negative
POC-CCA = point-of-care circulating cathodic antigen.
830
GASPARD AND OTHERS
facilities. Access to a flush toilet in their home was reported by
1,264 (82.3%) of the children. An additional 238 (15.5%) reported
using a toilet connected to a septic tank or latrine and 14 (9.1%)
more said they had access to shared sanitary facilities.
DISCUSSION
Considering the totality of the results from the series of tests
performed in these surveys, we were unable to confirm infection with S. mansoni infection for any of the children who
participated in the study. Although some children were positive by antigen testing, the proportion positive was consistent
with the percentage of false positives expected when using
the POC-CCA assay in areas with a very low prevalence of
schistosomiasis, as is the case with Saint Lucia.10,11 The small
number of children who had ELISA results near or above the
cutoff were negative in the confirmatory immunoblot and had
negative POC-CCA results. The one study participant who
had a low positive result in the antigen tests and immunoblot
was negative by ELISA and negative for both antigen and
antibody tests on follow-up testing, suggesting that the initial
results were false positives. Even if the child had been infected
in 2017 and naturally cleared his infection by 2018, although
the antigen tests may have turned negative, true antibody responses to worm antigens do not clear that quickly.15
The finding that none of the children were positive for
S. mansoni infection even though almost half of them reported
contact with fresh water suggests that schistosomiasis
transmission may have been interrupted on Saint Lucia.
However, additional work will be necessary to verify this
finding. For example, a survey of adults on Saint Lucia, especially those with occupational contact with fresh water,
would be important as persons with untreated infections can
continue to excrete eggs for decades. Similarly, a systematic
survey of snails in freshwater bodies is needed to determine
whether appropriate intermediate snail hosts remain on the
island, where they are located, and if any of them are infected.
As part of the Rockefeller Project, competitor snails that are not
suitable intermediate hosts for schistosomiasis transmission
were introduced and successfully replaced B. glabrata in several locations.16 People living near bodies of water where
B. glabrata remains would be more likely to have infection and
could be more intensively surveyed as part of the verification
process. Finally, a second serosurvey of school children is
needed to confirm the 2017 results and strengthen the evidence
against ongoing transmission.
Just as the earlier work on Saint Lucia was critical for elucidating the current global schistosomiasis control strategy
using mass drug administration, these surveys have the potential to inform guidelines to confirm elimination of transmission that are currently under development. Along with
showing negative results from human testing, the high level of
access to clean water and sanitation could be important
considerations for dossier preparation. Another important
aspect of the work on Saint Lucia was the formation of a
steering committee that included representatives from the
health, education, and agriculture sectors under the coordination of the Ministry of Health and Wellness. Engagement
by these stakeholders with a range of expertise and commitment to completing various assessments was necessary to
carry out the 2017 survey and will be needed to successfully
undertake the challenging process of demonstrating
elimination. As with any disease elimination effort, proving a
negative result is difficult. But, by building on the promising results obtained in this first countrywide survey of schoolchildren,
it may soon be possible to verify the elimination of schistosomiasis transmission on Saint Lucia as well as provide a template
for the process of schistosomiasis elimination in other countries.
Received December 6, 2019. Accepted for publication January 3,
2020.
Published online February 10, 2020.
Acknowledgments: This manuscript is dedicated to Joseph A. Cook
who provided medical care to the schistosomiasis patients on Saint
Lucia during much of the Research and Control Project and provided
advice regarding this survey. The authors would also like to recognize
the contributions of the field workers and nurses on Saint Lucia who
assisted with the sample collection, as well as the student participants
and their parents.
Financial support: This work was supported in part by the University of
Georgia Research Foundation, Inc., which was funded by the Bill &
Melinda Gates Foundation for the SCORE Project. This article will be
paid by Chronos for the Bill & Melinda Gates Foundation under
OPP50186, Schistosomiasis Consortium for Operational Research
and Evaluation (SCORE).
Disclosures: Maria Jesus Sanchez-Martin is a staff member of the
Pan American Health Organization.
Disclaimer: The funder had no role in the study design, data collection
and analysis, decision to publish, or preparation of the manuscript.
The author alone is responsible for the views expressed in this publication, and they do not necessarily represent the decisions or policies
of the Pan American Health Organization or the Centers for Disease
Control and Prevention.
Authors’ addresses: Janice Gaspard, Merlene Fredericks-James, and
Lydia Atkins, Ministry of Health and Wellness, Castries, Saint Lucia,
E-mails: janelva@gmail.com, mfredericks2020@yahoo.com, and lydatkins
@gmail.com. Madelaine M. Usey, Integrated Life Sciences Program, University of Georgia, Athens, GA, E-mail: madelaine.usey@uga.edu. Maria
J. Sanchez-Martin, Neglected Infectious Diseases, Pan American Health
Organization, Washington, DC, E-mail: sanchezmar@paho.org. Carl H.
Campbell Jr. and Daniel G. Colley, Schistosomiasis Consortium for
Operational Research and Evaluation, Center for Tropical and Emerging
Global Diseases, University of Georgia, Athens, GA, E-mails: ccamp@
uga.edu and dcolley@uga.edu. Paul L. A. M. Corstjens, Department of
Cell and Chemical Biology, Leiden University Medical Center, Leiden,
Netherlands, E-mail: p.l.a.m.corstjens@lumc.nl. Govert J. van Dam,
Department of Parasitology, Leiden University Medical Center, Leiden,
Netherlands, E-mail: g.j.van_dam@lumc.nl. W. Evan Secor, Division of
Parasitic Diseases and Malaria, Centers for Disease Control and
Prevention, Atlanta, GA, E-mail: was4@cdc.gov.
This is an open-access article distributed under the terms of the
Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution, and reproduction in any medium, provided
the original author and source are credited.
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