Under diagnosis of intestinal schistosomiasis in a referral hospital, North Ethiopia
Alemu et al. BMC Res Notes
Under diagnosis of intestinal schistosomiasis in a referral hospital, North Ethiopia
Megbaru Alemu 0
Eyob Zigta 1
Awoke Derbie 0
0 Department of Microbiology, Immunology and Parasitology, Bahir Dar University , Po. Box 79, Bahir Dar , Ethiopia
1 Department of Medical Parasitology and Vector Biology, Mekelle University , Mekelle , Ethiopia
Objective: The present cross-sectional study was aimed at determining the magnitude of under diagnosis of intestinal schistosomiasis among patients requested for routine ova/parasite examination at Ayder referral hospital. Results: A total of 280 stool samples were collected and only 5% of the patients were positive for ova of Schistosoma mansoni in the routine direct wet mount microscopy. On the other hand, 12.5% of the patients were positive for ova Schistosoma mansoni when the stool samples were processed by either Kato Kat or formol ether concentration techniques. Moderate test agreement (κ = 0.48) was recorded for wet mount. Formol-ether concentration (κ = 0.89) and Kato-Katz (κ = 0.92) showed excellent agreements with the 'Gold' standard. Direct wet mount technique exhibited the poorest sensitivity (35%) of detection of ova of Schistosoma mansoni. Hence, the Kato-Katz technique should be implemented in parallel with the direct wet mount microscopy for Schistosoma mansoni presumptive patients.
Wet mount; Formol ether concentration; Kato-Katz; Mekelle; Ethiopia
Human bilharziasis is caused by the trematode
species of Schistosoma mansoni, Schistosoma hematobium,
Schistosoma japonicum, Schistosoma intercalatum and
Schistosoma mekongi [
]. The disease is highly prevalent
throughout Africa, South America and several Caribbean
]. It is one of the most widespread of all human
parasitic diseases, ranking second only to malaria in
terms of its socioeconomic and public health importance
in tropical and subtropical areas [
]. Estimates suggest
that over 250 million people were infected and the
disease caused 11,700 deaths and a global burden of
3.3 million disability-adjusted life years [
In Ethiopia, schistosomiasis is widely spread in the
country where endemic areas are located in the
altitudinal range of 1200–2000 m above sea level [
spread of the disease also appears to have been facilitated
in areas which were originally non-endemic as a result of
the initiation of water-based development schemes [
High infection rates of S. mansoni were reported in the
hyper endemic areas of northwestern, northeastern and
northern parts of the country [
Examination of stool is the primary method of
diagnosing suspected Schistosoma mansoni infections. There
are several diagnostic techniques such as Kato-Katz,
wet-mount, and formol-ether concentration technique
(FECT). Despite its low sensitivity, the direct wet mount
is the only method employed for diagnostic purpose
of intestinal parasites in general and schistosomiasis in
particular in health institutions of Ethiopia while FECT
and Kato-Katz are reserved for research purpose [
The Kato-Katz method is characteristically rapid, easy to
perform and require minimal training. The formol-ether
concentration technique on the other hand is
time-consuming, and requires several materials.
Schistosome species lay only few numbers of eggs and
only two-third of the eggs are excreted intermittently
with stool, making single wet mount microscopy prone
for false negative results [
The reliable diagnosis of intestinal schistosomiasis
therefore requires a more rapid, economical, easy, and
sensitive method. Stool examination for intestinal
parasitosis is performed solely by wet mount procedure at
Ayder referral hospital. It has been a custom that
clinicians sending stool specimens of patients with
presumptive S. mansoni infection to Microbiology and
parasitology research laboratory whose stool
examination results turned negative in Ayder hospital laboratory.
Consequently, ova of S. mansoni were isolated from most
of the referred stool specimens in the research
laboratory. Hence, we sought to assess performance of wet
mount against the Kato-Katz and FECT in the diagnosis
of intestinal schistosomiasis and to recommend the best
technique in the hospital.
Study design and area
This cross-sectional study was conducted in Ayder
referral hospital, North Ethiopia, from August to October
2016. The Hospital provides referral and non-referral
services to more than 8 million populations in its
catchment areas. It provides a broad range of medical
services to both in and outpatients of all age groups. With
the total capacity of about 500 inpatient beds in four
major departments and other specialty units, the
hospital is also used as a teaching hospital for the College of
Health Sciences, Mekelle University. Stool examination
for intestinal parasitic infections in general and intestinal
schistosomiasis in particular is mainly based on direct
wet mount microscopy.
Sample collection and parasitological examination
Patients were provided with stool cups to bring adequate
stool samples. The routine direct wet mount microscopy
performed in the hospital laboratory. The Kato-Katz and
formol-ether concentration (FEC) methods, on the other
hand, were performed in microbiology and parasitology
The test procedures were carried out in accordance
with standard protocols reported by World Health
Organization (WHO) [
Kato‑Katz method An approximately 42 mg fecal
sample was sieved through a 200 µm Kato nylon screen mesh.
The stool was transferred into a 6 mm hole of a template
on a microscopic slide and covered with glycerol soaked
cellophane strip. The microscopic examination then
proceeded to identify schistosome eggs and to calculate the
number of eggs per gram (EPG) of feces [
]. Based on
egg counts, cut-off values for classification of the
intensity of infection were used. The intensity of S. mansoni
was classified into: light infection (1–99 EPG), moderate
(100–399 EPG) and heavy (≥ 400 EPG) [
Formol ether concentration technique (FEC)
Approximately 500 mg of feces was mixed with 10 ml of normal
saline and the mixed stool was strained via gauze into
a funnel. The strained contents were collected in a
centrifuge tube. About 2.5 ml of 10% formaldehyde (Loba
Chemie Pvt Ltd., 107, Wodehouse Road, Jehangir villa,
Mumbai-40005, India) and 1 ml of diethyl ether (Blulux
laboratories Pvt Ltd. 121005) was then added and
centrifuged at 1000g for 3 min. The supernatant was removed
and a drop of the sediment was covered with cover glass
for a microscopic investigation [
Wet mount preparation Fresh stool samples
(approximately 2 mg of stool) were put on a slide with wooden
applicator, emulsified with a drop of physiological saline
(0.85%) covered with a cover slide and examined at 10×
and 40× microscopic objectives [
Data entry and analysis
Data were entered and analyzed using SPSS version 20
statistical software. Estimation of the performance of the
three diagnostic tests was made by taking the combined
results of the wet mount, FEC and Kato-Katz tests as a
“Gold” standard diagnostic test, because stool
investigation for intestinal parasitosis lacks ‘Gold’ standard
Sensitivity, specificity, PPV (positive predictive value),
NPV (negative predictive value) and Kappa value of wet
mount, FEC and Kato-Katz techniques were computed
against the ‘Gold’ standard. The kappa score was used to
estimate the agreement between stool diagnostic tests
and the ‘Gold’ standard.
Data quality control
Laboratory technicians in charge of microscopic
investigations were blinded to the results of the wet mount,
FECT, and Kato-Katz. A different laboratory technician
was responsible for preparing and reading Kato-Katz and
FEC. In addition, results of the wet mount, Kato-Katz,
and FEC techniques were recorded on different sheets to
ensure strict blinding.
The overall prevalence of S. mansoni infection was 40
(14.3%) with a parasitic load ranging from 24 to 480 eggs
per gram of stool. S. mansoni infections were
predominantly light, 65% and moderate 35%. The peak prevalence
of S. mansoni infection was recorded for the 10–14 years
of age (17.6%) followed by those 15 and above years of
age (14.5%). Mean intensity of infection was also higher
in the age group 10–14 years (224 EPG). The overall
prevalence of infection was 12.7% for females and 15.8%
for male participants (P > 0.05). The mean intensity
of infection was also higher for males (201 EPG) than
females (169 EPG) (Table 1).
The prevalence of S. mansoni using wet mount,
Kato-Katz and FEC was 5, 12.5 and 12.5%, respectively
(Table 2). The detection rate when two techniques were
used at a time was 13.2% for wet mount and Kato-Katz,
12.9% for wet mount and FEC and 13.6% for Kato-Katz
and FEC. The detection rate was 14.3% when all the three
tests were used together (Table 2). Kato-Katz detected 23
samples that were negative by wet mount. Similarly, 21
samples were detected by FEC that were negative by wet
The sensitivity of wet mount, Kato-Katz and FEC in the
detection of S. mansoni infection was 35 (95% CI 20.6–
51.7), 87.5 (95% CI 73.2–95.8), 85 (95% CI 70.2–94.3),
respectively. Similarly, NPV of 90.2 (95% CI 88–92.1),
98 (95% CI 95.5–99.1) and 97.6 (95% CI 95–98.8) were
reported, respectively for the wet mount, FEC, and
Kato-Katz. Moderate test agreement (κ = 0.48) was
recorded for the wet mount. FEC (κ = 0.89) and
KatoKatz (κ = 0.92) on the other hand showed excellent
agreements with the Gold standard (Table 3).
The prevalence of S. mansoni infection in this study was
14.3%, which was higher than previous reports [
This might be explained in part by the nature of the study
participants, as symptomatic patients were recruited in
our study. The prevalence and intensity of S. mansoni
were found to be higher in males than females in the
current study. This goes in agreement with other studies in
]. The existence of more outdoor
activities and water exposure habits among males might have
contributed to these findings. The peak prevalence of S.
mansoni was recorded for the 10–14 age group which
was consistent with similar other findings .
Diagnosis of intestinal parasitosis is based on detection
of the eggs in stool samples examined through a
variety of parasitologic methods, and no single technique is
satisfactory. In Ethiopia, healthcare laboratories employ
the direct wet mount as the preferred stool
parasitological examination technique by virtue of its simplicity, low
cost, and rapidity.
Although the wet mount technique is the preferred
method in resource-limited settings [
], reliance on it as
the sole diagnostic tool in routine practice is very likely
to cause misdiagnosis of infections. This, in turn, leads to
grave clinical and public health consequences [
It is supported by our study in which wet mount
technique detected only one-third (14/40) of the study
participants infected by S. mansoni.
The Kato-Katz method is characteristically rapid, easy
to perform and require minimal training. Its high
sensitivity (87.5%) and specificity (100%) for the diagnosis of
S. mansoni compared to the wet mount is well noted in
our study. It was also slightly sensitive than formol-ether
concentration. Consistent results were reported by
]. The technique is a useful tool for the
quantification of egg counts to determine infection intensities.
These qualities make the Kato-Katz the most frequently
employed method in research works .
Time-consuming procedures, the requirement of
trained personnel and several materials/equipment make
the FEC technique the most expensive method to be
considered as an alternative for routine laboratory
diagnosis of intestinal schistosomiasis. However, our study
revealed that the sensitivity and specificity of FEC are not
much less than the Kato-Katz method for detecting eggs
of S. mansoni.
The prevalence of S. mansonia via wet mount, FEC,
and Kato-Katz was 5, 12.5 and 12.5%, respectively. Our
study revealed that the Kato-Katz and FEC techniques
had about threefold increased in the detection rate of S.
mansoni than the wet mount. Consistent findings were
reported in previous studies [
]. Our study revealed
that Kato-Katz (87.5%) was slightly sensitive than FEC
(85%) to detect S. mansoni. Similarly, Kato-Katz showed
the highest agreement with the Gold standard (κ = 0.92)
while wet mount showed the lowest agreement (κ = 0.48).
This showed that the use of Kato-Katz can ultimately
reduce misdiagnosis of intestinal schistosomiasis, and
reduce morbidity and mortality due to schistosomiasis.
This was supported by other findings in Ethiopia [
Direct wet mount technique exhibited the poorest
sensitivity of detection of ova of Schistosoma mansoni. Most of
the infections were predominantly light in the study area
which requires implementation of concentration
methods. Hence, the Kato-Katz technique should be
implemented in parallel with the direct wet mount microscopy
for Schistosoma mansoni presumptive patients.
Limitations of the study
In this study, we collected only a single stool sample and
hence we were unable to describe the variation of egg
counts as Schistosoma mansoni female worms undergo
intermittent egg excretion. Future investigations should
focus on the feasibility of the concentration techniques in
terms of the turn-around-time (TAT) in health facilities
that serve large number of outpatients.
FECT: formol ether concentration techniques; EPG: eggs per gram; WHO:
World Health Organization; PPV: positive predictive value; NPV: negative
MA designed the study and wrote the manuscript; EZ participated in the
design of the study, data collection and write-up of the manuscript; AD
participated in data analysis and revision of the manuscript. All authors read and
approved the final manuscript.
We are thankful to medical laboratory staff at Ayder referral hospital.
The authors declare that they have no competing interests.
Availability of data and materials
To produce findings of this study, the stated methods and materials were
applied. All the data were incorporated in the manuscript and no
supplementary files accompanied the submission. The original data supporting this
finding will be available at any time upon request.
Consent for publication
Individual data such as images and videos did not accompany this submission
and hence consent for publication is not applicable.
Ethics approval and consent to participate
Ethical approval was granted by Mekelle University, College of Health Sciences’
institutional review board (Reference Number ERC071/2015). Written consents
were signed by patients to collect stool specimens for Kato-Katz and FEC
processing. Stool samples reported negative by direct wet mount microscopy
and turned positive by Kato-Katz and/or FEC were communicated to the
hospital to trace patients for possible treatment.
The research project was not funded by any organization.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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