Background
Existing guidelines by world health organization (WHO) for control and elimination of Schistosomiasis recommends baseline evaluations of the prevalence of Schistosome infections to inform programmatic decisions on target populations and treatment frequency within endemic areas [
1]. In evaluating
Schistosoma mansoni, mapping by examinations for parasite eggs in the stool using Kato-Katz thick smear microscopic technique is usually the practice [
2]. This technique is usually preferred by control programmes in areas of moderate to high infection intensity and prevalence due to its ability to give both intensity and prevalence data for large sample of subjects. Due to its extensive use, it was established as part of WHO guidelines for morbidity control programmes [
3]. The guideline recommends examining one stool per subject using two separate Kato-Katz slides read by two microscopists when estimating prevalence and intensity for control programmes, this is believed to provide near true prevalence estimate in high-mean intensity areas [
4].
However, the Kato-Katz technique has showed day-to-day and intra-stool variability especially in areas with low intensity of infection and prevalence [
5‐
7], with the cause of day-to-day variation believed to be biological [
8], and the intra-stool variation attributed to sampling error due to the amount of stool evaluated [
9]. This has rendered the technique less useful especially in areas with lower rates of transmission with many studies now projecting less true prevalence estimates from this technique as the population mean intensity of infection decreases [
1,
10‐
13]. Programmes depending on single stool examination possibly lead to underestimated true infection prevalence with such observation resulting in an inaccurate allocation of control measures.
Owing to the reduced sensitivity of Kato-Katz technique in areas of low endemicity, improved diagnostic methods for accurate detection of
S. mansoni in the at-risk populations and monitoring progress of control programmes are desirable. Several studies have now documented the availability of indirect diagnostic tests like point-of-care circulating cathodic antigen (POC-CCA) as valuable alternatives to the direct parasitological methods for diagnosis of
S. mansoni [
6,
14,
15]. This test detects Schistosome glycoprotein in the host urine after being deposited into the bloodstream by actively feeding worms and subsequently cleared from the host’s kidneys [
14]. The Schistosome antigens; both CCA and circulating anodic antigen (CAA) are detectable in urine of infected individuals and they act as specific markers for the presence and intensity of infection [
6,
16,
17]. Studies that have been conducted in Kenya [
18], Uganda [
15,
19], and Ivory Coast [
16] to assess the performance of CCA urine dipstick and POC-CCA test among pre-school and school-aged children have both determined the usefulness of this technique in detection of
S. mansoni infection in those age groups. These findings have been corroborated further by systematic reviews that have found POC-CCA to be up to 6-fold accurate than Kato-Katz technique especially in areas of low prevalence [
5].
In Kenya, the ministries of health and of education began a national school-based deworming (NSBD) programme in the year 2012 in 27 counties identified as having a high prevalence of both soil-transmitted helminthes (STH) and Schistosome infections [
20,
21]. According to the Kenyan National School Health Policy, treatment for both STH and Schistosome infections is administered to all school-aged children including those out of school, based on the baseline prevalence in the identified areas so as to reduce infection [
21]. This national programme has been using Kato-Katz technique as the primary diagnostic tool in monitoring the impact of infection transmission over a five-year period (2012–2017). Having reduced the prevalence of
S. mansoni infection over the five years, and due to the growing concern over the reduced sensitivity of Kato-Katz technique, this current cross-sectional study was therefore designed to compare the performance of the stool-based Kato-Katz technique with the commercially available urine-based POC-CCA test with the view to inform decision-making by the programme in changing from Kato-Katz to POC-CCA test in its effort to control transmission of Schistosomiasis.
Results
Overall, 1899 children with mean age of 9.7 years (2–18 years) were surveyed in the pre-treatment survey, and 1878 children with mean age 9.4 years were surveyed during the post-treatment survey as shown in Table
1. All the pre-treatment surveys were conducted approximately 17 days before the treatment day and post-treatment surveys conducted 12–30 days after the treatment delivery. Information on sex was recorded for 98.8% of the children out of which 50.4% were male. The study did not necessarily survey same children during both pre- and post-treatment surveys and those children absent on the day of the survey were not included in the study; moreover, children who did not provide both stool and urine samples were excluded from further analysis.
Table 1
Number of schools and children examined by county
Bomet | 1 | 108 | 1 | 103 |
Bungoma | 1 | 108 | 1 | 105 |
Busia | 2 | 214 | 2 | 210 |
Homa Bay | 1 | 108 | 1 | 102 |
Kakamega | 2 | 212 | 2 | 213 |
Kisii | 1 | 105 | 1 | 103 |
Kisumu | 3 | 312 | 3 | 313 |
Kwale | 3 | 317 | 3 | 316 |
Mombasa | 1 | 98 | 1 | 108 |
Narok | 2 | 215 | 2 | 214 |
Taita Taveta | 1 | 102 | 1 | 91 |
Overall | 18 | 1899 | 18 | 1878 |
Table
2 compares the observed
S. mansoni prevalence using the two techniques in overall and by county. The prevalence of
S. mansoni infection was calculated with one Kato-Katz technique and compared with one POC-CCA technique. The observed prevalence using POC-CCA technique was 26.5% (95% CI: 24.6–28.6) during pre-treatment and 21.4% (95% CI: 19.6–23.4) during post-treatment compared to those observed when using Kato-Katz technique of 4.9% (95% CI: 4.0–5.9) and 1.5% (95%CI: 1.0–2.1) for pre- and post-treatment respectively. The observed prevalence for both pre- and post-treatment of
S. mansoni infection using POC-CCA technique were significantly higher (χ
2 = 135.58,
p < 0.001) than those observed using Kato-Katz technique.
Table 2
Comparison of S. mansoni prevalence (%) using POC-CCA and Kato-Katz techniques among school aged children
Overall | 4.9 (4.0–5.9) | 1.5 (1.0–2.1) | 69.8 | 26.5 (24.6–28.6) | 21.4 (19.6–23.4) | 19.4 | 0.11 (77.1%) |
Bomet | 0 | 0 | 0 | 61.9 (53.3–71.9) | 54.9 (46.0–65.5) | 11.3 | 0.00 (42.1%) |
Bungoma | 0 | 2.0 (0.5–7.7) | + | 8.6 (4.6–16.0) | 14.6 (9.1–23.2) | + | 0.06 (88.8%) |
Busia | 28.7 (23.2–35.5) | 2.9 (1.3–6.4) | 89.9* | 46.6 (40.3–53.9) | 23.3 (18.2–29.9) | 50.0* | 0.28 (71.4%) |
Homa Bay | 0 | 3.0 (1.0–9.1) | + | 8.3 (4.5–15.6) | 13.9 (8.5–22.5) | + | 0.13 (89.3%) |
Kakamega | 1.9 (0.7–5.0) | 5.8 (3.3–10.0) | + | 20.2 (15.4–26.5) | 36.8 (30.8–44.0) | + | 0.12 (73.4%) |
Kisii | 0 | 0 | 0 | 12.1 (7.1–20.6) | 15.0 (9.4–23.9) | + | 0.00 (86.1%) |
Kisumu | 7.1 (4.7–10.6) | 1.3 (0.5–3.5) | 81.4* | 34.3 (29.4–40.1) | 14.1 (10.7–18.6) | 58.9* | 0.15 (77.4%) |
Kwale | 0 | 0 | 0 | 6.6 (4.3–10.2) | 8.5 (5.9–12.3) | + | 0.00 (92.5%) |
Mombasa | 1.9 (0.3–13.4) | 0 | 100 | 9.7 (5.2–18.0) | 5.6 (2.6–12.2) | 42.1 | −0.01 (90.3%) |
Narok | 0.9 (0.2–3.7) | 0 | 100 | 47.4 (41.1–54.6) | 42.9 (36.7–50.1) | 9.5 | 0.01 (55.2%) |
Taita Taveta | 0 | 0 | 0 | 19.2 (12.8–28.8) | 2.4 (0.6–9.3) | 87.7 | 0.00 (88.5%) |
The number of children who tested positive or negative for each of the diagnostic methods is shown in Table
3, with the results showing that among the 1761 (92.7%) samples examined during pre-treatment, 446 discrepancies were recorded (27 false positives and 419 false negatives), while only 370 discrepancies being recorded during post-treatment.
Table 3
Comparative evaluation of the POC-CCA and the Kato-Katz parasitological examination for the diagnosis of S. mansoni infection
POC-CCA urine examination | Positive | 60 | 419 | 479 | 20 | 363 | 383 | 80 | 782 | 862 |
Negative | 27 | 1255 | 1282 | 7 | 1409 | 1416 | 34 | 2664 | 2698 |
Total | 87 | 1674 | 1761 | 27 | 1772 | 1799 | 114 | 3446 | 3560 |
Taking POC-CCA as the gold standard, Kato-Katz significantly correctly identified only 80 out of 862 POC-CCA - positive
S. mansoni infections; 9.3% Sn, (95% CI: 7.4–11.4; McNemar test = 782.0,
p < 0.001) and 2664 out of 2698 POC-CCA – negative samples; 98.7% Sp, (95% CI: 98.2–99.1; McNemar test = 34.0,
p < 0.001). The sensitivity of Kato-Katz was twice lower during post-treatment than pre-treatment (pre-treatment Sn = 12.5%, post-treatment Sn = 5.2%,
p < 0.001). Overall, Kato-Katz resulted in a slight/poor detection of
S. mansoni infection; k = 0.11,
p < 0.001, concordance = 77.1% (Table
4). In all those counties i.e. Bomet, Kisii, Kwale and Taita Taveta, where
S. mansoni prevalence was zero during both pre- and post-treatment by Kato-Katz technique, sensitivity of Kato-Katz was also found to be zero.
Table 4
Showing the performance measures of Kato-Katz by each survey round with POC-CCA as the gold standard
Pre-treatment | 12.5 (9.7–15.8) | 97.9 (97.0–98.6) | 6.0 (3.8–9.3) | 0.9 (0.9–0.9) | 69.0 (58.1–78.5) | 75.0 (72.8–77.0) | 0.14 (74.7%) |
Post-treatment | 5.2 (3.2–7.9) | 99.5 (99.0–99.8) | 10.6 (4.5–24.8) | 1.0 (0.9–1.0) | 74.1 (53.7–88.9) | 79.5 (77.6–81.4) | 0.07 (79.4%) |
Overall P-value** | 9.3 (7.4–11.4) Χ2m = 782.0, p < 0.001 | 98.7 (98.2–99.1) Χ2m = 34.0, p < 0.001 | 7.4 (5.0–10.92) - | 0.9 (0.9–0.9) - | 70.2 (60.9–78.4) - | 77.3 (75.9–78.7) - | 0.11 (77.1%) Z = 11.6, p < 0.001 |
Suppose otherwise we take Kato-Katz as the gold standard, the overall POC-CCA sensitivity was found to be 70.2% and specificity was 77.3%, with higher sensitivity and specificity noted during post-treatment, (Sn = 74.1%, Sp = 79.5% respectively, and p < 0.001).
Discussion
National Schistosomiasis control programmes need diagnostic techniques which are sensitive, specific, rapid and easy to perform at point-of-care. Kato-Katz technique has long been the mainstay test in
Schistosoma mansoni diagnosis in endemic areas by most epidemiological studies [
2,
14]. However, recent studies have since documented its poor sensitivity in evaluating
S. mansoni infection thus making it less useful especially in areas with lower rates of transmission [
5‐
7]. The low sensitivity can be attributed to the relatively small stool sample examined, fluctuations in daily egg excretion and the heterogeneous distribution of eggs within the stool sample [
30‐
32]. This study provides the first rigorous assessment of the performance of Kato-Katz technique in comparison to POC-CCA in a national Schistosomiasis control programme in selected areas with lower transmission rates in Kenya.
The number of children infected with
S. mansoni as determined by POC-CCA assay of one urine sample was found to be significantly higher than those determined by duplicate Kato-Katz thick smears of one stool sample and indeed 5-fold higher during pre-treatment and 14-fold higher during post-treatment, this finding is in agreement with previous studies [
1,
14,
30,
33,
34]. In fact, a recent systematic review by Kittur et al., [
5] noted that whenever
S. mansoni prevalence was above 50% by Kato-Katz then Kato-Katz and POC-CCA results yielded essentially the same prevalence. However, whenever the prevalence is below 50% by Kato-Katz then the POC-CCA prevalence was between 1.5 and 6-fold higher and could increase further as prevalence by Kato-Katz decreased.
Out of the 862 POC-CCA positive samples, Kato-Katz classified 782 (90.7%) as negative, a scenario which can be explained by the reasons mentioned above. On the other hand, of all the negative results by POC-CCA, Kato-Katz classified only 34 (1.3%) as positive indicating a good specificity for Kato-Katz technique.
We noted that interpretation of the POC-CCA result band and inter-reader variability especially when the result is a ‘trace’ is an issue in the use of this technique, the same challenge had also been documented by other studies [
35‐
37]. However, with better training in reading the result band, the challenge can be overcome. Other studies have suggested an addition of a comparison line on the assay to make reading of the result band easy and quick [
34].
In comparison to one POC-CCA exam used as the gold standard, the stool-based Kato-Katz technique had extremely low sensitivity especially during post-treatment, but however had higher specificity both at pre- and post-treatment. In overall, the results demonstrated a significantly slight/poor inter-rater agreement between the two techniques; k = 0.11,
p < 0.001, agreement = 77.1%. Our findings corroborate those of other studies done in Kenya [
18], Uganda [
15] and Ivory Coast [
14] where POC-CCA tests detected
S. mansoni infections in pre-school and school-aged at a higher sensitivity than the widely used Kato-Katz technique. Therefore, Kato-Katz and other direct diagnostic methods have inadequacies when it comes to accurate individual diagnosis which is further hampered by the fact that stools in young children are mostly diarrheic and renders the preparation of Kato-Katz thick smears difficult, hence challenging the accuracy of the diagnosis.
Finally, the study showed that even a single urine-based POC-CCA test seemed to be a more appropriate and effective screening tool for
S. mansoni evaluation compared to stool-based Kato-Katz smears in areas with low infection prevalence. The tool was more sensitive and up to 14-fold accurate than Kato-Katz method. Although, POC-CCA test has known limitations like inter-reader variability in deciding a ‘trace’ result [
30], more investigations can be conducted on how well the tool can distinguish between negative and ‘trace’ values. Even though the study found POC-CCA as a suitable alternative to Kato-Katz technique, it is known to provide limited intensity data and often no information on STHs infections [
38], therefore we recommend its use for
S. mansoni control programmes but with Kato-Katz for control programmes targeting both Schistosmiasis and STHs infections.
Conclusions
Most large-scale Schistosomiasis control programs are based on preventive chemotherapy which usually reduces the infection prevalence and intensity of Schistosomes [
39]. The frequent treatment normally results in lowered endemicity which goes hand-in-hand with reduced accuracy of Kato-Katz technique [
40]. Hence, the need of a more sensitive and specific diagnostic tool for examination of
S. mansoni after extensive mass treatment cannot be over-emphasized. This current study found POC-CCA method as more effective, and sensitive and it was up to 14-fold accurate than Kato-Katz. It was easy to use and less time consuming.
Acknowledgements
We are grateful to the Neglected Tropical Diseases Unit, Ministry of Health, Nairobi, Kenya and the County ministries of health and education for their unwavering support for this work. Additionally, we thank the school teachers and children who participated in this study for their support. Special thanks goes to all members of the study team and field personnel for their commitment towards this work. Finally, we want to thank CIFF for funding this work. CO is supported by CIFF through KEMRI-ESACIPAC as a Statistician, ES is also supported by CIFF through KEMRI-ESACIPAC as an Assistant Research Officer, and SMN and CM are supported by KEMRI as Chief Research Officers. This paper is published with the permission of the Director, KEMRI.