Impact of preventive chemotherapy on transmission of soil-transmitted helminth infections in Pemba Island, United Republic of Tanzania, 1994-2021.

Soil-transmitted helminth (STH) infections cause significant morbidity in children and women of reproductive age. The World Health Organization (WHO) recommends preventive chemotherapy (PC) of at-risk populations with anthelminthics to control these infections. Historically, STH are very intensively transmitted in Pemba Island (Zanzibar). A survey conducted in 1994 in 12 schools estimated a STH prevalence near to 100%. This extremely high prevalence induced the introduction of PC in the island; initially, however, PC was not regularly administered because of difficulties linked to drug procurement. A second STH survey, conducted in 2011, in 24 schools estimated a prevalence of STH of 89%; after this survey, PC was regularly administered until 2018. We conducted a survey in 2021 using the same method as that used in 2011. The prevalence of STH was evaluated at 80% (95% CI 78.1-81.5) and most of the STH cases were due to Trichuris trichiura. More than 32% (95% CI 30.3-34.0) of the children investigated had infections of moderate or heavy intensity. PC has been conducted for over 25 years in Pemba Island. However, despite its beneficial impact, both the prevalence and the intensity of STH infections remain high, and the intervention has been insufficient in controlling STH morbidity. This is probably due to a combination of irregular PC, climatic conditions favourable to STH transmission, the low sensitivity of T. trichiura to benzimidazoles, high population density and poor sanitation. Improvement of sanitation coverage remains a key measure to permanently reduce the prevalence and intensity of STH. Possible changes to the present PC approaches to better control STH in Pemba would be (i) to assure high coverage in all schools, (ii) to use mebendazole instead of albendazole given its better activity on T. trichiura and (iii) to use a combination of ivermectin and mebendazole to further increase anthelminthic efficacy on T. trichiura.

Background

The group of soil-transmitted helminths (STH) includes Ascaris lumbricoides, Trichuris trichiura and the two hookworm species Ancylostoma duodenale and Necator americanus. STH cause a relevant disease burden in tropical and subtropical countries [1] where levels of sanitation are inadequate. Current estimates indicate that over 1.5 billion people are infected with at least one of these helminth species [2]. STH infections cause severe morbidity that is proportional to the number of worms infecting the host [3]. Infections of moderate to heavy intensity (MHI) are considered to cause the greater part of STH morbidity [4].

In countries where STH are endemic, in addition to improvement in sanitation and behavioural change interventions, the World Health Organization (WHO) recommends preventive chemotherapy (PC), a periodic treatment with anthelminthic medicines, to different groups of populations at risk [5]. STH are historically known to be very intensively transmitted in Pemba Island (Zanzibar). A survey conducted in 1994 in 12 schools [6] estimated a STH prevalence near to 100%, with over 76% of individuals with MHI infections. A second STH survey conducted in 2011 in 24 schools [7] estimated a prevalence of STH of 89% (95% CI 88–91) with 29% (95% CI 27–30) of individuals infected at MHI.

Since 1994, PC activities have been conducted in the island. During 1994–2011, the records of PC were imprecise, but, generally, one or two rounds of deworming were distributed every year to schoolchildren depending on the availability and sufficiency of resources. Albendazole and ivermectin were also distributed annually to the entire population of Pemba Island in the context of the Global Programme to Eliminate Lymphatic Filariasis [8], which was implemented in Zanzibar by the Ministry of Health, Community Development, Social Welfare, Gender, Elderly and Children and which distributed albendazole and praziquantel in the context of the programme for morbidity control of schistosomiasis from 2001 to 2007 [9] and, subsequently, during implementation of the Zanzibar Elimination of Schistosomiasis Elimination (ZEST) project during 2012–2017 [10]. More precise data on PC activities conducted between 2011 and 2019 are available and presented in Table 1; during this period, preschool-aged children, school-aged children and adults were targeted. The coverage reported in Table 1 is that reported annually to WHO (calculated using the estimated total number of school-aged children in the area as denominator and the number of children treated as numerator). In 2019 and 2020, no PC activities were conducted due to problems with drug procurement and the interruption of the schistosomiasis programme.

Table 1

Preventive chemotherapy activities conducted in Pemba and their coverage according to reports from MoH Zanzibar to WHO.

Intervention target	Activity	2011	2012	2013	2014	2015	2016	2017	2018	2019	2020	2021
STH and schistosomiasis	Survey	X										X
	Number of rounds		2 (Apr–Nov)	2 (Jun–Nov)	1 (Apr)	2 (Jun–Dec)	2 (May–Nov)	2 (Mar–Nov)	2 (Mar–Nov)	0	0
	Medicine used		PZQ+ALB	PZQ+ALB	PZQ+ALB	PZQ+ALB	PZQ+ALB	PZQ+ALB	PZQ+ALB	NA	NA
	Population targeted		preSACSACAdults	preSACSACAdults	preSACSACAdults	preSACSACAdults	SACAdults	SACAdults	SACAdults	NA	NA
	Coverage children		*	*	82%	99%	49%	64%	68%	NA	NA
	Coverage adults				82%	67%	47%	53%	58%
Lymphatic filariasis	Number of rounds		0	0	1 (Nov.)	0	1 (May)	0	1	0	0
	Medicine used		NA	NA	IVR+ALB	NA	IVR+ALB	NA	IVR+ALB	NA	NA
	Population targeted		NA	NA	SACAdults	NA	SACAdults	NA	SACAdults	NA	NA
	Coverage		NA	NA	83%	NA	84%	NA	96%	NA	NA

ALB = albendazole; IVR = ivermectin; NA = not applicable; PreSAC = preschool-aged children; PZQ = praziquantel; SAC = school-aged children; STH = soil-transmitted helminthiases

* Details about coverage not available.

PC remains the cornerstone for morbidity control of STH infection and its impact should be evaluated periodically. The objective of this study was thus to assess the current distribution of STH in Pemba after more than 25 years of PC considering however that the intervention has not been consistently administered.

Method

Ethics statement

Ethical approval for the study was obtained from the Zanzibar Health Research Ethical Committee (ZAHREC), which works under the Zanzibar Health Research Institute (ZAHRI), on October 2020 (REF: No. ZAHREC/03/PR/OCT/2020/24). A formal written consent was obtained from the parents/guardian of the children enrolled in the study.

Time of survey conduction: The impact survey was conducted from 1 to 25 March 2021.

School selection and enrolment of children: The total number of public primary school in Pemba is 95. For comparison purposes, we selected for the present study the same 24 schools selected in 2011 when, in each district, 6 schools where randomly selected. (Fig 1).

Fig 1

Location of the 21 primary schools in the sample the number correspond to the school number in Table 2.

Base map in public domain from U.S. Geological Survey (USGS) via TopoView (https://ngmdb.usgs.gov/topoview/viewer/#10/-5.2079/39.9923), a tool created by the National Geologic Map Database Project (NGMDB) with a basemap from OpenStreetMap.

In each selected school, two grades were identified: standard 1 (age interval 6–8 years) and standard 3 (age interval 9–12 years). To ensure comparable numbers and equal participation for both sexes, in each school the children were grouped based on their sex and 70 children were randomly selected in each of the two groups. The study procedure was explained to the selected children, who were given a consent form and a stool container to take home. The specimens collected in the school the following morning were deposited in a cold box and transported to the laboratory for examination in the afternoon.

The specimens were microscopically examined for the presence of STH eggs using the Kato–Katz technique [11]. For each specimen, a single Kato–Katz smear was prepared and the number of eggs detected for each STH species was recorded. For quality control purposes, 10% of the slides examined every day were randomly selected and read by an experienced laboratory technician.

Data entry and analysis: Demographic and parasitology data were entered in EpiInfo version 3.5.1 and later imported to Excel for cleaning. The cleaned data were then imported to Stata 13 (College Station, Texas, USA) for analysis. For each STH species, the number of eggs counted in the slide was then multiplied by a factor of 24 to obtain an egg per gram of faeces. WHO thresholds [3] were used to categorize the intensity of infection as light, moderate or heavy. The Chi Square test (χ2) was used to assess association of categorical data and the probability of < 0.05 was considered as significant. The student t-test (t-test) was used to estimate group means. The proportion test (z-test) was used to estimate differences in the proportion of infection between different years.

Results

Age and sex distribution in the sample

A total of 3454 school children (1749 females and 1705 males) were initially selected, but only the 2474 (71.6%) children who provided stool samples and signed informed consent form were enrolled in the study. Overall, the mean age of enrolled children was of 8.0 years ± 1.8 standard deviation [SD] (95% confidence interval [CI] = 7.9–8.1) with minor difference between boys (8.2 ± 2.0 SD) and girls (7.8 ± 1.7 SD). The mean age of standard 1 children was 6.34 years, and the mean age of standard 3 children was 9.63 years.

STH prevalence in Pemba

The total prevalence measured was 80% (95% CI 78–81); Of the 2474 children enrolled, 1183 (47.8%) had infection of light intensity and 796 (32.2%) had MHI infection. T. trichiura was the more prevalent species (72.3% 95% CI 70–74) and hookworms the less prevalent (18% 95% CI 17–20). Table 2 presents the prevalence of STH infection across schools and districts in Pemba.

Table 2

Prevalence of STH infection across schools and districts in Pemba.

				Prevalence STH (%)
Region	District	N	School	Any STH	A. lumbricoides	Hookworms	T. trichiura
North				83.3	55.0	26.6	75.8
	Micheweni			90.0*	69.1*	37.5*	82.7*
		1	Konde A	80.6	58.3	18.5	71.4
		2	Konde B	76.9	55.6	36.8	66.7
		3	Kinowe	98.3	83.8	35	94
		4	Tumbe	97.4	78.1	36	91.2
		5	S/vyamboni	93.5	64.8	51.9	88
		6	Wingwi A	93.5	73.8	46.7	85.1
	Wete			76.6	40.9	15.7	68.8
		7	Kangagani	75	35.7	9.8	70.5
		8	K/minungwini	89.3	61.3	14.7	81.3
		9	Mzambarauni	69.4	32.4	11.1	58.3
		10	Piki	71.4	35.2	28.6	69.2
		11	Bagamoyo	85	57.5	16.7	67.5
		12	Kizimbani	69.2	23.1	13.2	65.9
South				75.4	43.5	10.0	67.7
	Chake-Chake			73.1	40.4	7.5	68.1
		13	Uwandani	74.1	42.6	1.9	66.7
		14	Ziwani	70.2	33.3	15.5	64.3
		15	Wesha	93.8	67.1	10.4	90.6
		16	Michakaini	49.4	20.1	0	43.2
		17	Pujini	82	48.4	8.2	79.5
		18	Chanjamjawiri	68.9	31.1	8.7	64.1
	Mkoani			77.7	46.5	12.5	67.4
		19	Chambani	90.3	67	11.7	75.7
		20	Ngwachani	71	38	7	52
		21	Mizingani	85	48	18	76
		22	Mtambile	70	38	8	67
		23	Ng’ombeni	56.8	22.1	4.2	50.5
		24	Mkanyageni	93	65.8	26.3	83.3

* Significantly different from other districts (p < 0.001).

Details on the prevalence of the different species and categories of intensity of infections are shown in Fig 2.

Fig 2

Categories of intensity of infection for each of the three STH parasites and for any infection from 1994 to 2021.

Prevalence of STH by sex and age group

The analysis of STH infection relative to sex showed that boys were slightly more infected than girls (Table 3), but we do not consider this difference significant from a clinical or epidemiological point of view.

Table 3

Proportions of boys and girls infected with different species of STH.

	Parasite species
	A. lumbricoides	Hookworm	T. trichiura	Any STH
Girls	45.2%*	15.4%*	71.9%	78.5%
Boys	55.6%*	22.5%*	72.8%	81.5%

* Difference between girls and boys significantly different with p < 0.01.

The analysis of STH worm infection in relation to school grade levels (Standard 1 to 3) revealed that the prevalence of the different STH species was similar among the two grades.

Prevalence of STH infection by region, district and school

The prevalence of STH by region, district and school is presented in Table 3.

Micheweni District had the highest STH prevalence (89%); in the other three districts of the island, the STH prevalence was similar (around 76%) (p < 0.001).

Michakaini School (Chacke-Chacke District) was the only school with a prevalence of any STH below 50%; seven schools (Chambani, Kinowe, Mkanyageni, S/vyamboni, Tumbe, Wesha and Wingwi A) had a STH prevalence of over 90%.

In all schools, the prevalence of T. trichiura was consistently higher (range 43–94%), followed by A. lumbricoides (range 22–83%); that of hookworm infections ranged between 0% and 36%.Mixed infections

As indicated above, 80% of the children were infected with at least one STH parasite species, 32% were infected with only one species, 33% with two STH species and 15% with three STH species.

Intensity of infection

The prevalence of the different classes of intensity is presented in Fig 1. The mean intensity of infections is presented in Table 4.

Table 4

Mean intensity of infection in eggs per gram by species.

STH species	Survey 2011	Survey 2021
A. lumbricoides	2 279 (CI 2205–2502)	5979 (CI 5445–6512)
T. trichiura	716 (CI 664–767)	702 (CI 600–805)
Hookworms	247 (CI 225–268)	105 (CI 89–121)

Trend of STH infection in 1994–2021

In 1994, the STH epidemiological data were collected before any large-scale control intervention; interestingly, only one child of the 3578 investigated tested negative for all three STH species.

Fifteen years of PC interventions (1995–2011), even if not consistently applied, reduced the prevalence of STH in schoolchildren from 100% to 88% and the prevalence of MHI infections from 76% to 29%. Additional PC interventions conducted between 2011 and 2021 further reduced the prevalence of STH to 79%. In parallel, the prevalence of MHI infection fell from 76% in 1994 to 32% in 2021. The trend of STH prevalence by STH species and the prevalence of MHI infections are presented in Fig 2.

In terms of STH species, the main prevalent species in 1994 were T. trichiura and hookworms, with prevalences of 96% and 94% respectively. In 2021 the prevalence of T. trichiura remains high at 73%, whereas that of hookworm has fallen to 19%.

Regarding reductions in intensity of infections of MHI, hookworm infections reduced consistently from 19% in 1994 to 1% 2021, T. trichiura infections from 41% in 1994 to 18% in 2021 and A. lumbricoides infections increased slightly from 20% in 1994 to 24% in 2021 (Fig 2).

Discussion

PC is advocated by WHO and is considered the cornerstone for morbidity control and, possibly, elimination of many neglected tropical diseases including STH infection [12]. In Pemba, the results of a PC intervention conducted for 25 years are not optimal: while the prevalence of hookworms has been significantly reduced and infections of MHI with this parasite almost eliminated, the prevalence and intensity of A. lumbricoides and, particularly, T. trichiura remain high.

These results differ from those reported by Mupfasoni and colleagues [12], who examined the result of impact surveys in 15 countries in Africa, America and Asia, which show that, for all species a prevalence reduction of more than two-thirds is normally obtained after 5 years of PC intervention.

The differences between our study and the results reported by Mupfasoni and colleagues are probably linked to the extremely high STH transmission in Pemba: prevalence near 100%, as observed in Pemba in 1994, was extremely rare even in the absence of PC interventions [13-16]. In our opinion, the consistently high levels of prevalence and the intensity of infection in Pemba result from several factors: (i) the favourable climatic conditions for STH transmission (in terms of vegetation index, rainfall, soil pH, soil moisture and mean temperature) [17]; (ii) the population density (387 persons/km2), which is among the highest in sub-Saharan Africa [18]; and (iii) the very poor sanitation coverage, especially in rural areas like Pemba (27% of households in Zanzibar use unimproved toilet facilities or have no toilet facilities at all) [19].

An additional possibility is the development by the parasites (particularly T. trichiura which is the predominant species) of resistance against albendazole as speculated previously [20-21]. On the one hand, further studies to elucidate the existence of mutations in the STH genome leading to resistance to benzimidazole derivatives are under way. On the other hand, a recent study from Keiser and collaborators [22] found that the egg reduction rate obtained with albendazole for trichuriasis in Pemba was of 57%, which is within the range of efficacy indicated by WHO as normal [23].

The observation of such high prevalence of STH infection among school-aged children also raises concerns about reported drug coverage and children’s compliance with treatment, as these two indicators play a critical role in achieving the objectives of PC interventions [24].

Another interesting aspect of this study is the different performance of the STH species over time: A. lumbricoides shows an important reduction in prevalence but a paradoxical increase in infection intensity, whereas hookworms and T. trichiura show a sustained decrease in prevalence and intensity. These results are surprising because the efficacy of albendazole and mebendazole is known to be better on A. lumbricoides than on hookworms [20] and the fact that in 1994 the prevalence of hookworms was higher than that of A. lumbricoides [6].

A possible explanation is the different transmission pattern of the STH parasites that entails for hookworms the development of larval stages and skin penetration (the large majority of hookworms in Pemba is N. americanus [25]). As consequence, the free-living stages of this parasite in the soil are probably more vulnerable than those of A. lumbricoides and T. trichiura eggs, which can survive in the environment for several years [26]. In this situation, a lower impact of the anthelminthic on the parasite while in the host may produce a larger negative impact on the transmission capacity of the parasite. This is confirmed by the review of Mupfasoni et al. [12] in which in 75% of the countries examined the reduction after over 5 years of PC was higher for hookworm (average 70%) than for A. lumbricoides (average 57%) and T. trichiura (average 47%).

The study conducted in Pemba has some limitations. First, the record of the interventions implemented before 2012 are incomplete, coverage of the groups at risk is also not always optimal, and the survey was conducted more than 3 years after the last PC intervention.

In conclusion, PC interventions were implemented for over 25 years in Pemba yet, despite their beneficial impact on prevalence and intensity of STH infections, the interventions did not sufficiently control STH morbidity. This is probably due to a combination of multiple factors: irregular PC, climatic conditions extremely favourable to STH transmission, the low sensitivity of T. trichiura to benzimidazoles, high population density and poor sanitation.

We consider that, when the STH baseline prevalence is extremely high, as in Pemba, it will be extremely difficult to reach the objectives set by WHO for 2030, [27] if PC intervention is not accompanied by substantial improvement of sanitation.

In 2019, Zanzibar launched a 10-year comprehensive cholera elimination plan, which included diagnosis and case management, vaccination, a surveillance system, health promotion activities, and plans to improve infrastructure for the provision of safe water and sanitation facilities. We hope that the sanitation component of this strategy, complementing PC efforts, will reduce STH transmission in the island. Possible changes in PC approaches to better control STH in Pemba would be (i) to assure high coverage and compliance in all schools (ii) to use mebendazole instead of albendazole because it has better activity on T. trichiura [28], which currently appears to be more problematic than hookworm, and (iii) to co-administer ivermectin with mebendazole to further increase drug efficacy [28].

We did not consider the extension of PC to the entire population because the higher cost of community distribution is estimated to be 10 times higher than that for distribution through the school system for school-aged children or during child health days for preschool-aged children [29] and because of the risk to stimulating drug resistance in the parasite if PC is administered for a long time to the entire population without refugia for the parasite [30].

Disclaimer

The authors alone are responsible for the views expressed in this article and they do not necessarily represent the views, decisions or policies of the institutions with which they are affiliated.

Dataset of 2011 survey.

(XLSX)

Click here for additional data file.

Dataset of 2021 survey.

(XLSX)

Click here for additional data file.

8 Feb 2022

Dear Dr. Montresor,

Thank you very much for submitting your manuscript "Impact of preventive chemotherapy on transmission of soil-transmitted helminth infections in Pemba Island, United Republic of Tanzania, 2011–2021" for consideration at PLOS Neglected Tropical Diseases. As with all papers reviewed by the journal, your manuscript was reviewed by members of the editorial board and by several independent reviewers. In light of the reviews (below this email), we would like to invite the resubmission of a significantly-revised version that takes into account the reviewers' comments.

In addition to the reviewers comments please add CI to the text and figures (to all values of % throughout the manuscript).

When comparing prevalence there should be mention of the p value and CI - only with this information the authors can make statements regarding whether the prevalence reduced or increased - comparing just the % as currently is done is not appropriate.

Also, given the comments of rev. 2, please review your manuscript following the STROBE guidelines and include a filled in form as a Supporting Information file. This will ensure that you provide more details and may help with more careful consideration of potential biases.

We cannot make any decision about publication until we have seen the revised manuscript and your response to the reviewers' comments. Your revised manuscript is also likely to be sent to reviewers for further evaluation.

When you are ready to resubmit, please upload the following:

[1] A letter containing a detailed list of your responses to the review comments and a description of the changes you have made in the manuscript. Please note while forming your response, if your article is accepted, you may have the opportunity to make the peer review history publicly available. The record will include editor decision letters (with reviews) and your responses to reviewer comments. If eligible, we will contact you to opt in or out.

[2] Two versions of the revised manuscript: one with either highlights or tracked changes denoting where the text has been changed; the other a clean version (uploaded as the manuscript file).

Important additional instructions are given below your reviewer comments.

Please prepare and submit your revised manuscript within 60 days. If you anticipate any delay, please let us know the expected resubmission date by replying to this email. Please note that revised manuscripts received after the 60-day due date may require evaluation and peer review similar to newly submitted manuscripts.

Thank you again for your submission. We hope that our editorial process has been constructive so far, and we welcome your feedback at any time. Please don't hesitate to contact us if you have any questions or comments.

Sincerely,

Susana Vaz Nery

Associate Editor

PLOS Neglected Tropical Diseases

Hélène Carabin

Deputy Editor

PLOS Neglected Tropical Diseases

***********************

In addition to the reviewers comments please add CI to the text and figures (to all values of % throughout the manuscript).

When comparing prevalence there should be mention of the p value and CI - only with this information the authors can make statements regarding whether the prevalence reduced or increased - comparing just the % as currently is done is not appropriate.

Reviewer's Responses to Questions

Key Review Criteria Required for Acceptance?

As you describe the new analyses required for acceptance, please consider the following:

Methods

-Are the objectives of the study clearly articulated with a clear testable hypothesis stated?

-Is the study design appropriate to address the stated objectives?

-Is the population clearly described and appropriate for the hypothesis being tested?

-Is the sample size sufficient to ensure adequate power to address the hypothesis being tested?

-Were correct statistical analysis used to support conclusions?

-Are there concerns about ethical or regulatory requirements being met?

Reviewer #1: Methods:

The objective of the study is clear. However, since the authors declare that the records of PC from 1994 to 2011 are imprecise it does not seem completely correct to speak about “more than 25 five years of PC”. The statement gives the idea of continuous and consistent PC campaigns during that period which is not the real scenario of the study. Being Pemba a high prevalence area where reinfection is very possible and PC should be implemented twice a year. It is not fear for the PC strategy to evaluate its effectiveness in a setting where: i) the coverage of the PC is imprecise (for example 2016: 47 – 90%) and ii) two years have passed since the last PC round was administered (2019 and 2020).

The design is appropriate for the objective. The diagnostic method used (Kato-Kats) is the recommended by WHO for diagnosis and monitoring of STH. An adequate quality control was also performed.

A definition and description of the formula used to calculate coverage is needed, as well as clarification about the source of the data used for that indicator.

More information is needed about population. What ages have the children of grades standard 2 and standard 3? Not all the PC interventions targeted preSAC children. Then children younger than 7 years might have never received PC. The inclusion of those children in the study would underestimate the impact of the PC.

The sample size is sufficient; actually the number of children analyzed is impressive.

No ethical concerns arise from reading of the study.

Reviewer #2: 1. One of my main concerns is about the content of methods section. While the methods used were appropriate for this survey, the content of methods section is too brief. More important information should be provided. For instance, description of the study area, study population, epidemiological characteristics of the targeted population pertaining to intestinal parasites transmission, students’ participation, samples delivery and transport to examination station, etc.

2. Also, provide information on the selection of schools and the procedures of sampling. It is not enough to just refer the reader to the 2011-study (2018 PhD thesis).

3. Please state the time frame of the survey. When was this study conducted?

Reviewer #3: See below

--------------------

Results

-Does the analysis presented match the analysis plan?

-Are the results clearly and completely presented?

-Are the figures (Tables, Images) of sufficient quality for clarity?

Reviewer #1: The results match the analysis plan and are completely presented.

It would be interesting to know which the hookworm species more prevalent in the island is. Obviously that could not be known from the Kato-kats results. But maybe that information is known from previous studies or from other laboratory techniques used for control purposes.

Figures 4 resume clearly the results. Figures 1, 2 and 3 seem repetitive. A map of the island showing the information presented in table 3 would be clarifying and eye-catching.

Reviewer #2: 1. Line 131: “495 (20%) where not infected,” this can be removed. Please indicate the overall prevalence of STH.

2. Line 142-143: Add the statistics. Was the difference significant or not? What species?

3. Prevalence of STH by sex: Provide the actual p values where needed. A row can be added to table 2 and p values for all parasite groups can be given.

4. Table 3: add rows for total for each district and report statistical tests.

5. Table 3: report statistical tests for difference between both regions, i.e. north and south.

6. Prevalence of STH infection by age: What about difference according to STH species? Please report this and provide statistics.

7. Prevalence of STH infection by age: this subsection can be merged with the “Prevalence of STH by sex”. Results according to age can be added to table 2.

8. Line 197: “23% in 1994 to 24% in 2021” but this is not a reduction! Please revise these percentages for Ascaris as they are incompatible with Figure 4.

9. It will be useful to display the prevalence % over the columns of all figures.

Reviewer #3: See below

--------------------

Conclusions

-Are the conclusions supported by the data presented?

-Are the limitations of analysis clearly described?

-Do the authors discuss how these data can be helpful to advance our understanding of the topic under study?

-Is public health relevance addressed?

Reviewer #1: The conclusions are supported by the data presented except for the statement: “the interventions did not sufficiently control STH morbidity” (lines 234 and 235). None of the results presented actually measure morbidity (anemia, malnutrition, stunting). Even though it is reasonable to deduce that the morbidity probably remains high with the very high prevalence and infection intensity found.

The limitations of the analysis are not clearly described. The limitations regarding coverage and records of the PC interventions should be discussed before stating: “In Pemba, the results of a PC intervention conducted for 25 years are not optimal”. Instead it could be more realistic to conclude that: The results of the PC are not optimal with the poor coverage achieved and targeting preSAC in only 4 of the 10 interventions recorded.

The results show a very interesting difference in response of each STH species to the PC. Trichuris trichiura remains with high prevalence but MHI proportion is reduced. Ascaris lumbricoides shows an important reduction in prevalence but a paradoxical increase in infection intensity. Hookworms show a sustained decrease in prevalence and intensity. These differences should be more fully discussed in the paper for two reasons: 1) they have morbidity implications, and 2) raise the need to establish individual transmission models for each STH species and not seek a single explanation for all. Particularly the findings on hookworms are interesting and require a possible explanation. The efficacy of albendazole is not higher against hookworms than against Ascaris lumbricoides. The reason for the different response of these two species might be an improvement in sanitation facilities for example.

The authors present a possible strategy to improve the results of PC in Pemba based on their results. The option of using mebendazole instead of albendazole should be more fully discussed presenting its advantages and disadvantages. With this strategy some little efficacy against Trichuris trichiura might be gained but a lot of efficacy against hookworms would be lost. The strategy of consistently targeting preSAC and adults in the PC interventions should be also discussed.

The relevance of the study for public health is addressed.

Reviewer #2: 1. Please describe the limitations of the study. For example, using a single faecal sample and single Kato-Katz smear, study design, etc.

Reviewer #3: see below

--------------------

Editorial and Data Presentation Modifications?

Use this section for editorial suggestions as well as relatively minor modifications of existing data that would enhance clarity. If the only modifications needed are minor and/or editorial, you may wish to recommend “Minor Revision” or “Accept”.

Reviewer #1: (No Response)

Reviewer #2: 1. The abstract in general is suitable for authors summary required by PLoS NTD; and another structured abstract can be provided. In the abstract, results should be provided clearly with proportions and statistics.

2. Abstract, first sentence: Why only focused on the STH impact on nutritional status? A more general statement can be used.

3. Line 39: “periodic treatment” can be replaced by periodic preventive chemotherapy or MDA.

4. Lines 41-43: This statement about the 2 surveys can be rewritten in a more general statement to indicate the continued burden over time. The sentence is repeated in the 3rd paragraph of introduction section.

5. Line 49: “… infections are higher, ..” or remin high!

6. Lines 50-52: Did you investigate these factors? This statement is suitable for authors summary but not for the main abstract section.

7. Lines 57-58: ” points ii & iii” did you investigate these points among the studied population?

8. Lines 60-61: Theoretically, Strongyloides stercoralis is also a STH species.

9. Table 1: correct “toto” in the title.

10. Table 1: Why PC activities were stopped in 2019 & 2020? Please indicate this in last paragraph of introduction, if applicable.

11. Table 1: Correct the “Number of rounds” for lymphatic filariasis in 2018. It cannot be 0!

12. Line 117: Correct “of ≤ 0.05” to “of < 0.05”.

13. Line 132: remove “only” and “at least”.

14. Line 220: considering point #iii, it is unclear whether the targeted schools and populations are rural or urban.

15. The results showed a small increase in the prevalence of MHI in 2021 compared to 2011 (Fig 2) and it seems to be due to Ascaris (Fig 4). This point can be discussed. Was it due to ceased PC activities in 2019-2020?

Reviewer #3: See below

--------------------

Summary and General Comments

Use this section to provide overall comments, discuss strengths/weaknesses of the study, novelty, significance, general execution and scholarship. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. If requesting major revision, please articulate the new experiments that are needed.

Reviewer #1: This is a very interesting study. Relevant for the area of helminthology and also for public health. It has two major strengths: 1) the long period of follow up and 2) the big sample size. Its weaknesses are: 1) the lack of a global picture since preSAC and adults (especially woman of childbearing age) were not studied and 2) the long period between the last PC intervention and the surveillance study.

Reviewer #2: This manuscript describes a large-scale survey that aimed to determine the prevalence of STH infections among schoolchildren in 4 districts of Pemba Island with the ultimate objective was to assess the impact of more than 25 years of PC on the burden of STH in Pemba. Overall, the study reported beneficial impact of the PC intervention; however, the impact was not optimal due to the lack of other interventions essential to sustain the impact of PC in any endemic area.

Overall, the topic is interesting and important. The findings shed light on the long-term impact of PC intervention in highly endemic areas and hence, worth publication. However, the manuscript has some drawbacks that should be addressed in a revised version.

Reviewer #3: General: Shaali Ame and co-authors report the results of a third soil-transmitted helminth (STH) survey among school children in Pemba, this time 25 years after initiation of preventive chemotherapy (PC) in the island. They found a very high Trichuris trichiura prevalence and high Ascaris lumbricoides prevalence while the hookworm prevalence decreased considerably. Various explanations for this persistence of high STH prevalences are discussed. This is a refreshingly short and clear manuscript, and only few comments are offered for consideration:

- Abstract and discussion: while climatic conditions are certainly ideal for STH this probably is not the key factor since they are equally good in many places with lower (albeit often still high) STH prevalences

- Abstract and discussion: high T. trichiura transmission is cited as an explanation for the consistently high STH prevalence. However, the transmission obviously is the outcome, not an explanation…

- Please state the selection criteria for the surveyed schools in 2011. How representative are they for the island?

- Is anything known about treatment coverage in the survey schools and communities? Overall coverage seems to be quite high, though…

- Have treatment coverage figures been verified (e.g. with post-treatment surveys)?

- In addition to the infection intensity classes it would be interesting to also add mean/median egg count figures (and compare them with 2011 values, if possible).

- Not all statistical test results seems to have been reported. E.g. the statistics of between-survey differences are not evident.

- No data is presented on STH prevalences among adults. Most likely, it is also high. In combination with a relatively high proportion of households without or with only unimproved sanitation, they are likely to contribute substantially to transmission. Hence, expanding deworming also to adults might be considered and should at least be discussed among possible options to reduce transmission in the short term.

- Carefully edit the text to eliminate typos, e.g. line 131 “where” should be “were”

--------------------

PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Hesham M. Al-Mekhlafi

Reviewer #3: Yes: Peter Steinmann

Figure Files:

While revising your submission, please upload your figure files to the Preflight Analysis and Conversion Engine (PACE) digital diagnostic tool, https://pacev2.apexcovantage.com. PACE helps ensure that figures meet PLOS requirements. To use PACE, you must first register as a user. Then, login and navigate to the UPLOAD tab, where you will find detailed instructions on how to use the tool. If you encounter any issues or have any questions when using PACE, please email us at figures@plos.org.

Data Requirements:

Please note that, as a condition of publication, PLOS' data policy requires that you make available all data used to draw the conclusions outlined in your manuscript. Data must be deposited in an appropriate repository, included within the body of the manuscript, or uploaded as supporting information. This includes all numerical values that were used to generate graphs, histograms etc.. For an example see here: http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001908#s5.

Reproducibility:

To enhance the reproducibility of your results, we recommend that you deposit your laboratory protocols in protocols.io, where a protocol can be assigned its own identifier (DOI) such that it can be cited independently in the future. Additionally, PLOS ONE offers an option to publish peer-reviewed clinical study protocols. Read more information on sharing protocols at https://plos.org/protocols?utm_medium=editorial-email&utm_source=authorletters&utm_campaign=protocols

4 Mar 2022

Submitted filename: Response to reviewer.docx

Click here for additional data file.

9 May 2022

Dear Dr. Montresor,

We are pleased to inform you that your manuscript 'Impact of preventive chemotherapy on transmission of soil-transmitted helminth infections in Pemba Island, United Republic of Tanzania, 1994–2021' has been provisionally accepted for publication in PLOS Neglected Tropical Diseases.

Before your manuscript can be formally accepted you will need to complete some formatting changes, which you will receive in a follow up email. A member of our team will be in touch with a set of requests.

Please note that your manuscript will not be scheduled for publication until you have made the required changes, so a swift response is appreciated.

IMPORTANT: The editorial review process is now complete. PLOS will only permit corrections to spelling, formatting or significant scientific errors from this point onwards. Requests for major changes, or any which affect the scientific understanding of your work, will cause delays to the publication date of your manuscript.

Should you, your institution's press office or the journal office choose to press release your paper, you will automatically be opted out of early publication. We ask that you notify us now if you or your institution is planning to press release the article. All press must be co-ordinated with PLOS.

Thank you again for supporting Open Access publishing; we are looking forward to publishing your work in PLOS Neglected Tropical Diseases.

Best regards,

Susana Vaz Nery

Associate Editor

PLOS Neglected Tropical Diseases

Hélène Carabin

Deputy Editor

PLOS Neglected Tropical Diseases

***********************************************************

Reviewer's Responses to Questions

Key Review Criteria Required for Acceptance?

As you describe the new analyses required for acceptance, please consider the following:

Methods

-Are the objectives of the study clearly articulated with a clear testable hypothesis stated?

-Is the study design appropriate to address the stated objectives?

-Is the population clearly described and appropriate for the hypothesis being tested?

-Is the sample size sufficient to ensure adequate power to address the hypothesis being tested?

-Were correct statistical analysis used to support conclusions?

-Are there concerns about ethical or regulatory requirements being met?

Reviewer #1: The objective of the study is clear.

The design is appropriate for the objective. The diagnostic method used (Kato-Kats) is the recommended by WHO for diagnosis and monitoring of STH. An adequate quality control was also performed.

A definition and description of the formula used to calculate coverage was given, as well as clarification about the source of the data used for that indicator.

The study population is sufficently described, details on the age of the chilfren enrolled is given.

The sample size is sufficient; actually the number of children analyzed is impressive.

No ethical concerns arise from reading of the study.

Reviewer #2: All are ok.

Reviewer #3: see below

**********

Results

-Does the analysis presented match the analysis plan?

-Are the results clearly and completely presented?

-Are the figures (Tables, Images) of sufficient quality for clarity?

Reviewer #1: The analysis presented match the analysis plan.

The results are clearly and completely presented. More important details about the findings of the surveillance were given in comparison to the previous version of the manuscript. For example the Table 4 showing the mean EPG of each STH species in 2011 and 2021 surveys is very illustrative. The map enhances the results presentation.

The images and tables are sufficient for clarity.

Reviewer #2: All are ok.

Reviewer #3: see below

**********

Conclusions

-Are the conclusions supported by the data presented?

-Are the limitations of analysis clearly described?

-Do the authors discuss how these data can be helpful to advance our understanding of the topic under study?

-Is public health relevance addressed?

Reviewer #1: The conclusions are supported by the data presented, except for the statement: “the interventions did not sufficiently control STH morbidity” (lines 265 and 267) since none of the results presented actually measure morbidity (anemia, malnutrition, stunting).

The conclusions are considerably improved in comparison to the previous version of the article. The limitations are fully and clearly described. An interesting explanation is presented for the differences found among STH species after regular PC campaigns.

The discussion about how the findings of this study can be helpful for Pemba island, and also for global health is one of the strengths of the study.

Public health relevance is addressed and possibility to generalize the findings is also discussed.

Reviewer #2: All are ok.

Reviewer #3: see below

**********

Editorial and Data Presentation Modifications?

Use this section for editorial suggestions as well as relatively minor modifications of existing data that would enhance clarity. If the only modifications needed are minor and/or editorial, you may wish to recommend “Minor Revision” or “Accept”.

Reviewer #1: (No Response)

Reviewer #2: None

Reviewer #3: see below

**********

Summary and General Comments

Use this section to provide overall comments, discuss strengths/weaknesses of the study, novelty, significance, general execution and scholarship. You may also include additional comments for the author, including concerns about dual publication, research ethics, or publication ethics. If requesting major revision, please articulate the new experiments that are needed.

Reviewer #1: This is a very interesting study. Relevant for the area of helminthology and also for public health. Addresses some key points of the WHO preventive chemotherapy recommendations that are under revision (the target group, the recommended coverage and drugs, etc). Manuscripts from different “real world” experiences with PC such as this enhance the discussion about how to improve current guidelines to accomplish STH control.

Reviewer #2: The authors have properly addressed my comments and I am satisfied with the way they have improved the manuscript.

Reviewer #3: The authors have adequately responded to the peer-review comments and the amended manuscript reflects their consideration of the comments.

Two minor points:

- line 153: the word "of" after the semicolon should not be capitalized

- line 298: delete "higher"

line numbers refer to the manuscript with track-changes

**********

PLOS authors have the option to publish the peer review history of their article (what does this mean?). If published, this will include your full peer review and any attached files.

If you choose “no”, your identity will remain anonymous but your review may still be made public.

Do you want your identity to be public for this peer review? For information about this choice, including consent withdrawal, please see our Privacy Policy.

Reviewer #1: No

Reviewer #2: Yes: Hesham M. Al-Mekhlafi

Reviewer #3: No

10 Jun 2022

Dear Dr. Montresor,

We are delighted to inform you that your manuscript, "Impact of preventive chemotherapy on transmission of soil-transmitted helminth infections in Pemba Island, United Republic of Tanzania, 1994–2021," has been formally accepted for publication in PLOS Neglected Tropical Diseases.

We have now passed your article onto the PLOS Production Department who will complete the rest of the publication process. All authors will receive a confirmation email upon publication.

The corresponding author will soon be receiving a typeset proof for review, to ensure errors have not been introduced during production. Please review the PDF proof of your manuscript carefully, as this is the last chance to correct any scientific or type-setting errors. Please note that major changes, or those which affect the scientific understanding of the work, will likely cause delays to the publication date of your manuscript. Note: Proofs for Front Matter articles (Editorial, Viewpoint, Symposium, Review, etc...) are generated on a different schedule and may not be made available as quickly.

Soon after your final files are uploaded, the early version of your manuscript will be published online unless you opted out of this process. The date of the early version will be your article's publication date. The final article will be published to the same URL, and all versions of the paper will be accessible to readers.

Thank you again for supporting open-access publishing; we are looking forward to publishing your work in PLOS Neglected Tropical Diseases.

Best regards,

Shaden Kamhawi

co-Editor-in-Chief

PLOS Neglected Tropical Diseases

Paul Brindley

co-Editor-in-Chief

PLOS Neglected Tropical Diseases