Prevalence and distribution of soil-transmitted helminth infections in India

Background Understanding the prevalence of soil-transmitted helminth infections is necessary to plan control strategies and focus on highly endemic regions for preventive chemotherapy and improved sanitation facilities. India is known to be endemic for soil-transmitted helminth infections. Methods To understand the prevalence, spatial distribution and identify high-risk zones, a systematic search of published literature was carried out based on PRISMA guidelines from the year 2000 to 2015. Results A careful screening of the identified literature yielded 39 studies that reported the prevalence of soil-transmitted helminth infections from 19 different states of India. Ascaris lumbricoides was the most prevalent parasite. Higher than 50% prevalence was reported from six states. Nearly 90% studies reported the prevalence of more than one parasite species in the same sample population. Conclusion This is the first study to comprehensively review the literature associated with soil-transmitted helminth infections from India giving a clear idea of its prevalence, distribution and high endemic areas.


Background
Soil-transmitted Helminths (STH) infects nearly 2 billion people of world's population with children being the most affected [1]. According to the World Health Organization (WHO) estimates, 870 million children live in the area of high prevalence. Africa, South Asia and South America are the most affected regions of the world [2]. India alone contributes nearly 25% to the total global cases with 220.6 million children in need of preventive chemotherapy [3]. STH infections rarely cause mortality with diarrhea, abdominal pain and low hemoglobin levels as the immediate outcome of infections, however, the long term effects of these infections are far more sinister as those with infections show reduced cognitive abilities, intellectual capacity and lower work productivity [4]. The warm and moist climate of tropical and subtropical countries provides the ideal environment for the survival of parasite eggs or larvae of these four STH, roundworm (Ascaris lumbricoides), whipworm (Trichuris trichiura) and hookworm (Necator americanus, Ancylostoma duodenale) [5].
The prevalence and control of STH infections is inextricably linked with water quality, sanitation, hygiene practices and socio-economic status in the affected areas [6]. Despite the fact that infection can be cured with either Albendazole or Mebendazole, eradication is difficult, given STH's fecooral and penetration-via-skin transmission pattern as the chances of reinfection are very high in population living in affected areas [7]. Control is achieved by targeted use of chemotherapy and improvement of sanitation, drinking water, use of pit-latrines instead of open defecation and good hygiene practices [8][9][10]. India, with support from WHO, launched a Global Programme for the Elimination of Lymphatic Filariasis in the year 2000 (http://nvbdcp.gov.in/ doc/guidelines-filariasis-elimination-india.pdf). Under this programme Diethylcarbamazine (DEC) and Albendazole were administered to people living in filarial endemic areas. In 2015 another nation-wide deworming programme was launched covering nearly 241 million children with STH infection or at risk of developing the infection (http://nrhm.gov.in/nrhm-components/rmnch-a/child-healthimmunization/ national-deworming-day-february-2017.html). For such targeted efforts a comprehensive knowledge of the prevalence pattern of these helminth infections needs to be assessed thoroughly. Several studies have been published from India by clinicians and researchers assessing the prevalence and epidemiology of STH infections in different states and municipalities. Such studies have the potential to guide governmental and non-governmental organizations (NGOs) to focus their efforts in highly endemic areas. The purpose of this review is to evaluate the prevalence of STH infections and identify high-risk areas in different regions of India, based on a comprehensive search and analysis of published literature. Such an exercise would help Governmental agencies and NGOs to focus on specific areas of high prevalence for preventive chemotherapy and improved sanitation practices.

Search Strategy and Data extraction
We did a review based on PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines to identify all relevant publications pertaining to the prevalence of STH infections in India. We systematically searched PubMed and Web of knowledge from January 1, 2000 to June 30, 2015. We did not search before the year 2000 because our goal was to inform decision-making rather than providing a historical perspective. In this regard, it is pertinent to mention that STH infections are often a function of sanitation, health care and economic condition. In the last few decades India has seen considerable improvement in sanitation with a reduction of 31% in open defection. Community based and school based mass deworming programs have been undertaken to reduce the burden of STH infections.
We used the following search terms anywhere in the articles: "soil transmitted helminth" or "ascaris" or "trichuris" or "whipworm" or "necator" or "ancylostoma" or "hookworm" AND "India". We searched without any bar on language or nature of studies. To identify additional studies, reference lists of publications were carefully screened. Initial assessment was based on review of title and abstract of all the studies. Full text of potentially relevant studies was further analyzed for STH prevalence data.
STH infections are largely asymptomatic in nature, and it is this asymptomatic population that carries the heaviest burden of infection. Only acute cases are reported in hospitals. In this context, we included studies reporting prevalence data from community based cross-sectional studies only. Retrospective analysis and hospital based observational studies were excluded. Any study that did not report either number of participants or age group of target population or method of parasitological testing or geographical location or had discrepancy in what was reported in the abstract and full text were all excluded from the final analysis. Cross-sectional studies with full-text availability, and reporting prevalence of at least one parasite in a defined population (location, age group, number of participants) were included in the review. In case time period of the study was not specified, the date of publication was considered instead. If more than one publication was reported from the same geographical area, then most recent publication was considered for final analysis.
The data extracted from the selected publications included first author, date of publication, date of survey, state and localities where the study was carried out, sample size and age, type of parasitological testing performed, study design and percentage prevalence of each parasite. All the data was entered in an excel file and double-checked.

Prevalence mapping
For prevalence mapping, locations where the studies were carried out were georeferenced using Google maps. A total of 49 unique locations were identified and georeferenced from where A. lumbricoides infections were reported. 39 unique locations were identified for T. trichiura and 35 locations were identified for hookworm infections.

Results
Initial searches identified 480 studies from PubMed and web of knowledge. After removing duplicates and irrelevant articles 127 studies were considered for full text review. Studies were excluded for not reporting prevalence data of parasites (42), lack of full text availability (10), mode of data collection (23) and incomplete data (10). Two studies were excluded as they reported data from the same geographical area; in each case only recently conducted study was considered. One study has discrepancy in what it reported in its full text and abstract. A total of 39 studies were identified that reported the prevalence of at least one STH infection among humans in India (Fig. 1). All 39 studies were cross sectional in nature with one study reporting data from pregnant females. Most of the reported data are from northern, northeastern, western and southern region of India, with a general lack of publication from northwest and southwest regions of India. Several studies (85%) reported prevalence of STH infection only in children. Smallest sample size was 25 and largest sample size was 3706. A total of 25,754 stool samples were screened for the presence of STH infection. A combination of Saline and iodine wet mount, Kato-Katz technique, salt flotation, formolether concentration, mini-FLOTAC and zinc sulphate concentration techniques were used for parasite detection. A total of 21 studies reported prevalence data for all three parasitic infections, 13 studies reported prevalence data for at least two parasites and 5 studies reported data only for a single parasite ( Table 1). The spatial distribution of STH was spread across whole of India with a total of 49 unique locations identified and georeferenced for the prevalence of A. lumbricoides (Fig. 2), 39 locations for T. trichiura (Fig. 3), and 35 locations for hookworm (Fig. 4). Prevalence of soil-transmitted helminths varied widely with A. lumbricoides infection ranging from 0.6 to 91%, T. trichiura ranging from 0.7 to 72% and hookworm ranging from 0.02 to 52%. A higher than 50% prevalence for A. lumbricoides was reported from 10 different locations scattered across six states, Jammu and Kashmir, Assam, Bihar, Tamil Nadu, West Bengal and Andhra Pradesh covering nearly 30% of India's population (Table 2) .
More than 50% prevalence for T. trichiura was reported from two different locations from the states of Assam and Andhra Pradesh and more than 50% prevalence for hookworm was reported from a single location from the state of Andhra Pradesh. Open defecation practices, lack of personal and community sanitation, lack of footwear wearing habit, poor maternal education, low literacy rate and poor socio-economic status were significant predictors of prevalence of STH infections. Jalaripet in Andhra Pradesh was a unique location from where two reports were published 7 years apart [40,50]. A weighted average indicated more than 50% prevalence for all three parasites (91.12% for A. lumbricoides, 71.5% for T. trichiura and 50.2% for hookworm). This was the only location from where such a high prevalence was reported of all three parasites by two studies despite the time gap. The states of Uttarakhand, Uttar Pradesh, Jharkhand, Manipur, Maharashtra and Puducherry reported a prevalence higher than 20%. Less than 20% prevalence was reported from another seven states of Delhi, Himachal Pradesh, Chandigarh, Madhya Pradesh, Rajasthan, Gujarat and Karnataka

Discussion
Soil-transmitted helminth infections continue to plague large parts of the world with India a significant contributor to the burden of disease [2]. Despite efforts to introduce usage of pit-latrines instead of open defecation, mass deworming program and improvement in water quality and sanitation, STH infections are still prevalent. A conducive climate for its growth, rapid and unplanned urbanization, social practices of open defecation and lack of community education and sanitation are some of the factors, which impedes control of infection in  far as 1923 [51], the data on STH infections remains scattered. This information has the potential to inform and develop a comprehensive approach to control STH infections and target highly endemic areas with greater urgency.
The present study is a detailed effort to assess the burden of STH infections by searching past and present published literature and analyzing the prevalence of roundworm, whipworm and hookworm in one of the most endemic country in the world. The study covers 15 years of  largely tr-   opical climate with high humidity and warm temperatures. Theses climatic conditions provide ideal environment for the survival of parasite eggs in moist soils, incidentally highest prevalence of STH infections was reported from such region of Tamil Nadu, Andhra Pradesh, Bihar, Assam, and West Bengal. A notable exception was Kashmir valley and Himachal Pradesh, which sees very cold and warm climates alternately, however, random urbanization without proper development of civic structure facilitates the spread of STH infections even in colder climates. Nearly 85% studies reported prevalence of STH infections in children, which form the disproportionately affected strata of society. Nearly 90% of the studies reported the prevalence of more than one parasite in same community, possibly because all the three helminths share similar climatic and socioeconomic niche. Several studies reported cases of co-infection, with other protozoal parasites. Southern, northern and eastern parts of India are the most affected regions with A. lumbricoides the commonest parasite reported. WHO recommends annual deworming for areas with higher that 20% prevalence and biannual deworming for areas with higher than 50% prevalence (http:// www.who.int/intestinal_worms/strategy/en/). Our study identifies six states with more than 20% prevalence and another six states with more than 50% prevalence rate, and in urgent need of chemotherapy. Very high prevalence was reported in Jalaripet in Andhra Pradesh implying that agriculture or farming based population may be at a greater risk due to higher exposure to contaminated soil and water at a regular basis [52,53]. Many of these states (Delhi, Rajasthan, Tamilnadu, Assam) from which prevalence data is available, are tourist destinations. Travel based spread of vector borne diseases is well documented [54]. Similarly an exposure of naïve individuals to STH infections carries the risk of introducing STH to new locations, although tourist are a population that does not usually share lack of sanitation and poor socio-economic status and they are much less at risk of STH transmission.
Data extraction and compilation are prone to bias, to this effect we have made every effort to identify and screen published literature with broad search queries, nonetheless many relevant studies were inaccessible due to lack of full text availability. We excluded all the studies that were reported in clinical patients as STH infections are largely asymptomatic and community based cross sectional studies are better in assessing the burden of infection. Furthermore, all the studies with incomplete information were excluded which emphasizes the systematic collection of data in cross sectional studies. Additionally there was lack of publications from central, north western and southwestern region of India, with maximum studies being published from states of Tamil Nadu, Maharashtra, Assam, Uttar Pradesh and Jammu Kashmir. Odisha, Chhattisgarh and newly formed state of Telangana contribute a large section of people living below poverty line, however prevalence of this region could not be determined due to absence of any published reports. Also the review relied completely on published literature where grey literature and studies with minimal or negative results may not have been included resulting in publication bias. Another important factor that might lead to under reporting of STH infection is lack of surveillance data since STH infections are asymptomatic in nature. Only those having symptoms are expected to seek clinical intervention. A cross sectional survey from every state or union territory will accurately predict the prevalence and burden of STH infections. WHO has recommended Kato-Katz method as the best and most reliable diagnostic tool with better efficacy, accuracy and predictive value than other techniques in resource poor settings [55]. However, only 15% studies reported the use of this method. Several states report less than 20% prevalence of STH infections. Kato-Katz technique has been shown to have reduced sensitivity in low transmission settings, which can be improved by taking more than one sample. Most of the studies relied on single stool examination, which may result in underreporting of the prevalence. Determination of prevalence and intensity of STH infection is an important tool for preventive chemotherapy and to assess the effect of ongoing deworming programs. Most of the studies have relied upon direct smear or in combination with flotation methods to determine the presence of helminth eggs. Adoption of Kato-Katz techniques with multiple stool samples will be helpful in identifying the actual prevalence of STH infections.
Despite these limitations, we were able to identify prevalence of STH infections in 19 states of India that covers nearly 84% of India's population, clearly identifying regions of high prevalence which requires focused efforts of mass deworming to reduce parasitic load. A concomitant improvement in sanitation level will be helpful in reducing cases of reinfection. An operational focus on children by utilizing school infrastructure is generally the best approach. However targeting and educating mothers about the problems associated with helminth infections and open defecation must be emphasized by community-based programs. Systematic collection of survey reports based on WHO recommended diagnostic methods for parasitological testing should be undertaken. Such efforts will be helpful in identifying regions of high endemicity and monitor ongoing deworming programs. Geospatial mapping based on soil characteristic have been very useful in determining STH burden in other parts of the world. Such an approach could be valuable when survey data based on stool test are not available. Lastly, an overall improvement of sanitation and behavioral changes in the attitude of people will go a long way in avoiding reinfection and interrupting transmission of STH infections.

Conclusion
The present study attempts to compile available data about the prevalence of STH infections in India. This analysis is based upon the reporting of STH prevalence data in cross sectional studies carried out in different parts of India. We have also georeferenced affected regions to identify populations in need of annual or biannual deworming to control STH infections. This study will hopefully provide a guide map for the control of STH infections by preventive chemotherapy. Furthermore, lack of studies from several parts of India requires urgent attention for the surveillance and prevalence determination of STH infection. An exhaustive knowledge of the burden of disease will be helpful in allocating resources, funding and designing survey strategies for the control and monitoring of STH infections in Indian subcontinent.