A Review of Strongyloides spp. Environmental Sources Worldwide

Strongyloides spp. are parasitic nematodes that are transmitted through the environment and are capable of causing disease. These nematodes affect an estimated 3–300 million humans worldwide. Identifying the environmental reservoirs of Strongyloides spp. is essential for the development of appropriate control strategies. This systematic literature review examined all published studies that identified Strongyloides stercoralis, Strongyloides fuelleborni, Strongyloides fuelleborni kellyi, and Strongyloides spp. from an environmental source. Most studies detected the nematode from dog and primate fecal samples. Other environmental sources identified were ruminants, cats, rodents, insects, water, soil, as well as fruit and vegetables. Most studies used microscopy-based identification techniques; however, several employed molecular-based techniques, which have become increasingly popular for the detection of Strongyloides spp. A limitation identified was a lack of studies that comprehensively screened all potential environmental samples in a region. Future research should undertake this holistic screening process to identify which environmental reservoirs pose the greatest significance to human health. Potential controls can be identified through the identification of environmental sources. Understanding where Strongyloides spp. is commonly found within the environment of endemic areas will inform environmental control strategies to reduce this neglected disease.


Introduction
Strongyloidiasis is a disease caused by parasitic nematodes of the genus Strongyloides. Within this genus, three species, Strongyloides stercoralis, Strongyloides fuelleborni, and Strongyloides fuelleborni kellyi are known to parasitize humans [1,2].
S. stercoralis, S. fuelleborni, and S. fuelleborni kellyi are capable of autoinfecting the host. This occurs after adult female parthenogenic nematodes within the infected human shed eggs. These eggs develop to larvae that are passed within the stool. A certain number burrow through the wall of the large intestine, thereby reinfecting the body. Infected individuals can have a low-level undetected infection for many years [3]. When this auto-infective life cycle becomes uncontrolled in immunocompromised, young, and elderly patients, a disseminated infection can develop. Disseminated infection occurs when the parasite travels throughout the body. This can result in sepsis, bacterial meningitis, or gastrointestinal hemorrhage [4]. The mortality rate from a disseminated infection and its comorbidities is estimated to be 80% [3]. The larvae that are passed within the stool are then capable of completing a free-living cycle, in which they molt twice to develop into filariform larvae. These infective filariform larvae are capable of then reinfecting humans, where they can be involved with the autoinfection cycle again [5].
Insects within the order Blattodea were identified in 2% of all studies, as seen in Table A1. Identified insects within this order include cockroaches from the genus Periplaneta spp. and Blattella spp. Four of the five identified studies reported Strongyloides spp. within populations of Blattodea in Africa. The remaining study identified Strongyloides spp. in Blattodea in Thailand. All studies identified infected insects within housing and food preparation areas [26][27][28][29]. The sample sizes of studies identifying insects within the order Blattodea ranged from 70 to 920, with the prevalence ranging from 1% to 81%.
Studies identifying environmental sources of S. stercoralis, S. fuelleborni, and Strongyloides spp. are distributed across the world. Areas with a large amount of research included Europe, Africa, and South East Asia, as seen in Figure 1. Areas lacking research include Oceania, and the Americas, as seen in Figure 1. Many published studies identified Strongyloides spp. within temperate regions as opposed to tropical regions, as seen in Figure 1.
Microscopy was the most commonly used identification technique (90%). However, molecular detection was more common in recent publications. For example, in 2018, 6 of the 14 papers identified employed molecular-based techniques; however, in 2011, 1 of 11 papers published used molecular-based techniques, as seen in Table A1. The size of each shape is mapped to the number of studies published in that country. Location of shapes does not represent exact location of study, but country in which the study was completed. Colored fill of shapes was assigned to a single source and is consistent across all helminth species. The size of each shape is mapped to the number of studies published in that country. Location of shapes does not represent exact location of study, but country in which the study was completed. Colored fill of shapes was assigned to a single source and is consistent across all helminth species.

Animals
Primates and domesticated or feral dogs (canids) adapt well to association with human settlements and cohabitation with humans, indicating the potential for transmission to humans. Contamination with feces from domesticated or synanthropic primates and dogs may lead to other environmental sources, such as water and soil, becoming reservoirs of Strongyloides spp. capable of causing infection. Most studies found in this review were based on primate and dog investigation, suggesting that these animals preferentially live closely with and benefit from humans. This habitual closeness presents a chance for environmental transmission of Strongyloides spp.

Canids
Studies that report parasites found in canid feces frequently investigate multiple parasites such as Ancylostoma spp., Giardia spp., and Strongyloides spp. These studies have often found low levels of S. stercoralis within otherwise highly parasitically infected populations [40,[48][49][50][51][52][53][54][55][56]. Infection occurs more frequently in canids when they are living stray. This might be a result of exposure to infective Strongyloides spp. larvae occurring more frequently to these dogs than dogs living within homes [57]. Mass drug administration (MDA) to stray dogs has been implemented successfully for the control of rabies; accordingly, it may be an option for the control of Strongyloides spp. [10]. Isolated or infrequent anthelmintic treatment increases infection rates and so considered treatment must be implemented [10,58]. Studies identifying canid feces as containing Strongyloides spp. commonly also screened the samples for other parasites. Sample sizes ranged from 35 to 3465. The highest prevalence was reported by Beknazarova et al. [59] who screened 35 canine fecal samples from Australia, of which 49% were positive for Strongyloides spp. This low sample size with a high positivity rate in comparison with other studies is representative of the inconsistent fecal shedding of Strongyloides spp. as well as the endemic location of the study. The lowest prevalence was reported by Ardelean et al. [60] with 1% of 3465 samples positive from dogs within Romania. Strongyloidiasis was observed most commonly in dogs three to six months of age in this study. This variance based on age and study location may be further impacted by the detection method. Ardelean et al. [60] reported high levels of Ancylostomidae spp. which is morphologically similar to Strongyloides spp., therefore making reliable identification with microscopy alone difficult.

Primates
Areas sparsely populated by humans increase roaming in primates due to the attractive food sources but offer a low threat from the decreased human numbers. More frequent entry to communities in search of food potentially increases the numbers of Strongyloides spp.-infected primate feces within these sparsely populated communities [13,14]. Terrestrial Papio primates were likely to excrete Strongyloides spp. larvae; however, arboreal Cercopithecus neglectus were less likely [11,12]. This may be due to less frequent contact with soil containing Strongyloides spp. larvae. The impact of human populations upon forests has led to an increased chance of interaction between humans and potentially infected primates. Hasegawa et al. [61] observed that degraded forest increased the chance of roaming and transfer of parasites.
Captive primates present an infection risk to handlers because anthelmintic treatment has been observed to not eliminate Strongyloides spp. larvae shedding within feces [15]. This may be due to the introduction of new individuals to groups, a phenomenon also observed within wild individuals [15]. This indicates the value in introducing physical environmental controls beyond anthelmintic drugs, especially in communities exposed to roaming wild primates.
Tourist sanctuaries provide an ideal environment for contact between primates and humans. Environmental controls such as fecal contamination removal can decrease helminthic infection in both primates and humans without interfering with natural behaviors [62]. Strongyloides spp. is unable to transfer either from animal to human or from human to human directly [63]. This further supports the importance of clearing feces because contact with the animals does not cause infection; however, contact with fecal matter can cause infection. Larger groups, such as those within tourist sanctuaries, are generally associated with higher parasitic species richness. Some variation of infection can be expected based on food availability and stress levels [64].
Studies of primates had sample sizes ranging from 7 to 3349, with prevalence also ranging from <1% to 100%. Prevalence within primate populations was reported to be higher than in canine populations. Hasegawa et al. [61] reported 100% prevalence within seven gorilla and chimpanzees from Uganda; whereas Li et al. [65] screened 3349 fecal samples and identified a prevalence rate of 6%. This variation in prevalence may be due to the inconsistent shedding of Strongyloides spp. larvae. Li et al. [65] employed microscopy whereas Hasegawa et al. [61] employed molecular techniques, which may account for differentiation in prevalence.

Ruminants
Strongyloides spp. has also been found within the feces of ruminants used in western farming settings including pigs, sheep, and cattle. All studies identifying Strongyloides spp. within farm-associated ruminants only identified the parasite to the species level [66,67]. These may have been genus specific, such as the pork threadworm, Strongyloides ransomi, or the more general Strongyloides papillosis. All studies used microscopy, a technique that can have low success in identifying Strongyloides spp. to species level. These recorded observations indicate the potential for infected ruminant feces to provide an environmental source of Strongyloides spp.

Rodents
Rodents are known to carry a range of communicable diseases. Strongyloides spp. has been found in several rodent species, including common house rats, and non-synanthropic rodents such as Hydrochoerus hydrochaeris. S. stercoralis has been identified in house rat feces in East Java, Indonesia, using microscopy [20]. The area in which S. stercoralis was identified in house rat feces is an area with poor sanitation and hygiene. People reported a large house rat population within these areas [20]. In such cases, where a zoonotic pathogen is identified, control of the offending carrier can be employed. The sample sizes of rodents were low in comparison with other sources, with the highest sample size being 502 [19]. The highest prevalence was within a population of Rattus norvegicus within a Brazilian slum [17]. The rodents sampled within this study had particularly high levels of infection with helminths; in all except five, helminths were present within their feces [17]. This prevalence of 97% from a sample size of 299 is representative of animals living within an area highly contaminated by human waste.

Insects
Increasing urbanization has allowed for synanthropic dependence to increase within insect populations. Densely urbanizing areas lead to an increase in available food for insects, and areas with poor sanitation and hygiene practices attract disease-carrying insects such as those within the order Blattodea (cockroaches) and Diptera (flies). Filth flies present a source of helminth transmission. Their preference for consuming wet, rotting substances indicates a high probability for the consumption and carriage of Strongyloides spp. Carriage of Strongyloides spp. has been observed on the external body of flies despite frequent preening and cleaning [68]. Fetene and Worku [23] identified S. stercoralis within Chrysomya rufifacies, Musca sorbens, and Lucilia cuprina. C. rufifacies were identified largely within butcheries and defecating grounds; M. sorbens was found more frequently within the market collection sites. Furthermore, Musca domestica, a species always found in association with humans, has been observed to carry Strongyloides spp. [24]. The presence of these flies within human food areas presents a potential transmission route for Strongyloides spp. larvae. Prevalence was higher in the internal structures of flies than on the external surface of flies [25]. This observation is further supported by the preference of these flies for consumption of wet substances. Insects within the order Blattodea, commonly known as cockroaches, also present a transmission source. Parasite prevalence has been found to be associated with housing type. Low-cost housing with pit latrines as well as housing in close proximity to dumpsites was reported to contain higher levels of carrier cockroaches [26,28,29]. Through the introduction of environmental controls such as fly screens or nets, movement of carrier insects can be decreased [27]. Sample sizes for both orders were high. Studies found low prevalence with the exception of Morenikeji et al. [28] who reported 81% in 70 cockroaches.

Water
Contamination of water is also a potential source of helminth transferal. Pollution of water sources with human and agricultural waste can render water sources unsuitable for use as drinking and irrigation water. In areas where water access is limited, contaminated water may be employed for these uses [69]. Waste stabilization ponds, chlorination, or activated sludge treatment systems may be suitable approaches for reducing helminth levels; however, many studies monitoring wastewater treatment methods have provided contradictory results [70][71][72][73]. Some studies identified standard treatment techniques as adequate for removing larvae; however, others did not. Frequent monitoring of treated waste water is important because treated water has been identified as containing higher than acceptable levels of helminths including Strongyloides spp. [74]. To date, studies have focused on the helminth burden of treated water instead of comparing treated with untreated levels. Through focusing on untreated and treated waters from the same area, reduction in burden levels of treated water may be better understood.
Untreated water used for drinking can contain Strongyloides spp., particularly when water is sourced from storm water or collected rainwater [75]. According to one study, when water runoff moves into drinking water sources such as rivers, it can carry Strongyloides spp. larvae with it [75]. Bore and ground-water contamination can also occur and has been identified [76,77]. Jonnalagadda and Bhat [77] found that improper washing of vessels used to collect and store water can lead to helminth contamination. Implementation of appropriate washing and sanitation education in areas with high contamination risk may decrease incidences of infection.
Prevalence of Strongyloides spp. within non-potable water was higher than in potable water; this was expected because most sources of non-potable water were collected from wastewater treatment facilities [71]. The prevalence of Strongyloides spp. within non-treated wastewater was between 40-100%. Treated waste water intended for use on crops for human consumption had a much lower prevalence (2%); however, it was still observed to be present, supporting the importance of monitoring water intended for reuse [70].

Fruit and Vegetables
The rough nature of green, leafy vegetables surfaces means that adhesion of parasitic larvae and eggs occurs easily when these vegetables are either washed with contaminated water or come into contact with contaminated human fecal-based fertilizers (i.e., night soil) [30]. Studies identified S. stercoralis contamination most frequently within leafy, rough-surfaced vegetables such as lettuces, cabbage, celery, spinach, and carrot [30][31][32][33][34][35][36][37]. This correlation may be due to these vegetables growing close to or in the ground, which may lead to increased contamination from fertilizers [78,79]. Market vendors commonly wash vegetables prior to purchase, and consumption of raw vegetables such as salad leaves is frequently noted [31,79,80]. There is an increasing focus on the study of vegetables, washing water, and farm soil to determine where in the food chain parasites are being introduced [33].
Prevalence of Strongyloides spp. within fruit and vegetable samples was generally low, ranging from <1% to 46%. Ogbolu et al. [81] found S. stercoralis in 46% of fresh vegetables sold at open markets in Nigeria. The application of night soils and untreated wastewater is common within low-income nations may have led to the high level of prevalence [81]. Lower prevalence was also reported within Nigeria, between <1% and 19%. This variation may be due to differences in handling of samples, treatment during farming, and cross contamination [30,31].

Soil
Increasing urbanization has led to an ever-increasing amount of waste. Modern waste includes not only fecal waste but waste produced in the form of rubbish. Dumpsites and landfills are commonly employed to deal with this large amount of waste; Strongyloides spp. contamination can occur throughout the nearby environments. Dumps and landfills pose a transmission risk due to the ability of Strongyloides spp. larvae to survive effectively in the soil [82].
The texture and chemistry of soil also plays a role in the prevalence of Strongyloides spp. larvae. Moisture levels in soils increases the incidence of rhabditiform larvae developing into filariform larvae [83]. This requirement for moisture is supported by the findings of a study of helminth larvae during the wet season, which found that no larvae were located during the dry season despite contamination [39]. High sand and silt content soils favor the survival of Strongyloides spp. and other helminth larvae. This is due to the high porosity of these soils, which allows larvae to move effectively through the soils towards sources of nutrition and moisture [84].
Strongyloides spp. is transmitted from soil-based sources to humans through skin-to-soil contact; however, the behavior of purposefully ingesting soil known as geophagy can also lead to soil-based infections. Geophagy is culturally accepted and common in sub-Saharan Africa. This behavior is common in pregnant women; S. stercoralis infections have been observed, along with other soil-transmitted helminth infections, in these women [85,86]. Geophagy may be undertaken as a method for diet supplementation in low-income areas. Notably, these areas are more likely to have helminth-contaminated soil, which leads to an increased chance of infection.
Prevalence of Strongyloides spp. within soil was between 1% and 20%. Ivoke et al. [85] screened 797 pregnant women for parasitic infections related to geophagy. The prevalence of infection was 1% within this cohort. This low prevalence is likely due to picking soils specifically for consumption. Higher prevalence was observed in soils sampled in soil directly from areas densely populated with poor health infrastructure [41].

PCR and Microscopy
Identification of Strongyloides spp. nematodes can be undertaken using several methods. These fall into either techniques involving the identification of Strongyloides spp. larvae using a microscope or molecular-based techniques. Microscopy presents several problems but it accounts for most larval identification-based techniques (90%) as presented in this literature review. This study did not exclude papers based on year; accordingly, this overrepresentation of microscopy is likely a result of the recent introduction of polymerase chain reaction (PCR) techniques. Recent papers have increasingly employed PCR as accessibility to the equipment increases. In 2018, 5 of the 14 papers published employed molecular-based techniques; in contrast, in 2011, only 1 of 11 studies published employed molecular-based techniques, as seen in Table A1. This increasing use of molecular-based techniques is expected, and its use will allow for more accurate identification of species of Strongyloides spp. The strengths of microscopy include that it can be employed within the field or where resources are limited; however, microscopy alone cannot reliably differentiate S. stercoralis from S. fuelleborni. The reliance on microscopy-based techniques is hazardous because both species are morphologically similar [34]. Molecular techniques allow for the accurate identification of Strongyloides spp. to the species level; however, set up of molecular protocols can be expensive. Each technique has strengths and weaknesses; when looking at all published works, a consideration of the identification techniques allows for more accurate assessment of reports.

Global Distribution
Globally, published research has mainly focused in countries within Africa, Europe, and South East Asia. Commonly, Strongyloides spp. is reported as a tropical disease; however, Strongyloides spp. was often also reported in temperate regions such as Europe, as seen in Figure 1 [83,87]. This may be due to a lack of resources within low-income nations, leading to an overrepresentation of the generally higher income countries within Europe. Australia and the Americas both lacked studies looking at the environmental sources of Strongyloides spp. (Figure 1). This indicates a need for more research into the environmental transmission of S. stercoralis, S. fuelleborni, and Strongyloides spp. within these areas.

Materials and Methods
This systematic literature review is based on an adapted version of the PRISMA statement. This tool allows for the transparent and reliable reporting of evidence. A systematic search of the databases Scopus and Web of Science was undertaken, and all articles published prior to 2019 were included. Key words used in searches included Strongyloides spp., strongyloidiasis, tap water, soil, insect, zoonotic, and waste, as seen in Table 1. A search strategy was developed to ensure a transparent and complete literature review of all identified environmental sources of Strongyloides spp. was completed. This strategy is as follows; All non-English documents were excluded from the search.
To be included, published data must have reported Strongyloides spp. in one of the three spp. capable of human infection or to the genus level because these studies cannot be excluded as identifying disease-causing Strongyloides spp. The document must have reported this presence within an environmental source. Documents were excluded if they were reviews, reports of humans with Strongyloides spp. infection with no mention of contributing environmental source, or lab-based studies, as seen in Figure 2.
First, all titles and abstracts of all papers were manually reviewed to ensure the papers met inclusion criteria. If it was unclear from titles and abstracts if papers met the criteria, they were included for full text review. Papers that were unclear were included. Papers were then read as full text and compared against inclusion and exclusion criteria. Articles that met these criteria were included in the study. All papers included in the study had key points extracted and recorded including the environmental source reported, species of Strongyloides spp. observed, detection method used, and the country from which the sample was taken.

Conclusions
Although Strongyloides spp. is considered a soil-transmitted helminth, there are several environmental sources that can potentially provide a route of transmission of the disease. Understanding the potential sources, combined with the adoption of environmental controls for Strongyloides spp. is likely to decrease transmission and therefore infections. Animals such as dogs, primates, and insects, as well as soil, water, and fruit and vegetables have all been reported to contain Strongyloides spp. larvae, capable of perpetuating infection within humans who have come into contact. Future research is needed to undertake a holistic screening of all environmental sources within endemic areas to identify those which pose the greatest significance to human health. By understanding the established and recorded environmental reservoirs of S. stercoralis, S. fuelleborni, and S. fuelleborni kellyi, better environmental controls can be implemented.

Conclusions
Although Strongyloides spp. is considered a soil-transmitted helminth, there are several environmental sources that can potentially provide a route of transmission of the disease. Understanding the potential sources, combined with the adoption of environmental controls for Strongyloides spp. is likely to decrease transmission and therefore infections. Animals such as dogs, primates, and insects, as well as soil, water, and fruit and vegetables have all been reported to contain Strongyloides spp. larvae, capable of perpetuating infection within humans who have come into contact. Future research is needed to undertake a holistic screening of all environmental sources within endemic areas to identify those which pose the greatest significance to human health. By understanding the established and recorded environmental reservoirs of S. stercoralis, S. fuelleborni, and S. fuelleborni kellyi, better environmental controls can be implemented.