Freshwater snails as the intermediate host of trematodes in Iran: a systematic review

Freshwater snails, as the first intermediate hosts of trematodes, can cause health hazards in animals and humans. Recently, the World Health Organization has included Iran in a list of 6 countries known to have serious problems with fascioliasis. In addition, cercarial dermatitis is a job-related disease that is seen often in paddy workers, agricultural labourers, and fishermen in Iran, particularly in Mazandaran Province. Many studies have been conducted in Iran to survey larval trematodes in freshwater snails. However, to the best of our knowledge, no comprehensive data exist regarding infections in gastropods. Therefore, the aim of the present study was to estimate the types and prevalence of cercarial infections in snails in Iran. Electronic English-language and Persian-language databases were searched to identify 24 published articles reporting the prevalence of trematode infections in snails (9 species from 6 families) in various provinces of Iran. In total, 4.4% of gastropods were infected with the larval stages of trematodes. According to the studies reviewed in this meta-analysis‚ the highest infection prevalence was found in Radix auricularia (9.9%). Twelve larval species of trematodes were identified, and the highest prevalence of cercariae was found for Echinostomatidae cercariae (4.3%). Among the provinces explored, West Azerbaijan had the highest prevalence of infected snails (16.9%). The presence of trematodes in snails could pose a serious health problem in Iran. Thus, further studies are necessary to characterize these infections in other provinces.


Search process
To evaluate trematode cercariae infections in snails in various regions of Iran, we carried out a systematic review of the literature (full-text) published online in the English and Persian languages. Records from 10 databases (PubMed, Science Direct, Web of Science, Scopus, EBSCO, Google Scholar, IranMedex, Magiran, Irandoc, and SID), starting from the first study available (1974) through February 2018, were searched ( Figure 1).
To avoid missing any articles, the references of all papers were thoroughly checked. We searched for keywords or subject headings including snail, gastropoda, Mollusca, trematode, Trematoda, and Iran.

Inclusion and exclusion criteria
All Persian-language and English-language studies that determined the larval stages of trematodes in snails, either microscopically or molecularly, in Iran were included in this review. Studies in which the type of snail infection, the number of infected snails, or the surveyed province was not individually considered were excluded from the review.

Study selection
Two authors (SD and EK) independently assessed the titles and abstracts of the articles. The full texts of relevant studies were reviewed independently to ensure their eligibility for inclusion in this review. Disagreements were resolved by discussion or by a third researcher (MM).

Quality assessment
The quality of each study was independently evaluated by 2 authors (SD and MM) according to Newcastle-Ottawa Scale [11] (Supplementary Material 2). This tool includes 3 main sections, the first section is graded on a scale containing 5 stars and mainly assesses the methodological quality of an individual study. The second section is graded from 2 stars and focuses on the comparability of the study. Finally, the third section, which is graded from 3 stars, deals with the outcomes and statistical analysis of the original study. Studies with a score of 50.0% and above were considered to have high methodological quality.

Data extraction
Information regarding the species of snails, year of publication, areas of study, number of snails examined, number of infected snails, prevalence of infection, method of study, stage of development, parasites isolated, and first author was extracted from the articles and entered into an Excel spreadsheet (Table 1). areas worldwide, causing severe morbidity and mortality. In Asia, human infections are mainly observed in Iran, and to a lesser extent in Vietnam. The World Health Organization has recently included Iran in a list of 6 countries that are known to have significant problems with fascioliasis [6]. Opisthorchis viverrini and Clonorchis sinensis are common liver flukes found in fish-eating mammals, especially cats and dogs, although humans can become infected by consuming undercooked freshwater fish containing metacercariae of these parasites. Fasciolopsis buski, the giant intestinal fluke, is the etiological agent of fasciolopsiasis in many mammals, particularly humans. Fasciolopsiasis occurs through the consumption of raw or undercooked aquatic plants containing the metacercariae. Paragonimus westermani, a lung fluke, has a wide variety of mammalian hosts, and can also infect humans who ingest insufficiently cooked crabs or crayfish contaminated with the encysted parasite [4].
The dynamics of ecosystems containing snails should be monitored in multiple areas, so that knowledge about the distributions of both the snail population and parasitic diseases in those areas may help to control the snail population, thereby improving community health.
Approximately 350 snail species are known to be of probable medical or veterinary importance. Among the intermediate hosts of trematodes, the species belonging the genera Biomphalaria, Bulinus (water), and Oncomelania (amphibious) are important in the transmission of human schistosomes. In addition, the most important intermediate hosts of liver flukes are members of the genus Lymnaea, which may be either aquatic or amphibious [7].
According to the available resources, various species of snails are found in different parts of Iran; but the fauna of Iran and relevant parasitic infections have not been extensively examined, unlike in other parts of the world [8].
Thus, collecting data on the prevalence of infections with different cercariae in snails is essential for estimating the risk of parasitic diseases in different parts of Iran. In recent years, some studies have been carried out on trematode infections in snails in Iran, but further research is necessary to understand these infections more completely. No relevant systematic review has been conducted about the prevalence of these infections in Iran. We therefore considered it necessary to summarize all available studies on such infections in freshwater snails in the form of a review. The aim of the present study was to estimate the types and prevalence of cercarial infections in snails in Iran. Moreover, this review provides easy access to the available literature and also considers the gaps left by previous studies, which would be beneficial for future researchers.

Data analysis
StatsDirect statistical software (StatsDirect Ltd., Cambridge, UK) was used for data analysis. For each study, the standard deviation of the prevalence was calculated using a binomial distribution formula. The degree of heterogeneity among the results was determined based on the Q test and the I 2 indicator. I 2 indicates the percentage variation between studies attributed to heterogeneity compared to chance. I 2 values ≤ 50.0% indicate low heterogeneity, and I 2 values between 50.0% and 75.0% suggest moderate heterogeneity. Values > 75.0% indicate high heterogeneity across studies, meaning that pooled prevalence must have considered by using a random-effects model [12]. To investigate possible sources of heterogeneity, a meta-analysis was performed on some snail species and provinces. Point estimations of the prevalence of infections with 95% confidence intervals (CIs) are shown in forest plots. The size of each square illustrates the weight of each study, while the crossed lines indicate the CIs. The bias in the results was examined by Begg and Egger tests.

RESULTS
Of the 2,408 studies identified through the initial search, 24 were included in this systematic review, as shown in Table 1 [8,. A flowchart describing the study design is shown in Figure 1.
A total of 98,235 freshwater snails of 9 different species from the families Lymnaeidae, Physidae, Planorbidae, Viviparidae, Thiaridae, and Melanopsidae were collected and identified from different parts of Iran. The prevalence of larval trematodes in the various snail species and areas investigated is shown in Table 1. Additionally, the total quality of all primary studies was greater than 4, indicating that the quality of the included papers was medium to high. Considerable variation was present in the number of trematode cercariae-infected snails across studies; the heterogeneity was quite high (Q= 4,187.3, p< 0.001), and the I² obtained was 99.2% ( Figure 2).
Using a random-effects model for the meta-analysis, 4.4% (95% CI, 2.8 to 6.3) of the collected gastropods were found to be infected with fluke larvae.

Analysis of bias in the findings
Publication bias was analyzed using an Egger funnel plot with 95% CIs. The results of the test (Egger bias: 6.3 [95% CI, 3.3 to 9.3]; p < 0.001) strongly suggested publication bias ( Figure 3).

Infections in gastropods
The gastropods in all studies belonged to Radix auricularia, Lymnaea palustris, Lymnaea    tode infections in R. auricularia was 9.9% (p < 0.001, Q = 3,953.6), which was greater than that of L. stagnalis and L. palustris, at 3.9% (p < 0.001, Q = 343.9) and 1.2% (p < 0.001, Q = 16.8), respectively ( Figure 4). Because other infected snails were investigated in only a few studies, it was not possible to carry out a meta-analysis. Therefore, we present the frequency of infected snails with larval stages of trematodes in Table 2.

Prevalence of larval stages of trematodes in the collected gastropods
Twelve species of trematode larvae were identified based on systematic keys, biometric examinations, experimental infections (cercariae isolated from snail were transferred into glass petri dishes containing natural water for metacercariae formation; wild-type laboratory mice were orally inoculated with metacercariae, euthanized after 8 weeks, and their livers, peritoneum walls, and abdo-   (Table 3). However, in some studies, the larval stage and parasite species were not evaluated, and only trematode larva or furcocercariae were reported. Among the studies included in the meta-analysis, the highest prevalence of cercariae was found for EC and XC (4.3% and 4.1%, respectively). The prevalence of other infections is shown in Table 4.
Since the other larval stages were surveyed only in a limited number of studies, it was not possible to conduct a meta-analysis; instead, we used the average number of larval stages, as shown in Table 5.
We found that R. auricularia was infected with the largest number of larval trematodes (EC, XC, MC, DC, CC, FL, and ScF). Oth-       Table 3.

Prevalence of infected gastropods in different provinces of Iran
Freshwater snails were studied in 5 provinces of Iran-West Azerbaijan, Mazandaran, Guilan, Khuzestan, and Chaharmahal-Bakhtiari-that are important for agriculture.

DISCUSSION
To our knowledge, this is the first systematic review of cercarial infections in snails in Iran. In this systematic review and meta-analysis, the overall prevalence of freshwater gastropods infected with cercariae in Iran was estimated to be 4.4% (95% CI, 2.8 to 6.3). The prevalence of cercarial infections in gastropods has been reported worldwide; similar studies in Turkey, Pakistan, and Iraq have reported high prevalence rates of these infections (7.3%, 14.8%, and 30.7%, respectively) [36][37][38]. The prevalence in countries such as Tanzania (1.3%) and France (1.9%) is lower [39,40], while the prevalence in Nepal (4.3%) is similar to that found in our study [41].
Among the snail species surveyed in this meta-analysis, R. auricularia (9.9%) was the most commonly infected. Some gastropods were not included in this meta-analysis because an inadequate number of articles investigated them. Since sequencing of ribosomal DNA ITS-2 demonstrated that Lymnaea (Radix) gedrosiana, which has been reported in many European countries [42], is a synonym of R. auricularia (the haplotype reported in Bandar-Anzali, northern Iran), in this study, R. auricularia was employed instead. Notably, the larval stages of Echinostomatidae, Monostome, Diplostomidae, Clinostomidae, Plagiorchiidae, Fasciola, and schistosomes were found in R. auricularia.
The prevalence of infections in L. stagnalis, which functions as a host of various trematode larvae, such as Plagiorchiidae, Fasciola, and schistosome larvae, was reported to be 3.9%. Soldánová et al. [43] reported that 15.1% of L. stagnalis samples were infected by species from the families Echinostomatidae, Diplostomidae, Plagiorchiidae, Schistosomatidae, and Telorchiidae in the Ruhr River of Germany.
Despite the broad range of data related to the prevalence of cercariae in snails, only 4 types of cercariae could be evaluated by a meta-analysis because of the insufficient number of studies on oth- er cercariae. In this review, the prevalence of EC infection in R. auricularia, M. tuberculata, and Melanopsis spp. was 4.3%. This rate is lower than that reported by Abdul-Salam (6.1%) in a study on Clypeomorus bifasciata in Kuwait [44]. In Bangladesh, EC was also isolated from R. auricularia [45].
In general, the presence of EC in various snails indicates the important role of the gastropods in transmission of EC to birds, including aquatic wild birds as well as domesticated birds, in Iran and other countries. Ranjbarbahadory et al. [46], in a study on gastrointestinal helminths of native turkeys in Amol, Iran, recorded a prevalence of Echinostoma of 11.0%. Humans become infected by eating the infected freshwater gastropods. Echinostomiasis is endemic in East and Southeast Asia; however, Ahmadi et al. [47] recorded a 0.9% prevalence of Echinostoma spp. in rehabilitation centers in Mazandaran Province in northern Iran.
In this study, the prevalence of XC in various gastropod species was found to be 4.1%. Parasites isolated from L. stagnalis, L. palustris, V. bengalensis, and M. tuberculata were from the Plagiorchiidae family. In a study of freshwater resources in Poland, the Plagiorchiidae family was detected in Lymnaeid gastropods [48]. Species of Plagiorchis (family: Plagiorchiidae) have been reported as intestinal trematodes in birds, reptiles, and mammals. Human infections due to Plagiorchis spp. are rare; at present, only 11 cases have been found in humans worldwide [49]. Furthermore, XC generally belong to the Heterophidae, Opisthorchidae, and Fascolidae families, which underscores the veterinary relevance of these snails. It is very unlikely for humans to be infected by these parasites under normal conditions [50].
In Iran, especially in the coastal areas of the Caspian Sea and Persian Gulf, animal fascioliasis has been prevalent over the past 50 years. Despite the high incidence of livestock infections in the southern regions over the past decades, infections in humans have often occurred in the northern provinces, especially in Guilan Province [51]. The 2 species of Fasciola show variant Lymnaeid snail hosts. F. gigantica is transmitted by species of the Radix genus, while F. hepatica is mostly transmitted by species of the Galba/Fossaria genera [42]. In France, L. neotropica, L. viatrix var. ventricosa, and Galba truncatula are known to be hosts of F. hepatica [52]. It is noteworthy that human fascioliasis has emerged as a public health problem in Kermanshah Province in western Iran; hence, the verification of new regions of human fascioliasis requires complementary investigations [53].
Both mammalian and bird schistosomes are causative agents of human cercarial dermatitis, but bird schistosomes, mostly Trichobilharzia species, are responsible for the majority of dermatitis outbreaks reported both in Iran and worldwide [13,54]. Recently, in a study conducted in Mazandaran Province, Iran in 2016, Fakhar et al. [55] reported that all the examined samples of nasal schistosomes were grouped in a sister clade to the European Trichobilharzia regenti.
In Mazandaran Province, Gohardehi et al. [13] showed that the prevalence of infections with Trichobilharzia spp. among migratory birds (ducks) was 15.8% and the most infected snail was R. au-ricularia. In addition, in 2013, Rahimi-Esboei et al. [56] reported a high prevalence (77.5%) of cercarial dermatitis among paddy field workers in Mazandaran Province, thereby proving it to be a health hazard in the area. In this review, schistosome furcocercariae were detected in 5 snail species: R. auricularia, L. palustris, L. stagnalis, P. planorbis, and M. tuberculata, with a prevalence of 2.7%. The prevalence of furcocercariae in Bulinus globosus (1.1%) in Tanzania [39] was less than that found in our study. Bulinus spp. act as intermediate hosts in the life cycle of S. haematobium; despite their remarkable abundance, only a single study, by Farahnak et al. [28], has surveyed infections by trematodes in Bulinus spp. [57].
In this study, different cercariae were detected in several provinces of Iran because of variations in rainfall, humidity, and temperature in each province. Among the provinces evaluated in this meta-analysis, the highest and lowest proportions of cercarial infections were observed in West Azerbaijan (16.9%) and Khuzestan (1.7%) Provinces, respectively. Temperatures of 25-30°C, annual rainfall exceeding 100 mm, and relative humidity > 65.0% are favorable for the growth and shedding of cercariae [58]. These conditions facilitate their development and growth in host snails, and they change from miracidia to sporocysts, rediae, and eventually cercariae. The climate in West Azerbaijan is mainly influenced by the rain-bearing winds of the Mediterranean Sea and Atlantic Ocean [59]. The mean temperature ranges from 13.7°C (winter) to 22.5°C (summer), with an average annual rainfall of 300 mm-800 mm that occurs in 2 primary rainy seasons (March to June and October to November) and a humidity of 30-80%, indicating that it has a moderate climate. Additionally, West Azerbaijan contains 30 permanent and seasonal wetlands and many suitable habitats for birds. This province is also geographically positioned along the migration path of birds that migrate from northern latitudes to southern latitudes every year at the beginning of the autumn and cold seasons. In previous studies in the province, snails were collected from May through November. The most prevalent cercarial types in the province, according to the reports by Imani-Baran et al. [33] and Yakhchali et al. [35], were EC (276 of 370), T. szidati, and T. franki (100 of 320) from R. auricularia, respectively. Thus, contamination by these two trematodes may be significant in West Azerbaijan. In Chaharmahal-Bakhtiari, 105 of 350 snails surveyed by Rivaz et al. [27] were infected with Plagior chiidae cercariae.
Some limitations should be considered when interpreting the results of our study, including publication bias. The present metaanalysis only included published studies; we did not search for unpublished studies or original data. Thus, the findings of the Egger funnel plot should be interpreted with caution, because of the few numbers of included studies in combination with the high heterogeneity across the studies, which could limit the ability to assess publication bias.
Few studies have been conducted in Iran, compared to other parts of the world, of the distribution of types of snails and the prevalence of trematode infections in intermediate hosts using conventional microscopic and molecular methods. Therefore, due to the limited number of studies carried out in 5 provinces, we could not conduct a meta-analysis of all surveyed provinces, snails, and trematode infections.

CONCLUSION
This is the first study to provide information on the distribution of various snails, their trematode infections, and their potential to cause zoonotic diseases in Iran. According to the present review, snail control is essential for reducing the prevalence of diseases such as echinostomiasis, fascioliasis, and cercarial dermatitis in humans. Implementation of snail control depends on several factors, such as the infection level in the final hosts (domestic animals or people), the freshwater snail habitat, transmission pattern, snail species, and ecological concerns.
Future studies are needed to characterize the prevalence of trematodes in snails in different provinces of Iran. It is also recommended that more accurate methods should be used for identifying cercariae at the species level, to promote a better understanding of the epidemiological conditions of these infections in different provinces.