Apropos: critical analysis of molluscicide application in schistosomiasis control programs in Brazil

Schistosomiasis is a snail-transmitted infectious disease affecting over 200 million people worldwide. Snail control has been recognized as an effective approach to interrupt the transmission of schistosomiasis, since the geographic distribution of this neglected tropical disease is determined by the presence of the intermediate host snails. In a recent Scoping Review published in Infectious Diseases of Poverty, Coelho and Caldeira performed a critical review of using molluscicides in the national schistosomiasis control programs in Brazil. They also described some chemical and plant-derived molluscicides used in China. In addition to the molluscicides described by Coelho and Caldeira, a large number of chemicals, plant extracts and microorganisms have been screened and tested for molluscicidal actions against Oncomelania hupensis, the intermediate host of Schistosoma japonicum in China. Here, we presented the currently commercial molluscicides available in China, including 26% suspension concentrate of metaldehyde and niclosamide (MNSC), 25% suspension concentrate of niclosamide ethanolamine salt (SCNE), 50% niclosamide ethanolamine salt wettable powder (WPN), 4% niclosamide ethanolamine salt dustable powder (NESP), 5% niclosamide ethanolamine salt granule (NESG) and the plant-derived molluscicide “Luowei”. These molluscicides have been proved to be active against O. hupensis in both laboratory and endemic fields, playing an important role in the national schistosomiasis control program of China. Currently, China is transferring its successful experiences on schistosomiasis control to African countries. The introduction of Chinese commercial molluscicides to Africa, with adaptation to local conditions, may facilitate the progress towards the elimination of schisosomiasis in Africa. Electronic supplementary material The online version of this article (doi:10.1186/s40249-017-0246-x) contains supplementary material, which is available to authorized users.


Introduction
Schistosomiasis, caused by the blood fluke parasites of Schistosoma genus, is a snail-transmitted infectious disease affecting over 200 million people worldwide [1]. Since the geographic distribution of this neglected tropical disease is determined by the presence of the intermediate host snails of the parasite, snail control is considered to be effective to interrupt the transmission of the disease [2]. A recent quantitative analysis of schistosomiasis control outcomes captured from 83 countries and territories over the past 100 years, showed that the extensive snail control contributed to a reduction of schistosomiasis prevalence by 92%, while the control programs with little or no snail control interventions achieved a 37% reduction, which highlights the critical role of snail control in the schistosomiasis control programs [3].
In a recent Scoping Review published in in the journal Infectious Diseases of Poverty, Coelho and Caldeira [4] performed a critical review of using molluscicides in the national schistosomiasis control programs in Brazil. They also described some chemical and plant-derived molluscicides used in China. From the initiation of the national schistosomiasis control program until now, the national schistosomiasis control strategy of China has shifted from transmission control strategy based on snail control between mid-1950s and early 1980s, praziquantel chemotherapy based-morbidity control strategy during the period from mid-1980s through 2003, to the current integrated control strategy since 2004, and snail control has been an important part of the national schistosomiasis control strategy throughout the control programs [5]. Currently, molluscicide treatment remains the predominant approach for snail control in China [6]. In addition to the molluscicides described by Coelho and Caldeira, a large number of chemicals, plant extracts and microorganisms have been screened and evaluated for molluscicidal actions against Oncomelania hupensis, the intermediate host of S. japonicum. Here, we summarized the currently commercial molluscicides available in China, which have been proved to be active against O. hupensis in both laboratory and endemic fields.

26% suspension concentrate of metaldehyde and niclosamide (MNSC)
MNSC, a mixture of 25% niclosamide and 1% metaldehyde, is prepared by grinding niclosamide, metaldehyde, glycerol, sodium carboxymethyl cellulose, sodium benzoate and distilled water in a ball miller to yield a suspension formulation. In the laboratory, immersion with MNSC at 0.125 mg/L and higher concentrations for 24 h achieved 100% snail mortality, with a 24 h LC 50 of 0.058 mg/L and a 48 h LC 50 of 0.044 mg/L. Spraying with MNSC at 2 g/m 2 resulted in 94, 100 and 100% snail mortality at 1, 3, and 7 days, respectively [7]. In the marshland and lake regions, the field immersion test showed 97.5-100%, 98.89-100% and 100% snail mortality following treatment with MNSC at 2 g/m 3 for 24, 48 and 72 h, while the field spraying test revealed 67.81-95.53%, 72.8-89.68%, and 86.67-100% snail mortality following treatment with MNSC at 2 g/m 2 for 1, 3, and 7 days, respectively [7]. In the mountainous regions, spraying with MNSC at 6 g/m 2 resulted in 92.88-94.46% snail mortality at 7 days, 90.47-93.79% at 15 days and 88.68-94.27% at 30 day, respectively [8]. Compared with the 50% wettable powder of niclosamide ethanolamine salt (WPN), the first-choice chemical molluscicide used for snail control in China since 1990s on the recommendation of WHO [9], MNSC reduces the toxicity of niclosamide to the environment and fish [7]; however, the exact effect of MNSC on the environment and the mechanisms underlying the molluscicidal efficacy of the drug against O. hupensis snails remain to be investigated.
In addition, MNSC was reported to be active against Lymnaea species [10], and 25% suspension concentrate of niclosamide (SCN) has shown toxicity to both adult and juvenile Biomphalaria glabrata [11]. These exciting findings encourage the investigations of the molluscicidal efficacy of 26% MNSC against Biomphalaria spp. and Bulinus spp..

25% suspension concentrate of niclosamide ethanolamine salt (SCNE)
SCNE is a novel suspension formulation of niclosamide which contains 25% niclosamide ethanolamine salt. In the laboratory, immersion with SCNE at 0.125 mg/L and higher doses for 24 h and a longer period of time resulted in 100% snail mortality, with 0.058, 0.044 and 0.039 mg/L for 24, 48, and 72 h LC 50 values, respectively, and spraying with SCNE at 1 to 8 g/m 2 achieved 90-100% snail mortality at 1 day, 97-100% at 3 days, and 99-100% at 7 days, respectively, depending on drug doses [12]. In a marshland and lake region, the field immersion test showed 100% snail mortality following the treatment with SCNE at 2 g/m 3 for 24, 48, or 72 h, and the field spraying test revealed 74.51-88.81% snail mortality at 1 day, 80.34-98.26% at 3 days and 87.38-100% at 7 days depending on the drug doses given (1 to 8 g/m 2 ) [12]. This formulation provides a novel choice of chemical molluscicides for the field application in the endemic foci of China; however, further studies are required to investigate the environmental toxicity and the mechanisms of action of SCNE.

WPN
With the initiation of the World Bank Loan Project for Schistosomiasis Control in China since 1992, WPN was introduced and currently it is the most widely used chemical molluscicide in China [13]. The sensitivity of O. hupensis to WPN varies in the schistosomiasis-endemic regions of China [14], and a recent meta-analysis showed 77% (95% CI: 68 to 86%), 83% (95% CI: 77 to 89%) and 88% (95% CI: 82 to 92%) snail mortality after spraying WPN at a dose of 2 g/m 2 for 3, 7 and 15 days in the marshland regions [15]. Excitingly, no evidence of resistance to WPN has been detected in O. hupensis after more than 2 decades of repeated, extensive application in the main endemic foci of China [16].
Recently, de Novo transcriptome analysis revealed 254 differentially expressed unigenes in WPN-treated O. hupensis snails, which were associated with cell structure defects and inhibition of neurohumoral transmission and energy metabolism that may cause snail death [17]. In addition, WPN treatment resulted in damages to cell structures and organelles of O. hupensis snails, suggesting the suppression of the movement ability and effects on liver and energy metabolism; in parallel, activities of carbohydrate metabolism-associated enzymes were inhibited, and activities of stress response-associated enzymes were increased followed by a reduction to lower levels than those of the H 2 O-treated group [18]. This shift of carbohydrate metabolism patterns led to insufficient energy supply and lactic acid accumulation [18]. Variations of nitric oxide synthase (NOS), alanine aminotransferase (ALT), and superoxide dismutase (SOD) during the WPN treatment suggested a stress response of O. hupensis snail to the molluscicide at early stages and later fatal damage in the liver and the nervous system [18]. However, the environmental issues related to the use of WPN have not been clearly demonstrated to date.

5% niclosamide ethanolamine salt granule (NESG)
The use of niclosamide suspension formulations requires water, and the powder formulation suffers from the drug drift and the resultant environmental pollution, as well as potential damages to workers spraying the powder. NESG was therefore developed in order to overcome these problems [23]. Laboratory test showed that spraying with NESG at 0.5, 1 and 2 g/m 2 resulted in 85-98% snail mortality at 1 day, 93-99% at 3 days and 96-100% at 7 days, respectively, depending on drug doses [23]. In the marshland region, 7 days treatment with 2 g/m 2 NESG achieved 85.42% snail mortality and a 93.1% reduction in the density of living snails [24], and in the mountainous region, spraying with NESG at 2 g/m 2 resulted in 72.41% snail mortality at 7 days, 74.61% at 15 days and 73.24% at 30 day, and reduced the density of living snails by 72.26% at 7 days, 74.09% at 15 days and 75.91% at 30 day, respectively [25]. NESG is a breakthrough niclosamide formulation of chemical molluscicides in China, and this agent greatly expands the application range of niclosamide formulations. Further studies to investigate the environmental toxicity of this formulation seem justified.
In addition, "Luowei" was found to be active against Pomacea canaliculata [33], and the molluscicidal efficacy of "Luowei" against Biomphalaria and Bulinus is under evaluation.

Conclusions
During the past two decades, a large number of novel molluscicides have been screened and developed in China, and many of them have been made commercial. In addition to chemical and plant-derived agents, the molluscicidal action of microorganisms against O. hupensis has been recently paid more and more attention [34][35][36].
Currently, China is transferring its successful experiences on schistosomiasis control to African countries [37,38]. The introduction of Chinese commercial molluscicides, with adaptation to African local conditions, may facilitate the progress towards the elimination of schisosomiasis in Africa [39].