Double impact: natural molluscicide for schistosomiasis vector control also impedes development of Schistosoma mansoni cercariae into adult parasites

Abstract

Background: Schistosomiasis has been reported in 78 endemic countries and affects 240 million people worldwide. The digenetic parasite Schistosoma mansoni needs fresh water to compete its life cycle. There, it is susceptible to soluble compounds that can affect directly and/or indirectly the parasite's biology. The cercariae stage is one of the key points in which the parasite is vulnerable to different soluble compounds that can significantly alter the parasite's life cycle. Molluscicides are recommended by the World Health Organization for the control of schistosomiasis transmission and Euphorbia milii latex is effective against snails intermediate hosts.

Methodology/principal findings: We used parasitological tools and electron microscopy to verify the effects of cercariae exposure to natural molluscicide (Euphorbia milii latex) on morphology, physiology and fitness of adult parasite worms. In order to generate insights into key metabolic pathways that lead to the observed phenotypes we used comparative transcriptomics and proteomics.

Conclusions/significance: We describe here that the effect of latex on the adult is not due to direct toxicity but it triggers an early change in developmental trajectory and perturbs cell memory, mobility, energy metabolism and other key pathways. We conclude that latex has not only an effect on the vector but applies also long lasting schistosomastatic action. We believe that these results are of interest not only to parasitologists since it shows that natural compounds, presumably without side effects, can have an impact that occurred unexpectedly on developmental processes. Such collateral damage is in this case positive, since it impacts the true target of the treatment campaign. This type of treatment could also provide a rational for the control of other pests. Our results will contribute to enforce the use of E. milii latex in Brazil and other endemic countries as cheap alternative or complement to mass drug treatment with Praziquantel, the only available drug to cure the patients (without preventing re-infection).

Fig 1. Sex ratio of Schistosoma mansoni (male parasites / female parasites) in function of parasite load.

The parasite load was measured in 18 infected control mice and 57 mice infected with 150 exposed cercariae per mouse. (A) Parasite load in control group: linear regression R² = 0.85, Y = 4.0 E^-3x + 87.13; (B) Parasite load in Swiss Webster mouse infected with latex-exposed S. mansoni: linear regression R² = 0.002, Y = 5.2 E^-5x - 1.7752. Every point represents perfusion of worms from all mice of single cage (5-20 mice). The cages were also used for Kato-Katz.

Fig 2. Hepatic granuloma of mice infected with Schistosoma mansoni.

The samples were randomly chosen among 18 mice from control group and 57 mice from exposed group. (A) Hepatic granulomas in control group. (B): Hepatic granulomas in Swiss Webster mouse infected with latex-exposed S. mansoni. Bar: 200 μm.

Fig 3. Scanning electron micrographs of adult male Schistosoma mansoni.

Oral and ventral suckers of parasites from control (A) and latex-exposed groups (B); Tegument of worms from control (C) and latex-exposed groups (D); Gynecophoral channel of worms with spines from control (E) and without spines for latex-exposed ones (F).

Fig 4. Scanning electron micrographs of adult female Schistosoma mansoni.

Oral and ventral suckers of female parasites from control (A) and latex-exposed groups (B).

Fig 5. 2D gels of adult females (left) and males (right) of Schistosoma mansoni.

For each sex, spots over-expressed in latex-exposed group as compared to control group are indicated in red while under-expressed ones are represented in green. Protein identification of spots by MS/MS is available in S3 Table.

Fig 6. Pathways affected in Schistosoma mansoni adult worms after exposure to latex at cercarial stage.

Anabolic and catabolic pathways affected only in male (left, blue), only in female (right, red) or in both sexes (middle, purple). Pathways down-expressed are circled by a dotted lines and pathways over-expressed are circled with full lines.