A Study on a Cercarial Assay for Detection of Praziquantel-Resistance in Alexandria (Egypt)

Background: Praziquantel (PZQ) has been used extensively and successfully in national schistosomiasis control programs in Egypt and there are reports of emerging PZQ resistance. Objective: The aim of this work was to use an in vitro assay for detection of PZQ resistance in Egyptian field and laboratory strains of Schistosoma mansoni cercariae. Methodology: The assay was performed on ordinary glass slides. Cercariae were exposed to a final concentration of 0, 5×10−7 M or 5×10−6 M PZQ. They were examined and counted at 0, 20, 40 and 60 minutes. In some samples the assay was coupled with scanning electron microscopy to detect any PZQinduced tegumental changes in cercariae. Results: After exposure to 5×10−7 M or 5×10−6 M of PZQ, there was a gradual decrease in the proportion of unaffected cercariae. The effect of the exposure time on the percentage of unaffected cercariae of the laboratory and field strains was significant in the two concentrations of PZQ. The low concentration of the drug did not succeed to cause significant differences between the two isolates of cercariae throughout a 60minute exposure time. Surprisingly, the high concentration of PZQ showed a significant difference in the response of the two isolates where the field strain was more affected after exposure to PZQ for 60 minutes. The results of the SEM showed that all cercariae exposed to PZQ for 30 or 60 minutes were influenced, as blebs appeared in the body tegument, unlike the unexposed cercariae. Conclusion: The in vitro assay results indicated that PZQ resistance may not constitute a real problem in the field isolates of S. mansoni cercariae in Alexandria. However, why the field isolates showed significantly higher susceptibility to PZQ needs further investigations.