In vitro activity of usnic acid potassium salt against different developmental stages of Schistosoma mansoni: An ultrastructural study

•Effects of unic acid potassium salt on evolutionary stages of S. mansoni were studied.•In vitro effect of PS-UA was dose and time-dependent.•LC100 at the 24 h for schistosomules and young worms were 12.5 and 200 μM respectively.•PS-UA induced ultrastructural changes in both the evolutionary stages of S. mansoni.•PS-UA is a promising compound that could be used for the development of new schistosomicidal agent. Currently, the control of schistosomiasis is based on a single drug, praziquantel, which is effective against all species of Schistosoma but only in the adult stage, presenting a schistosomicidal deficit at the other developmental stages of the parasites. Recently our research group has demonstrated that the potassium salt of usnic acid (PS-UA) presented schistosomicidal property against couples of adult worms of S. mansoni. Thus, the present study seeks to report for the first time the in vitro activity of PS-UA against different developmental stages of S. mansoni (schistosomules and young worms). As schistosomicide parameters, we evaluated motility, mortality, cell viability of the worms and tegument changes by scanning electron microscopy (SEM). After 3 h exposure, PS-UA was lethal to schistosomules at concentrations of 100 and 50 μM, whereas for concentrations 25 and 12.5 μM, 38 and 18% of mortality and 62 and 24% changes in motility, respectively, were reached. Yet for schistosomules, concentration of 25 μM caused 90 and 100% of death after 6 and 12 h, respectively. In the concentration of 12.5 μM at intervals of 12 and 24 h mortality was 68 and 100%, respectively. For young worms, after 3 h of exposure at concentrations of 200 and 100 μM caused 57 and 27% mortality, respectively. After 12 and 24 h, these concentrations caused mortality of 90 and 100% and 47 and 60% respectively. After 24 h, concentrations of 50 and 25 μM caused 80 and 30% change in motility, respectively. However, at the 12.5 μM concentration no change was observed. In addition, PS-UA reduced the cellular viability of young worms by 50.98% and 85.87% at concentrations of 100 and 200 μM, respectively. In both stages of worms and at different exposure intervals, PS-UA caused alterations such as: dorsoventral contraction, peeling, swelling, blisters, erosion, exposure of subtegumental tissue and disintegration of tegument. According to the results, changes in motility and mortality caused by PS-UA against schistosomules and young worms were concentration and time-dependents, also