Praziquantel is the main pillar of schistosomiasis control. The number of countries administering praziquantel within the frame of preventive chemotherapy programmes to school-aged children and other at-risk populations is increasing [28, 32]. Since no real alternative drug against schistosomiasis is currently available [15, 21, 24], there is a need to closely monitor the efficacy of praziquantel, particularly in areas where high coverage is reached and treatment is repeated frequently. Standard protocols for drug efficacy assessment and monitoring do not yet exist, but are needed, so that the potential development of drug resistance can be detected early on [26, 29]. For recommendation of the most appropriate time point to assess CR and ERR of praziquantel against S. haematobium infection, we studied the dynamics of S. haematobium egg output and associated infection parameters over a 62-day period following treatment with a standard dose of 40 mg/kg praziquantel in a group of school-aged children in south Côte d’Ivoire.
S. haematobium infection prevalence and egg output post-treatment
We found that both CR and ERR were highly dependent on the time of measurement post-treatment. The CR was highest in week 6 (between day 36 and day 42) post-treatment (92.9%). The AM and GM ERR increased sharply in the first two weeks after treatment and showed a constantly high level from week 3 post-treatment onwards (>97.5%). These findings are in line with observations from Gabon, where a significant reduction in daily S. haematobium egg counts between day 3 and day 9 post-treatment was observed, among schoolchildren treated with praziquantel and observed for 35 days post-treatment . A study from South Africa found a high GM ERR (95.3%), but only a moderate CR (57.9%) three weeks post-treatment . In Cameroon, observed CRs at weeks 3 and 6 were 40-50% and 83%, respectively, after praziquantel administration , corroborating our findings. Hence, we recommend that CRs be assessed at around day 36 post-treatment, whereas ERRs can be assessed considerably earlier, from day 15 post-treatment onwards. However, it should be noted that our results were obtained in a high prevalence setting without prior praziquantel treatment, and hence care is indicated in generalizing our findings to other areas with different infection intensity profiles that have been subjected to prior large-scale administration of praziquantel.
The tendency of finding increased egg output and a higher number of S. haematobium-positive children in weeks 7 and 8 post-treatment is possibly due to schistosomula that were not affected by praziquantel, and hence have further developed, matured and paired up in the weeks after treatment to start oviposition . Reinfection might have also occurred, but it is unlikely that one finds S. haematobium eggs in the urine of newly infected individuals, as it takes more than 60 days from infection to oviposition . Since our study started during the dry season in March 2010, when the transmission of S. haematobium should be low, the effect of juvenile worms might not be as strong as during high transmission season . An increase in S. mansoni egg excretion already four weeks post-treatment was observed in a preceding study conducted in western Côte d’Ivoire that followed a similar design as the work presented here . Since the development of S. haematobium worms takes considerably longer than that of S. mansoni, whilst the period of insusceptibility is the same in both species, egg output might be seen at a later stage for S. haematobium than for S. mansoni[22, 23, 39]. However, it has been suggested that insusceptibility of schistosomula to praziquantel is less pronounced in S. haematobium than in S. mansoni. A study from Cameroon did not find lower CRs 9 weeks post-treatment compared to 6 weeks post-treatment and therefore suggested good efficacy of praziquantel against all stages of S. haematobium. In view of our findings and those of others, we suggest that CR should be assessed before week 7 post-treatment, and we encourage further research on drug susceptibility of immature S. haematobium worms.
Notably, the slight decrease in prevalence observed in week 9 of our study is likely due to the fact that only one instead of two urine samples per child were collected. Indeed, previous research carried out in Côte d’Ivoire, South Africa and Zimbabwe has shown that the more intense the sampling effort post-treatment, the lower the observed CR [34, 40–42]. Future standard operating procedures should therefore include advice on the sampling strategy for drug efficacy monitoring. We suggest the microscopic examination of at least two urine samples, collected between 10:00 and 14:00 over consecutive days, for determining praziquantel efficacy in terms of CR.
It is important to note that S. haematobium and S. mansoni often co-exist [43–45]. In such co-endemic settings, mainly for operational and practical reasons, it would be useful to have a single time point for assessment of praziquantel treatment efficacy. According to our data pertaining to S. haematobium and recommendations made by Scherrer and colleagues for S. mansoni, the optimal time points for assessment of CRs differ between the two schistosome species (i.e. week 3 post-treatment for S. mansoni and week 6 post-treatment for S. haematobium). As a compromise, perhaps, CRs of both infections could be measured in weeks 4 or 5 post-treatment. However, it needs to be considered that at this time point, S. haematobium infection prevalence might not be at a minimum, and hence care is indicated with regard to drug efficacy evaluation and monitoring of potential resistance development. If morbidity control rather than elimination of schistosomiasis is the goal of the treatment intervention, assessing ERRs in both species at one time point will be a useful strategy. Indeed, egg counts of both species have been shown to be constantly low in both species from week 3 post-treatment onwards . We conclude that ERRs can be assessed at a single time point from day 14 post-treatment onwards.
S. haematobium egg output during the first days post-treatment
The observation that S. haematobium egg counts first dropped rapidly within one day but increased again until the end of the first week post-treatment before they dropped sharply and remained at low levels from week 3 onwards seems an artefact at first sight. However, this phenomenon has also been described for S. mansoni egg output after treatment with praziquantel . One possible explanation is that not all adult worms die immediately after treatment but some might be paralysed and reproduce heavily before dying within weeks 1–3 post-treatment. Another explanation might be that praziquantel has a direct effect on the host tissue, leading to a decreased release of eggs into the bladder. Praziquantel might also have a direct effect on S. haematobium eggs. Indeed, it has been described that praziquantel in very high doses can lead to hatching of eggs in the tissue . If this finding is translatable to regular treatment doses with praziquantel, the temporary “disappearance” of schistosome eggs might not be due to dying adult worms but rather due to early hatching of miracidia. Further research is needed to elucidate the exact mechanisms of praziquantel on eggs, different developmental stages of the worms and reactions in the human body.
Other infection parameters post-treatment
In addition to studying egg output dynamics using the urine filtration method, we assessed the evolution of S. haematobium-associated infection parameters after treatment with praziquantel. As shown in previous studies, the prevalence of microhaematuria, proteinuria and leukocyturia decreased after treatment with praziquantel [47–50]. Proteinuria and leukocyturia showed a transient minimum around week 5 post-treatment. Both parameters increased again towards the end of our 2-month observation period, which might point to new eggs being excreted by surviving worms and triggering inflammation and lesions in the urinary tract. Microhaematuria remained more common than the other parameters until the end of the study and many children were still passing blood in urine without excreting S. haematobium eggs. Towards the end of our study, four children had macrohaematuria, but they did not pass any or only very few eggs in their urine. Possible explanations for the remaining macrohaematuria are as follows. First, two of the children were girls, aged 13 and 15 years, and hence the observed blood might be due to menstruation. Second, one 13-year-old boy had likely high exposure levels, since he presented with a heavy infection before treatment and had a very quick increase in egg counts shortly after treatment.
Our findings of lasting egg-negative haematuria after treatment are in line with a study from Kenya where the prevalence of microhaematuria decreased more slowly than the prevalence of eggs detected by microscopy and where it took 4 to 6 months until the lowest levels were reached [51, 52]. Lesions in the urinary tract with small bleeds might still be present even after egg output has terminated. Indeed, ultrasound examination has shown that urinary tract pathology resolves at a slower rate than reduction in egg output during the first months after treatment [52, 53]. As for microhaematuria, a decrease of proteinuria and leukocyturia 5 to 6 weeks post-treatment might serve as an additional parameter for drug efficacy and morbidity reduction assessment in settings of high endemicity where initial prevalences of microhaematuria, proteinuria and leukocyturia are elevated [18, 54].