Different approaches to modeling the cost-effectiveness of schistosomiasis control

Impact of two rounds of praziquantel mass drug administration on Schistosoma mansoni infection prevalence and intensity: a comparison between community wide treatment and school based treatment in western Kenya

This study compared the effectiveness of the community-wide treatment and school-based treatment approaches in the control of Schistosoma mansoni infections in villages with ⩾25% prevalence in western Kenya. Stool samples from first year students, 9-12year olds and adults (20-55years) were analyzed by the Kato-Katz technique for S. mansoni eggs. After two rounds of treatment, S. mansoni prevalence and intensity levels significantly declined in both treatment approaches. Prevalence comparisons between the two approaches did not show any significant differences following treatment. However, infection intensity levels in the 9-12year old school-attending pupils were significantly higher in the community-wide treatment arm than in the school-based treatment arm. Nevertheless, significant reductions in S. mansoni infection prevalence and intensity levels were achieved among school-age children regardless of the treatment approach used.

Impact and cost-effectiveness of a comprehensive Schistosomiasis japonica control program in the Poyang Lake region of China

Schistosomiasis japonica remains a significant public-health problem in China. This study evaluated cost-effectiveness of a comprehensive schistosomiasis control program (2003–2006). The comprehensive control program was implemented in Zhangjia and Jianwu (cases); while standard interventions continued in Koutou and Xiajia (controls). Incurred costs were documented and the schistosomiasis comprehensive impact index (SCI) and cost-effectiveness ratio (Comprehensive Control Program Cost/SCI) were applied. In 2003, prevalence of Schistosoma japonicum infection was 11.3% (Zhangjia), 6.7% (Jianwu), 6.5% (Koutou), and 8.0% (Xiajia). In 2006, the comprehensive control program in Zhangjia and Jianwu reduced infection to 1.6% and 0.6%, respectively; while Koutou and Xiajia had a schistosomiasis prevalence of 3.2% and 13.0%, respectively. The year-by-year SCIs in Zhangjia were 0.28, 105.25, and 47.58, with an overall increase in cost-effectiveness ratio of 374.9%–544.8%. The SCIs in Jianwu were 16.21, 52.95, and 149.58, with increase in cost-effectiveness of 226.7%–1,149.4%. Investment in Koutou and Xiajia remained static (US$10,000 unit cost). The comprehensive control program implemented in the two case villages reduced median prevalence of schistosomiasis 8.5-fold. Further, the cost effectiveness ratio demonstrated that the comprehensive control program was 170% (Zhangjia) and 922.7% (Jianwu) more cost-effective. This work clearly shows the improvements in both cost and disease prevention effectiveness that a comprehensive control program-approach has on schistosomiasis infection prevalence.

Projecting the long-term impact of school- or community-based mass-treatment interventions for control of Schistosoma infection

Background

Schistosomiasis remains a significant health burden in many areas of the world. Morbidity control, focused on limiting infection intensity through periodic delivery of anti-schistosomal medicines, is the thrust of current World Health Organization guidelines (2006) for reduction of Schistosoma-related disease. A new appreciation of the lifetime impact of repeated Schistosoma infection has directed attention toward strategies for greater suppression of parasite infection per se, with the goal of transmission interruption. Variations in drug schedules involving increased population coverage and/or treatment frequency are now undergoing field trials. However, their relative effectiveness in long-term infection suppression is presently unknown.

Methodology/Principal Findings

Our study used available field data to calibrate advanced network models of village-level Schistosoma transmission to project outcomes of six different community- or school age-based programs, as compared to the impact of current 2006 W.H.O. recommended control strategies. We then scored the number of years each of 10 typical villages would remain below 10% infection prevalence (a practicable level associated with minimal prevalence of disease). All strategies that included four annual treatments effectively reduced community prevalence to less than 10%, while programs having yearly gaps (‘holidays’) failed to reach this objective in half of the communities. Effective post-program suppression of infection prevalence persisted in half of the 10 villages for 7–10 years, whereas in five high-risk villages, program effects on prevalence lasted zero to four years only.

Conclusions/Significance

At typical levels of treatment adherence (60 to 70%), current WHO recommendations will likely not achieve effective suppression of Schistosoma prevalence unless implemented for ≥6 years. Following more aggressive 4 year annual intervention, some communities may be able to continue without further intervention for 8–10 years, while in higher-risk communities, annual treatment may prove necessary until eco-social factors fostering transmission are removed. Effective ongoing surveillance and locally targeted annual intervention must then become their mainstays of control.

Debate persists about how best to prevent disease caused by Schistosoma parasites. Current guidelines focus on suppressing morbidity by limiting average intensity of infection during childhood. However, this may not be sufficient to cure infection or prevent reinfection, leaving risk for sub-clinical morbidities such as growth stunting and anemia. More intensive programs involving broader coverage and/or more frequent treatments could potentially cure most infections and even prevent their return. Because such programs' effectiveness is not currently known, we used computer simulation (grounded by past treatment program results) to project short- and long-term impact in communities where Schistosoma are common. We estimated that 4 annual treatments (delivered community-wide or only to school-age children and high-risk adults) could effectively reduce local prevalence below 10%. Programs with gap years were less effective, particularly in high-risk communities. If a program was successful, infection stayed <10% for 7–10 years in low risk communities. However, rapid resurgence (within 1–5 years) was projected for high risk villages. We conclude that, given the networked transmission of Schistosoma, annual treatment programs of sufficient duration can have secondary benefits, i.e., long-term suppression in some areas. However, high risk areas will need continuing surveillance and frequent retreatment to truly minimize their risk for disease.

A research agenda for helminth diseases of humans: modelling for control and elimination

Mathematical modelling of helminth infections has the potential to inform policy and guide research for the control and elimination of human helminthiases. However, this potential, unlike in other parasitic and infectious diseases, has yet to be realised. To place contemporary efforts in a historical context, a summary of the development of mathematical models for helminthiases is presented. These efforts are discussed according to the role that models can play in furthering our understanding of parasite population biology and transmission dynamics, and the effect on such dynamics of control interventions, as well as in enabling estimation of directly unobservable parameters, exploration of transmission breakpoints, and investigation of evolutionary outcomes of control. The Disease Reference Group on Helminth Infections (DRG4), established in 2009 by the Special Programme for Research and Training in Tropical Diseases (TDR), was given the mandate to review helminthiases research and identify research priorities and gaps. A research and development agenda for helminthiasis modelling is proposed based on identified gaps that need to be addressed for models to become useful decision tools that can support research and control operations effectively. This agenda includes the use of models to estimate the impact of large-scale interventions on infection incidence; the design of sampling protocols for the monitoring and evaluation of integrated control programmes; the modelling of co-infections; the investigation of the dynamical relationship between infection and morbidity indicators; the improvement of analytical methods for the quantification of anthelmintic efficacy and resistance; the determination of programme endpoints; the linking of dynamical helminth models with helminth geostatistical mapping; and the investigation of the impact of climate change on human helminthiases. It is concluded that modelling should be embedded in helminth research, and in the planning, evaluation, and surveillance of interventions from the outset. Modellers should be essential members of interdisciplinary teams, propitiating a continuous dialogue with end users and stakeholders to reflect public health needs in the terrain, discuss the scope and limitations of models, and update biological assumptions and model outputs regularly. It is highlighted that to reach these goals, a collaborative framework must be developed for the collation, annotation, and sharing of databases from large-scale anthelmintic control programmes, and that helminth modellers should join efforts to tackle key questions in helminth epidemiology and control through the sharing of such databases, and by using diverse, yet complementary, modelling approaches.

Schistosomiasis in the People's Republic of China: the era of the Three Gorges Dam

The potential impact of the Three Gorges Dam (TGD) on schistosomiasis transmission in China has invoked considerable global concern. The TGD will result in changes in the water level and silt deposition downstream, favoring the reproduction of Oncomelania snails. Combined with blockages of the Yangtze River's tributaries, these changes will increase the schistosomiasis transmission season within the marshlands along the middle and lower reaches of the Yangtze River. The changing schistosome transmission dynamics necessitate a comprehensive strategy to control schistosomiasis. This review discusses aspects of the epidemiology and transmission of Schistosoma japonicum in China and considers the pathology, clinical outcomes, diagnosis, treatment, immunobiology, and genetics of schistosomiasis japonica together with an overview of current progress in vaccine development, all of which will have an impact on future control efforts. The use of synchronous praziquantel (PZQ) chemotherapy for humans and domestic animals is only temporarily effective, as schistosome reinfection occurs rapidly. Drug delivery requires a substantial infrastructure to regularly cover all parts of an area of endemicity. This makes chemotherapy expensive and, as compliance is often low, a less than satisfactory control option. There is increasing disquiet about the possibility that PZQ-resistant schistosomes will develop. Consequently, as mathematical modeling predicts, vaccine strategies represent an essential component in the future control of schistosomiasis in China. With the inclusion of focal mollusciciding, improvements in sanitation, and health education into the control scenario, China's target of reducing the level of schistosome infection to less than 1% by 2015 may be achievable.

The role of 'passive chemotherapy' plus health education for schistosomiasis control in China during maintenance and consolidation phase

In order to explore the possibility of further optimising schistosomiasis control during the maintenance and consolidation phase in China, two highly endemic villages were selected to compare the strategy of 'passive chemotherapy' plus health education to that of mass chemotherapy singly. Emphasis was placed on treatment coverage with praziquantel among individuals infected with Schistosoma japonicum and costs incurred for treating an infected person. The results show that the former strategy was almost as good as the latter producing treatment coverage rates among egg-positives of 96.2-97.1% during 2 years, while corresponding rates of 100% were achieved in the village where mass chemotherapy was employed. Importantly, the cost of the former strategy was only about half that of mass chemotherapy, i.e. 49.0% in the first year and 54.6% in the following. Moreover, 'passive chemotherapy' together with health education can conveniently be integrated into the primary health care system making it an attractive strategy for schistosomiasis control during the maintenance and consolidation phase.

An economic evaluation of the national schistosomiasis control programme in China from 1992 to 2000

The World Bank Loan Project, by far the largest effort in China for schistosomiasis control since control activities were initiated in the mid 1950s, was carried out for a 9-year period commencing in 1992 in the 8 provinces where Schistosoma japonicum remained endemic when the project started. To evaluate its impact, a retrospective economic evaluation was done in 2001. Six representative counties, i.e. Huarong in Hunan province, Qianjiang in Hubei province, Yugan in Jiangxi province, Tongling in Anhui province, Xichang in Sichuan province and Dali in Yunnan province, were selected for the study. The total financial input in these counties from 1992 to 2000 was RMB Yuan 90.334 million with the World Bank loan accounting for 40.9%. Control efforts resulted in reduction of human prevalence rates in the six counties from 0.7-9.0% in 1992 to 0.1-2.7% in 2000. With regard to S. japonicum infection in bovines, a high reduction was observed in Qianjiang, and smaller decreases were noted in four counties, while there was an increase in Dali. In general, the areas infested by the intermediate host snail fluctuated around the initial level. The net benefit-cost ratio was 6.20, which means that this project gained US$ 6.20 for every dollar spent. The correlation coefficients of the net benefit-cost ratio to the human and bovine infection rates at the beginning of the project were 0.55 and 0.66, respectively. It is conceivable that further progress in schistosomiasis control is an important feature for sustained growth of the local economy, particularly in areas where control of the disease has been most challenging.

Heterogeneous model of schistosomiasis transmission and long-term control: the combined influence of spatial variation and age-dependent factors on optimal allocation of drug therapy

Prior field studies and modelling analyses have individually highlighted the importance of age-specific and spatial heterogeneities on the risk for schistosomiasis in human populations. As long-term, large-scale drug treatment programs for schistosomiasis are initiated in subSaharan Africa and elsewhere, optimal strategies for timing and distribution of therapy have yet to be fully defined on the working, district-level scale, where strong heterogeneities are often observed among sublocations. Based on transmission estimates from recent field studies, we develop an extended model of heterogeneous schistosome transmission for distributed human and snail population clusters and age-dependent behaviour, based on a 'mean worm burden + snail infection prevalence' formulation. We analyse its equilibria and basic reproduction patterns and their dependence on the underlying transmission parameters. Our model allows the exploration of chemotherapy-based control strategies targeted at high-risk behavioural groups and localities, and the approach to an optimal design in terms of cost. Efficacy of the approach is demonstrated for a model environment having linked, but spatially-distributed, populations and transmission sites.

Community-directed treatment with ivermectin in two Nigerian communities: an analysis of first year start-up processes, costs and consequences

OBJECTIVES

To determine the start-up processes, costs and consequences of community-directed treatment with ivermectin (CDTI) in two onchocerciasis endemic rural towns of Southeast Nigeria; namely Achi and Nike. The other objectives were to discover the community-financing mechanisms, local ivermectin distribution strategies and communities' organisational capacity to handle the programme.

METHODS

Structured questionnaires, informal interviews, observations, discussions with community members at general village assemblies and community outreach lectures were used at different stages of the study.

RESULT

The towns had the organisational capacity to implement the programme. Coverage with ivermectin was between 31-73% in Achi (mean = 58.6%), and 36.6-72% in Nike (mean = 61.95%). The unit financial costs were $0.17 in Nike and $0.13 in Achi, but the unit aggregate cost was $0.37 in Nike and $0.39 in Achi. When research costs were removed, the unit aggregate cost was $0.22 in Achi and $0.20 in Nike. Provider's financial costs and communities' non-financial costs were the biggest contributors to the aggregate cost. The cost would decrease in subsequent years since the research cost and parts of the mobilisation and training costs would not be incurred after the first year.

CONCLUSION

Governments and sponsors of CDTI should find means of continuously strengthening the programme and providing technical support to the communities. As both CDTI and communities are dynamic entities, continuous health education campaigns are needed to keep reminding the people of the benefit of long-term ivermectin distribution, together with the need for community ownership of the programme.

Mathematical modelling of schistosomiasis japonica: comparison of control strategies in the People's Republic of China

We present the first mathematical model on the transmission dynamics of Schistosoma japonicum. The work extends Barbour's classic model of schistosome transmission. It allows for the mammalian host heterogeneity characteristic of the S. japonicum life cycle, and solves the problem of under-specification of Barbour's model by the use of Chinese data we are collecting on human-bovine transmission in the Poyang Lake area of Jiangxi Province in China. The model predicts that in the lake/marshland areas of the Yangtze River basin: (1) once-yearly mass chemotherapy of humans is little better than twice-yearly mass chemotherapy in reducing human prevalence. Depending on the heterogeneity of prevalence within the population, targeted treatment of high prevalence groups, with lower overall coverage, can be more effective than mass treatment with higher overall coverage. Treatment confers a short term benefit only, with prevalence rising to endemic levels once chemotherapy programs are stopped; (2) depending on the relative contributions of bovines and humans, bovine treatment can benefit humans almost as much as human treatment. Like human treatment, bovine treatment confers a short-term benefit. A combination of human and bovine treatment will dramatically reduce human prevalence and maintains the reduction for a longer period of time than treatment of a single host, although human prevalence rises once treatment ceases; (3) assuming 75% coverage of bovines, a bovine vaccine which acts on worm fecundity must have about 75% efficacy to reduce the reproduction rate below one and ensure mid-term reduction and long-term elimination of the parasite. Such a vaccination program should be accompanied by an initial period of human treatment to instigate a short-term reduction in prevalence, following which the reduction is enhanced by vaccine effects; (4) if the bovine vaccine is only 45% efficacious (the level of current prototype vaccines) it will lower the endemic prevalence, but will not result in elimination. If it is accompanied by an initial period of human treatment and by a 45% improvement in human sanitation or a 30% reduction in contaminated water contact by humans, elimination is then possible.

The costs and coverage of a strategy to control schistosomiasis morbidity in non-enrolled school-age children in Egypt

Schistosomiasis still constitutes a major public health problem in some endemic parts of rural Egypt, particularly in school-aged children. The school-based health programme for schistosomiasis control adopted by the Egyptian Ministry of Health and Population (MOHP) focuses on treating enrolled schoolchildren. Children who are not enrolled or who do not attend regularly (out-of-school) do not have access to this service, in spite of evidence that the prevalence and intensity of infection are higher in these children than among children who attend school regularly. The aim of the present study was to test an intervention to extend the existing routinely applied school-based treatment to the out-of-school children. This paper reports on the costs and the coverage obtained by the intervention under 2 alternative delivery strategies: selective versus mass chemotherapy for out-of-school children. The intervention reached 88.5% of the out-of-school children at a relatively low cost per infected child of 2.29 L.E. (US$0.67) and 2.02 L.E. (US$0.59) for selective and mass chemotherapy, respectively. The calculations also suggest that, if the government were to implement such a programme, it would be more efficient to offer mass than selective chemotherapy at observed levels of prevalence. Sensitivity analysis showed that selective chemotherapy would be more cost-effective in areas where the prevalence of infection in out-of-school children was < 43%.

Is there immunity to Schistosoma japonicum?

The Oriental schistosome, Schistosoma japonicum, unlike the other two major schistosomes that infect humans (S. mansoni and S. haematobium), is a zoonotic species. The transmission dynamics and the potential effects of host-related regulatory factors, including immunity, are likely to be distinct for this parasite. Here, Allen Ross and collaborators from Australia, China and the Philippines discuss recently published and established epidemiological and laboratory data bearing on anti-infection immunity to Asian schistosomiasis, and contrast these findings with the emerging picture of development of anti-infection immunity against the African schistosomes. Implications for vaccines and other control strategies for schistosomiasis japonica are also discussed.

A comparative analysis of the cost-effectiveness of treatment based on parasitological and symptomatic screening for Schistosoma mansoni in Burundi

OBJECTIVES

To assess the cost-effectiveness of three alternative screening strategies in delivering treatment to patients presenting with symptoms suggestive of S. mansoni, using data collected from 17 PHCCs in the Rusizi Plain, Burundi. The three strategies were: (1) screening all symptomatic patients using a Kato-Katz smear and treating only the ones found positive; (2) treating all symptomatic patients or (3) treating only those presenting with symptoms of severe diarrhoea (blood in stool).

METHODS

The database consisted of 41 051 visits of symptomatic patients to the 17 PHCCs during 1990. Effectiveness was measured as the number of infected patients treated (gold standard: 25-mg Kato-Katz smear). Cost-effectiveness ratios (CERs) (cost in US$ per infected patient treated) were used to compare the control strategies. Sensitivity analysis was performed to assess the effect of drug price and prevalence of infection on the CERs.

RESULTS

The overall prevalence of S. mansoni infection was 9.5% (95% CI = 9.1%, 9.9%). Treating only those with severe symptoms of infection would have left 92% of infected patients untreated. Performing a Kato-Katz smear to confirm the diagnosis was more cost-effective than treating all symptomatic patients on a presumptive basis, with CERs of 4.2 US$ and 12.43 US$ per infected person treated, respectively. Sensitivity analysis showed that, for a cost of 0.99 US$ per dose of praziquantel, the Kato-Katz option remained the most cost-effective approach for prevalences under 76%. For a drug price of 0.21 US$ per dose, both strategies would have become equivalent. The latter value varied extensively per PHCC (range 0.17 US$ to 0.51 US$), due to the different prevalences of infection (range 0.5% to 34.3%).

CONCLUSION

We found that using severe diarrhoea as an indicator for infection was not appropriate, and that screening symptomatic patients with the Kato-Katz method remained the most cost-effective approach in the given setting. The CER of treating all symptomatic patients on a presumptive basis depended very much on the drug price and the endemic level.

Developing strategies to control schistosomiasis morbidity in nonenrolled school-age children: experience from Egypt

Schistosomiasis is a major health problem in school-age children in much of the tropical world. They harbour the most intense infections for both Schistosoma mansoni and S. haematobium. In Egypt, the Ministry of Health and Population (MOHP) has implemented a successful school-based treatment programme in which children are screened and those found to be infected treated with praziquantel. High nonenrolment rates in some rural areas have a negative impact on the coverage of this programme and on its ability to reduce transmission in the community. The main aim of our study was to introduce and test a simple intervention to extend treatment to nonenrolled children using the routine MOHP schistosomiasis treatment programme. Twenty villages or ezbas in Tamia district, Fayoum governorate, with 8 schools and 1901 nonenrolled children were targeted. 88.5% of nonenrolled children attended schools to avail themselves of treatment. Coverage rates were significantly higher for girls (P < 0.001). These results are important for countries where schistosomiasis is endemic. They suggest that offering interventions in schools may not only improve the health of school attendees but also be an affordable way of extending services to out-of-school children.