Preventive anthelmintic chemotherapy--expanding the armamentarium

Preventive chemotherapy and anthelmintic resistance of soil-transmitted helminths - Can we learn nothing from veterinary medicine?

Current parasite control programs in veterinary species have moved away from mass anthelmintic treatment approaches due to the emergence of significant anthelmintic resistance (AR), and the availability of few classes of anthelmintics. A number of parallels between livestock and human helminths exist that warn of the risk of AR in human soil-transmitted helminthiases, yet current public health interventions continue to prioritize mass treatment strategies, a known risk factor for AR. This review discusses the existing parallels between human and animal helminth biology and management, along with current public health recommendations and strategies for helminth control in humans. The effectiveness of current recommendations and alternative management strategies are considered.

Discovery of genomic intervals that underlie nematode responses to benzimidazoles

Parasitic nematodes impose a debilitating health and economic burden across much of the world. Nematode resistance to anthelmintic drugs threatens parasite control efforts in both human and veterinary medicine. Despite this threat, the genetic landscape of potential resistance mechanisms to these critical drugs remains largely unexplored. Here, we exploit natural variation in the model nematodes Caenorhabditis elegans and Caenorhabditis briggsae to discover quantitative trait loci (QTL) that control sensitivity to benzimidazoles widely used in human and animal medicine. High-throughput phenotyping of albendazole, fenbendazole, mebendazole, and thiabendazole responses in panels of recombinant lines led to the discovery of over 15 QTL in C. elegans and four QTL in C. briggsae associated with divergent responses to these anthelmintics. Many of these QTL are conserved across benzimidazole derivatives, but others show drug and dose specificity. We used near-isogenic lines to recapitulate and narrow the C. elegans albendazole QTL of largest effect and identified candidate variants correlated with the resistance phenotype. These QTL do not overlap with known benzimidazole target resistance genes from parasitic nematodes and present specific new leads for the discovery of novel mechanisms of nematode benzimidazole resistance. Analyses of orthologous genes reveal conservation of candidate benzimidazole resistance genes in medically important parasitic nematodes. These data provide a basis for extending these approaches to other anthelmintic drug classes and a pathway towards validating new markers for anthelmintic resistance that can be deployed to improve parasite disease control.

The treatment of roundworm (nematode) infections in both humans and animals relies on a small number of anti-parasitic drugs. Resistance to these drugs has appeared in veterinary parasite populations and is a growing concern in human medicine. A better understanding of the genetic basis for parasite drug resistance can be used to help maintain the effectiveness of anti-parasitic drugs and to slow or to prevent the spread of drug resistance in parasite populations. This goal is hampered by the experimental intractability of nematode parasites. Here, we use non-parasitic model nematodes to systematically explore responses to the critical benzimidazole class of anti-parasitic compounds. Using a quantitative genetics approach, we discovered unique genomic intervals that control drug effects, and we identified differences in the effects of these intervals across compounds and doses. We were able to narrow a major-effect genomic region associated with albendazole resistance and to establish that candidate genes discovered in our genetic mappings are largely conserved in important human and animal parasites. This work provides new leads for understanding parasite drug resistance and contributes a powerful template that can be extended to other anti-parasitic drug classes.

Building a global schistosomiasis alliance: an opportunity to join forces to fight inequality and rural poverty

Schistosomiasis, one of the 17 neglected tropical diseases listed by the World Health Organization, presents a substantial public health and economic burden. Of the 261 million people requiring preventive chemotherapy for schistosomiasis in 2013, 92% of them lived in sub-Saharan Africa and only 12.7% received preventive chemotherapy. Moreover, in 2010, the WHO reported that schistosomiasis mortality could be as high as 280 000 per year in Africa alone.In May 2012 delegates to the sixty-fifth World Health Assembly adopted resolution WHA65.21 that called for the elimination of schistosomiasis, and foresees the regular treatment of at least 75% of school age children in at-risk areas. The resolution urged member states to intensify schistosomiasis control programmes and to initiate elimination campaigns where possible.Despite this, in June 2015, schistosomiasis was indicated to have the lowest level of preventive chemotherapy implementation in the spectrum of neglected tropical diseases. It was also highlighted as the disease most lacking in progress. This is perhaps unsurprising, given that it was also the only NTD with access to drug donations but without a coalition of stakeholders that collaborates to boost commitment and implementation.As a consequence, and to ensure that the WHO NTDs Roadmap Targets of 2012 and World Health Assembly Resolution WHA65.21 are met, the Global Schistosomiasis Alliance (GSA) has been set up. Diverse and representative, the GSA aims to be a partnership of endemic countries, academic and research institutions, international development agencies and foundations, international organizations, non-governmental development organizations, private sector companies and advocacy and resource mobilisation partners. Ultimately, the GSA calls for a partnership to work for the benefit of endemic countries by addressing health inequity and rural poverty.

Efficacy and reinfection with soil-transmitted helminths 18-weeks post-treatment with albendazole-ivermectin, albendazole-mebendazole, albendazole-oxantel pamoate and mebendazole

Background

Preventive chemotherapy with albendazole or mebendazole is the current strategy to control soil-transmitted helminth (STH) infections (i.e. Ascaris lumbricoides, hookworm and Trichuris trichiura). STH reinfections, in particular A. lumbricoides and T. trichiura occur rapidly after treatment with the standard drugs. However, their low efficacy against T. trichiura, made an accurate assessment of reinfection patterns impossible.

Methods

In 2013 a randomised controlled trial was conducted on Pemba Island, Tanzania. School-aged children diagnosed positive for T. trichiura, were randomly allocated to (i) albendazole-ivermectin; (ii) albendazole-mebendazole; (iii) albendazole-oxantel pamoate; or (iv) mebendazole. Here we report the efficacy [cure rates (CR) and egg-reduction rates (ERR)], reinfection rates and new infections determined 18 weeks post-treatment.

Results

For a total of 405 children complete baseline and follow-up data were available. Similar to the efficacy determined after 3 weeks, 18 weeks after treatment albendazole-oxantel pamoate showed a significantly higher efficacy against T. trichiura (CR: 54.0 %, 95 % CI: 43.7–64.0; ERR: 98.6 %, 95 % CI: 97.8–99.2) compared to the other treatment arms. Children treated with albendazole-oxantel pamoate or albendazole-ivermectin had fewer moderate infections compared to children treated with albendazole. The reinfection rates 18 weeks post-treatment among all treatment arms were 37.2 % for T. trichiura (95 % CI: 28.3–46.8), 34.6 % for A. lumbricoides (95 % CI: 27.3–42.3) and 25.0 % for hookworms (95 % CI: 15.5–36.6).

Conclusion

The moderate reinfection rates with STHs 18 weeks post-treatment support the concept of regular anthelminthic treatment in highly endemic settings. Combination chemotherapy might achieve decreased morbidity in children since in the albendazole plus oxantel pamoate and albendazole plus ivermectin treatment arms only few moderate T. trichiura infections remained. Further trials should investigate the long term efficacy of albendazole-oxantel pamoate (i.e. 6 and 12 month post-treatment) and after several rounds of treatment in order to develop recommendations for appropriate control approaches for STH infections.

Trial registration

Current Controlled Trials ISRCTN80245406

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1406-8) contains supplementary material, which is available to authorized users.

Current WHO protocols for mass drug administration in helminth control

Soil transmitted helminths (STH), comprising Ascaris, Trichuris, Strongyloides and the hookworms remain a significant cause of morbidity amongst people in many parts of the world, including Australia. Other important helminth infections include lymphatic filariasis (LF), schistosomiasis and onchocerciasis. Preventive chemotherapy (mass drug administration [MDA]) campaigns are frequently conducted for these helminth infections in endemic areas, but the target population groups, duration of campaigns, cointerventions (e.g. vector control) criteria for inclusion, drugs used and doses of drugs differ.

Monitoring the efficacy of drugs for neglected tropical diseases controlled by preventive chemotherapy

In the last decade, pharmaceutical companies, governments and global health organisations under the leadership of the World Health Organization (WHO) have pledged large-scale donations of anthelmintic drugs, including ivermectin (IVM), praziquantel (PZQ), albendazole (ALB) and mebendazole (MEB). This worldwide scale-up in drug donations calls for strong monitoring systems to detect any changes in anthelmintic drug efficacy. This review reports on the outcome of the WHO Global Working Group on Monitoring of Neglected Tropical Diseases Drug Efficacy, which consists of three subgroups: (i) soil-transmitted helminthiases (ALB and MEB); (ii) onchocerciasis and lymphatic filariasis (IVM); and (iii) schistosomiasis (PZQ). Progress of ongoing work, challenges and research needs for each of the four main drugs used in helminthic preventive chemotherapy (PC) are reported, laying the ground for appropriate implementation of drug efficacy monitoring programmes under the co-ordination and guidelines of the WHO. Best practices for monitoring drug efficacy should be made available and capacity built as an integral part of neglected tropical disease (NTD) programme monitoring. Development of a disease-specific model to predict the impact of PC programmes, to detect outliers and to solicit responses is essential. Research studies on genetic polymorphisms in relation to low-efficacy phenotypes should be carried out to identify markers of putative resistance against all NTD drugs and ultimately to develop diagnostic assays. Development of combination and co-administration of NTD drugs as well as of new drug entities to boost the armamentarium of the few drugs available for NTD control and elimination should be pursued in parallel.

Neglected tropical diseases: now more than just 'other diseases'--the post-2015 agenda

Neglected tropical diseases (NTDs) have become recognised as important health problems facing at least a billion people in the low-income countries and the poorest communities in middle-income countries. WHO plays a leading role in developing strategies to address these diseases, pharmaceutical companies provide drug donations to treat or control the NTDs and many partners from different constituencies have become increasingly committed to their control or elimination. This review looks to the post-2015 agenda and emphasises that despite the progress made over recent years, if the targets established are to be achieved, then not only will additional financial resources be required to up-scale treatments and increase access, but increased applied and operational research will be necessary to address problems and human capacity in NTD skills will need to be strengthened. Continuing advocacy for the relevance of control or elimination of NTDs must be placed in the context of universal health coverage and access to donated essential medicines for the poor as a right. The evidence that investment in NTD interventions are cost-effective and impact not only on health, but also to enhance socio-economic development, must be refined and promulgated. The global burden of disease attributable to NTDs requires reassessment to appropriately define the true burden, while the potential for unexpected events, political, climatic, environmental as well as biological, have the potential to reduce future progress towards the agreed post-2015 targets. NTD progress towards the WHO Roadmap targets and the fulfilment of the World Health Assembly Resolution 66.12 of 2013 demand continued commitment from all partner constituencies when challenges emerge.