Single-sex schistosome infections of definitive hosts: Implications for epidemiology and disease control in a changing world
A substantial proportion of the world’s disease burden is caused by infectious agents that lead to high mortality, morbidity, and reduced productivity among many millions of people and their animals. Guided by the Millennium Development Goals (MDGs) and the subsequent Sustainable Development Goals (SDGs), much progress has been made in reducing the burden of human infectious diseases since 2000, although challenges remain and new questions emerge.
Human schistosomiasis is a neglected tropical disease (NTD) caused by blood flukes of the genus Schistosoma. The parasitic disease remains a public health problem in the majority of 78 tropical and subtropical countries, with approximately 261 million people in need of treatment. After nearly 70 years of major multidisciplinary control efforts, great success has been achieved against Schistosoma japonicum within China. Across sub-Saharan Africa, efforts to control S. mansoni and S. haematobium through large-scale preventive chemotherapy (PC) with praziquantel (PZQ) have also had a substantial impact on preventing or relieving morbidity and improving global health, particularly amongst the poorest. The success of such activities has in part led to a revision of WHO’s strategic plan and a vision for ‘a world free of schistosomiasis’, with elimination as a public health problem and complete interruption of transmission in selected regions by 2025.
However, recent reports of prevalence levels greater than previously thought, re-emergence of schistosomiasis in previously controlled regions, and emergence of zoonotic hybrid Schistosoma within previously uninfected regions, as well as indications of reduced drug efficacy amongst populations under high PC pressure, all serve to highlight that schistosomes are highly complex multi-host parasitic organisms. Many essential characteristics of their biology and epidemiology remain unknown.