Abstract
Schistosomiasis is the second most important human parasitic disease in terms of socioeconomic impact, causing great morbidity and mortality, predominantly across the African continent. For intestinal schistosomiasis, severe morbidity manifests as periportal fibrosis (PPF) in which large tracts of macro-fibrosis of the liver, visible by ultrasound, can occlude the main portal vein leading to portal hypertension (PHT), sequelae such as ascites and collateral vasculature, and ultimately fatalities. For urogenital schistosomiasis, severe morbidity manifests as pathology throughout the urinary system and genitals, and is a definitive cause of squamous cell bladder carcinoma. Preventative chemotherapy (PC) programmes, delivered through mass drug administration (MDA) of praziquantel (PZQ), have been at the forefront of schistosomiasis control programmes in sub-Saharan Africa since their commencement in Uganda in 2003. However, despite many successes, ‘biological hotspots’ (as distinct from ‘operational hotspots’) of both persistent high transmission and morbidity remain. In some areas, this failure to gain control of schistosomiasis has devastating consequences, with not only persistently high infection intensities, but both “subtle” and severe morbidity remaining prevalent. These hotspots highlight the requirement to revisit research into severe morbidity and its mechanisms, a topic that has been out of favor during times of PC implementation. Indeed, the focality and spatially-structured epidemiology of schistosomiasis, its transmission persistence and the morbidity induced, has long suggested that gene-environmental-interactions playing out at the host-parasite interface are crucial. Here we review evidence of potential unique parasite factors, host factors, and their gene-environmental interactions in terms of explaining differential morbidity profiles in the human host. We then take the situation of schistosomiasis mansoni within the Albertine region of Uganda as a case study in terms of elucidating the factors behind the severe morbidity observed and the avenues and directions for future research currently underway within a new research and clinical trial programme (FibroScHot).
Highlights
Species of the genus Schistosoma are digenetic trematodes and the causative agents of the Neglected Tropical Disease (NTD) schistosomiasis; a parasitic disease that ranks second only to malaria in terms of socioeconomic impacts
Schistosomiasis is a multifactorial disease in which several host and parasite factors including their interaction with the environment may act at each step of the parasite’s life cycle [287, 288]
While the reasons behind discrepancies in clinical outcome remain ambiguous and the relationship between infection intensity and organ-related morbidity is complex, cumulative evidence is showing that as well as hostspecific factors shaping the genetic composition of schistosome populations [156, 190, 191], parasite genetic differences may be important in the development of human host morbidity [53, 133, 175, 176, 289], and the characteristics of morbidity responses to them [150]
Summary
Species of the genus Schistosoma are digenetic trematodes and the causative agents of the Neglected Tropical Disease (NTD) schistosomiasis; a parasitic disease that ranks second only to malaria in terms of socioeconomic impacts. Studies have revealed that the disease severity can vary considerably at both the geographical and individual level for both S. mansoni [42,43,44,45] and S. haematobium [46,47,48], associated in part, but not always, in focality to accompanying high transmission and infection intensities [39, 45, 49,50,51,52] Epidemiological evidence suggests such variation in morbidity could be partially explained by, among other factors, the immunological and genetic background of the endemic communities, their nutritional status and/or the length of time individuals have been exposed [45, 53,54,55,56,57]. We take the situation of schistosomiasis mansoni within the Albertine region of Uganda as a case study in terms of elucidating the potential factors behind the severe morbidity observed to date and the avenues and direction for research currently underway within a new clinical trial programme (FibroScHot)
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