Schistosomiasis around Siavonga, on the shores of Lake Kariba, Zambia

An update on snail and trematode communities in the Sanyati Basin of Lake Kariba: New snail and trematode species but no human schistosomes

BACKGROUND

The construction of Lake Kariba brought about a rise in the incidence of schistosomiasis in its surrounding towns of Kariba (Zimbabwe) and Siavonga (Zambia). After extensive control programs in Kariba, schistosomiasis prevalence dropped significantly. The objective of this study was to revisit the same localities sampled by Chimbari et al. (2003), and provide an update on the snail community and prevalence of trematodes in the Northern shore of Lake Kariba while focusing on planorbid species.

METHODS

Monthly sampling of snails at 16 sites along the Northern shoreline of Lake Kariba, near Kariba town, was undertaken for one year. Minimum one specimen per morphotype was identified using molecular barcoding (sequencing a fragment of cytochrome c oxidase I subunit (COI)). The infection status of snails was assessed by Rapid Diagnostic PCRs (RD-PCR), and trematode infections were genotyped by sequencing COI and 18S rDNA markers.

RESULTS

We collected and identified seven snail species: Bulinus truncatus, Bulinus forskalii, Gyraulus sp., Physella acuta, Bellamya sp., Radix affinis plicatula and Pseudosuccinea columella. Physella acuta was the most abundant snail species (comprising 56.95% of the total snail count) and present at all sites. The B. truncatus population was found to be infected with the stomach fluke Carmyerius cruciformis, a Petasiger sp. and a trematode species belonging to the family Notocotylidae. No Schistosoma sp. infections were detected in our collected snail specimens.

CONCLUSIONS

We report B. truncatus as an intermediate snail host for Carmyerius cruciformis, and the presence of three non-schistosome trematode species that have not been reported in Lake Kariba before. Furthermore, we detect a possible shift in the snail community when compared to the report by Chimbari et al. (2003): this is the first record of Gyraulus sp. in Lake Kariba, and we did not observe the previously reported B. pfeifferi, B. globosus and Radix natalensis. Although this shift in snail communities might have contributed to the absence of Schistosoma spp. detection in this study, further monitoring of final and intermediate hosts across the Kariba basin is essential to prove a decrease of schistosomiasis in the area.

Invasive snails, parasite spillback, and potential parasite spillover drive parasitic diseases of Hippopotamus amphibius in artificial lakes of Zimbabwe

Background

Humans impose a significant pressure on large herbivore populations, such as hippopotami, through hunting, poaching, and habitat destruction. Anthropogenic pressures can also occur indirectly, such as artificial lake creation and the subsequent introduction of invasive species that alter the ecosystem. These events can lead to drastic changes in parasite diversity and transmission, but generally receive little scientific attention.

Results

In order to document and identify trematode parasites of the common hippopotamus (Hippopotamus amphibius) in artificial water systems of Zimbabwe, we applied an integrative taxonomic approach, combining molecular diagnostics and morphometrics on archived and new samples. In doing so, we provide DNA reference sequences of the hippopotamus liver fluke Fasciola nyanzae, enabling us to construct the first complete Fasciola phylogeny. We describe parasite spillback of F. nyanzae by the invasive freshwater snail Pseudosuccinea columella, as a consequence of a cascade of biological invasions in Lake Kariba, one of the biggest artificial lakes in the world. Additionally, we report an unknown stomach fluke of the hippopotamus transmitted by the non-endemic snail Radix aff. plicatula, an Asian snail species that has not been found in Africa before, and the stomach fluke Carmyerius cruciformis transmitted by the native snail Bulinus truncatus. Finally, Biomphalaria pfeifferi and two Bulinus species were found as new snail hosts for the poorly documented hippopotamus blood fluke Schistosoma edwardiense.

Conclusions

Our findings indicate that artificial lakes are breeding grounds for endemic and non-endemic snails that transmit trematode parasites of the common hippopotamus. This has important implications, as existing research links trematode parasite infections combined with other stressors to declining wild herbivore populations. Therefore, we argue that monitoring the anthropogenic impact on parasite transmission should become an integral part of wildlife conservation efforts.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-021-01093-2.

Invasion and Dispersal of Biomphalaria Species: Increased Vigilance Needed to Prevent the Introduction and Spread of Schistosomiasis

Biological invasion is a matter of great concern from both public health and biodiversity perspectives. Some invasive snail species may trigger disease emergence by acting as intermediate hosts. The geographic distribution of Schistosoma mansoni depends on the presence of susceptible species of Biomphalaria freshwater snails that support the parasite's transformation into infective stages. Biomphalaria spp. have shown strong local and global dispersal capacities that may increase due to the global warming phenomenon and increases in the development of agricultural and water projects. Should intermediate hosts become established in new areas then this will create potential transmission foci. Examples of snail invasions that have had an impact on schistosomiasis transmission include the introduction of Biomphalaria tenagophila to Congo and B. glabrata to Egypt. The current spread of B. straminea in China is causing concern and needs to be monitored closely. An understanding of the mode of invasion and distribution of these snails as well as their experimental susceptibility to S. mansoni will predict the potential spread of schistosomiasis. Here we review the invasion patterns of Biomphalaria snails and factors that control their distribution and the impact that invasion may have on intestinal schistosomiasis transmission. In addition, we propose some possible surveillance responses for optimum control strategies and interventions. Whenever possible, swift action should be taken to contain any new occurrence of these intermediate snail hosts.

Gender-based differences in water, sanitation and hygiene-related diarrheal disease and helminthic infections: a systematic review and meta-analysis

BACKGROUND

Qualitative evidence suggests that inadequate water, sanitation and hygiene (WASH) may affect diarrheal and helminthic infection in women disproportionately. We systematically searched PubMed in June 2014 (updated 2016) and the WHO website, for relevant articles.

METHODS

Articles dealing with the public health relevance of helminthic and diarrheal diseases, and highlighting the role of gender in WASH were included. Where possible, we carried out a meta-analysis.

RESULTS

In studies of individuals 5 years or older, cholera showed lower prevalence in males (OR 0.56; 95% CI 0.34-0.94), while Schistosoma mansoni (1.38; 95% CI 1.14-1.67), Schistosoma japonicum (1.52; 95% CI 1.13-2.05), hookworm (1.43; 95% CI 1.07-1.89) and all forms of infectious diarrhea (1.21; 95% CI 1.06-1.38) showed a higher prevalence in males. When studies included all participants, S. mansoni and S. japonicum showed higher prevalence with males (OR 1.40; 95% CI 1.27-1.55 and 1.84; 95% CI 1.27-2.67, respectively). Prevalence of Trichiuris and hookworm infection showed effect modification with continent.

CONCLUSIONS

Evidence of gender differences in infection may reflect differences in gender norms, suggesting that policy changes at the regional level may help ameliorate gender related disparities in helminthic and diarrheal disease prevalence.

A meta-analysis of changes in schistosomiasis prevalence in Zambia: implications on the 2020 elimination target

Schistosomiasis affects more than 4 million school-aged children in Zambia, mostly in rural communities due to unsafe water and inadequate sanitation facilities. Although several studies were done in Zambia between 1976 and 2019, empirical estimates of the disease burden remain unavailable. Therefore, appraisal of the current schistosomiasis burden is pertinent in the re-evaluation of schistosomiasis-control strategies in Zambia. A random-effect model was used to estimate the prevalence of schistosomiasis infection in Zambia across different age groups for the period between 1976 and 2019. A literature search was done in the following databases: PubMed, ISI Web of Science, Google Scholar, CINAHL, and African Journals Online. Twenty-eight studies with relevant prevalence data were identified and included in the analysis. The pooled prevalence estimate of Schistosoma haematobium and Schistosoma mansoni across studies for the entire period was 35.5% (95% CI: 25.8-45.9) and 34.9% (95% CI: 20.7-50.6), respectively. Prevalence estimates among school-aged children for S. haematobium and S. mansoni were 32.2% (95% CI: 21.1-44.7) and 18.1% (95% CI: 3.0-38.4), respectively. The reported pooled prevalence estimate for S. haematobium among the adults was 54% (95% CI: 23.2-83.7). Only two studies collected information from preschool aged children. Substantial heterogeneity (I² = 100%, p < 0.0001) was observed among the studies. Although a reduction in disease prevalence was observed from 1990 to 2010, this was not sustained after 2010. In this meta-analysis, S. haematobium was more prevalent compared to S. mansoni, with more cases observed among school-aged children (SAC). Thus, control programs should target age groups that are highly infected or are at high risk of infection.

A cascade of biological invasions and parasite spillback in man-made Lake Kariba

Parasite spillback, the infection of a non-indigenous organism by a native parasite, is a highly important although understudied component of ecological invasion dynamics. Here, through the first analysis of the parasite fauna of lymnaeid gastropods of Lake Kariba (Zimbabwe). We illustrate how the creation of an artificial lake may lead to a cascade of biological invasions in which an invasive aquatic plant promotes the proliferation of invasive gastropods, which in turn alters the epidemiology of trematodiases of potential medical and veterinary importance. Using a new multiplex Rapid Diagnostic PCR assay, we assessed the prevalence of Fasciola sp. infections in the gastropod populations. Both gastropod hosts and trematode parasites were identified using DNA barcoding. We provide the first record of the invasive North-American gastropod Pseudosuccinea columella in Lake Kariba. This species was found at 14 out of 16 sampled sites and its abundance was strongly positively correlated with the abundance of the invasive South-American water hyacinth (Eichhornia crassipes). About 65% of the P. columella specimens analysed were infected with a hitherto unknown Fasciola species. Phylogenetic analyses indicate close affinity to Fasciola hepatica and F. gigantica, which cause fasciolosis, an important liver disease affecting both ruminants and humans. In addition, another non-native Lymnaeid species was found: a Radix sp. that clustered closely with a Vietnamese Radix species. Radix sp. hosted both amphistome and Fasciola trematodes. By linking an invasion cascade and parasite spillback, this study shows how both processes can act in combination to lead to potentially important epidemiological changes.

Schistosomiasis in Zambia: a systematic review of past and present experiences

Background

The speedy rate of change in the environmental and socio-economics factors may increase the incidence, prevalence and risk of schistosomiasis infections in Zambia. However, available information does not provide a comprehensive understanding of the biogeography and distribution of the disease, ecology and population dynamics of intermediate host snails. The current study used an information-theoretical approach to understand the biogeography and prevalence schistosomiasis and identified knowledge gaps that would be useful to improve policy towards surveillance and eradication of intermediate hosts snails in Zambia.

Methods

To summarise the existing knowledge and build on past and present experiences of schistosomiasis epidemiology for effective disease control in Zambia, a systematic search of literature for the period 2000–2017 was done on PubMed, Google Scholar and EBSCOhost. Using the key words: ‘Schistosomiasis’, ‘Biomphalaria’, ‘Bulinus’, ‘Schistosoma mansoni’, ‘Schistosoma haematobium’, and ‘Zambia’, in combination with Booleans terms ‘AND’ and ‘OR’, published reports/papers were obtained and reviewed independently for inclusion.

Results

Thirteen papers published in English that fulfilled the inclusion criteria were selected for the final review. The papers suggest that the risk of infection has increased over the years and this has been attributed to environmental, socio-economic and demographic factors. Furthermore, schistosomiasis is endemic in many parts of the country with infection due to Schistosoma haematobium being more prevalent than that due to S. mansoni. This review also found that S. haematobium was linked to genital lesions, thus increasing risks of contracting other diseases such as HIV and cervical cancer.

Conclusions

For both S. haematobium and S. mansoni, environmental, socio-economic, and demographic factors were influential in the transmission and prevalence of the disease and highlight the need for detailed knowledge on ecological modelling and mapping the distribution of the disease and intermediate host snails for effective implementation of control strategies.

Electronic supplementary material

The online version of this article (10.1186/s40249-018-0424-5) contains supplementary material, which is available to authorized users.

Transmission of schistosomiasis in Kariba, Zimbabwe, and a cross-sectional comparison of schistosomiasis prevalences and intensities in the town with those in Siavonga in Zambia

Given that the two communities lie only 10 km apart, on the northern shore of Lake Kariba, it is surprising that human schistosomiasis now appears to be a much less important health problem in Kariba town (Zimbabwe) than in Siavonga town (Zambia). In an attempt to explain this difference, the level and sites of Schistosoma haematobium and S. mansoni transmission in Kariba, and the prevalences and intensities of human infection with these parasites in both communities, have now been investigated. In a longitudinal study, a cohort of 378 schoolchildren, 150 subsistence fishermen and 42 commercial fishermen from Kariba town was screened three times for schistosome infection, at 6-month intervals. Sixteen human-water contact sites in or near the town were surveyed for intermediate host snails every month for 1 year. Finally, the results of screening 660 Kariba schoolchildren (in January 2001) and 527 Siavonga schoolchildren (in July 2002) were compared. In the longitudinal study, 9.0% of the schoolchildren, 7.3% of the subsistence fishermen and 0% of the commercial fishermen were each found positive for S. haematobium at least once. The corresponding values for S. mansoni were 2.5%, 12.5% and 26.3%, respectively. The results indicated that, each year in Kariba, 2.4% and 2.0% of schoolchildren and 18.2% and 5.2% of fishermen were infected with S. haematobium and S. mansoni, respectively. Although both Bulinus globosus and Biomphalaria pfeifferi were found at 14 of the 16 water-contact sites, snails infected with schistosomes that could infect mammals were only found at three of the sites. The problem of schistosomiasis in Kariba town appears to be greater among fishermen than schoolchildren, all transmission probably occurring in Lake Kariba. As expected, the overall prevalences of S. haematobium and S. mansoni infection among Siavonga schoolchildren (19.4% and 33.5%, respectively) were far higher than the corresponding values for Kariba schoolchildren (7.1% and 2.1%, respectively). The marked differences in the prevalence of human schistosomiasis between Kariba and Siavonga appear to be attributable to the better water and sanitation facilities and a history of schistosomiasis-control activities in Kariba town.

Schistosomiasis and water resources development: systematic review, meta-analysis, and estimates of people at risk

An estimated 779 million people are at risk of schistosomiasis, of whom 106 million (13.6%) live in irrigation schemes or in close proximity to large dam reservoirs. We identified 58 studies that examined the relation between water resources development projects and schistosomiasis, primarily in African settings. We present a systematic literature review and meta-analysis with the following objectives: (1) to update at-risk populations of schistosomiasis and number of people infected in endemic countries, and (2) to quantify the risk of water resources development and management on schistosomiasis. Using 35 datasets from 24 African studies, our meta-analysis showed pooled random risk ratios of 2.4 and 2.6 for urinary and intestinal schistosomiasis, respectively, among people living adjacent to dam reservoirs. The risk ratio estimate for studies evaluating the effect of irrigation on urinary schistosomiasis was in the range 0.02-7.3 (summary estimate 1.1) and that on intestinal schistosomiasis in the range 0.49-23.0 (summary estimate 4.7). Geographic stratification showed important spatial differences, idiosyncratic to the type of water resources development. We conclude that the development and management of water resources is an important risk factor for schistosomiasis, and hence strategies to mitigate negative effects should become integral parts in the planning, implementation, and operation of future water projects.

The control of schistosomiasis and soil-transmitted helminths in East Africa

As a result of support from the Bill and Melinda Gates Foundation, schistosomiasis and intestinal or soil-transmitted helminth infections have been the subject of national control programmes in three Eastern and Southern African countries: Uganda, the United Republic of Tanzania and Zambia. Here, we review the significant progress made in their control efforts and highlight the different approaches being adopted to ensure programme effectiveness and sustainability. Although a positive start has been made to reduce morbidity resulting from schistosomiasis and soil-transmitted helminth infections in these countries, it is imperative that support is identified to sustain the programmes until these infections are no longer a public health problem and children can therefore be given a healthy start to life.

Snail-parasite compatibility and prevalence of Schistosoma haematobium on the shores of Lake Kariba, Zambia

Reduced snail-parasite compatibility might be at least partially responsible for the decrease observed, over the last three decades, in the prevalence of urinary schistosomiasis on the shores of Lake Kariba, Zambia. To explore this possibility, the prevalences of urinary schistosomiasis in schoolchildren were investigated in three disparate areas of Zambia (Lake Kariba, Lake Bangweulu and Lusaka), and attempts were made to infect the snails that developed from the eggs of snails collected from each of these sites with Schistosoma haematobium from each of the sites. The prevalence of S. haematobium infection in schoolchildren ranged from 0% around Lake Bangweulu to 76% around Lake Kariba. The F1 progeny of Siavonga (Lake Kariba) snails showed good compatibility with the local parasite but were also susceptible to all of the geographical strains of S. haematobium tested. The interaction between the S. haematobium and S. mansoni found along the shores of Lake Kariba may favour S. mansoni, since prevalences of urinary schistosomiasis tend to be relatively high where S. mansoni is absent. Although Lake Bangweulu was confirmed to be an area of very low endemicity for urinary schistosomiasis, the snails bred from snails collected in this area were compatible with isolates of S. haematobium from the same region and also those from the other regions. In general, sympatric snail-parasite combinations were the most successful. All the snails were identified as Bulinus globosus, on the basis of common enzyme profiles, although polymorphism was evident for malate dehydrogenase (MDH): two samples had type-1 MDH and the rest were of type 3. Isoenzyme patterns for both acid phosphatase (AcP) and MDH could be used to distinguish between infected and uninfected snails.