The latest Parasitology Paper of the Month is “Detecting hybridization in African schistosome species: does egg morphology complement molecular species identification?” by Nele A. M. Boon, Wouter Fannes, Sara Rombouts, Katja Polman, Filip A. M. Volckaert and Tine Huyse.

In 2014 I swapped rainy Belgium for field work in Senegal as part of my PhD on the molecular epidemiology of schistosomiasis. I stayed for four months in Richard-Toll, a friendly Sahel town, on the banks of the Senegal River, close to a big freshwater lake named Lake de Guiers. These water bodies provide an oasis of refreshment in this otherwise very dry region. Water is crucial for irrigation, bathing and washing. Unfortunately, it is also a source of schistosomiasis, a major but neglected tropical disease that is hard to control. On contact with water, people are infected by the larval stage of the Schistosoma worm that is released by freshwater snails.

We studied schistosomiasis in Pakh, a rural village bordering Lake de Guiers, together with the local field team. Interestingly, human schistosomes with hybrid ancestry have been found in this region. They are the result of crosses between Schistosoma haematobium, infecting humans, and S. bovis, infecting cattle. Hybrid parasites are a cause of concern because it is not known which impact they have on the epidemiology and morbidity of schistosomiasis. We therefore collected parasite eggs from urine and stool samples to investigate the genetic constitution of the hybrid parasites in greater detail. Some eggs were immediately stored on ethanol, others were stimulated to hatch by exposing them to light in bottled water. The resulting miracidia were subsequently fixed on filter paper for DNA preservation. While trying to catch eggs and miracidia under the binocular we never noticed eggs with typical ‘hybrid’ morphologies. However from experiments we knew that first-generation hybrid schistosomes have an egg morphology distinct from the parental species.

Back in Belgium, this inspired me to further study the ‘classical’ question: what is the egg morphology of hybrid schistosomes? Wouter, a researcher at the Africa museum in Tervuren, made pictures and performed measurements on the eggs. Together with Sara, an enthusiastic bachelor student, I performed a molecular bar coding study to assign each of the measured egg to species status. The results confirmed that (in Senegal) eggs of hybrid ancestry have the same morphology as S. haematobium eggs.

This study points to the importance of careful field work. The observations from laboratory experiments do not necessarily correspond with the observation or the situation in the field. We hope that our results warn for ‘false expectations’. Even when schistosome eggs look perfectly normal, i.e. like a ‘pure species’ they may have hybrid ancestry. Unraveling the nature of this hybridization phenomenon seems more complex than previously thought. Improving the detection and understanding of hybridization between schistosome species will be crucial to improve the control of this disease.
The success of our project is directly linked to collaboration with the Pakh villagers. We became so involved locally that we asked the villagers if we could do something for them in return. The ready-to-use electric wiring in the local hospital had no permanent power source. We arranged for a solar panel that we paid for with crowd funding organized in Belgium involving family, friends and colleagues. The field-team of Richard-Toll helped with the local organization and took care of installation.


PAR potm Apr17_villager

Picture of a meeting with the villagers of Pakh. Informing and discussing is a major part of field work.

PAR potm Apr17_irrigation

Irrigation canal in Pakh that is used by human and cattle for several activities (bathing, drinking, washing). It is a transmission site for both human (S. haematobium) and bovine (S. bovis) schistosomiasis.

PAR potm Apr17_solarpanel

Photo solar panel before its installation on the hospital.


Read the full article “Detecting hybridization in African schistosome species: does egg morphology complement molecular species identification?” in full for free until 30th April 2017.

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