Conserving These Endangered Prawns Is A Tasty Way To Protect Human Health
Restocking rivers in tropical and subtropical Africa with a large endangered freshwater prawn not only provides locals with a protein-rich food source, but it also breaks the deadly life cycle of schistosomiasis
The African river prawn, Macrobrachium vollenhovenii, is a large commercially important freshwater prawn that lives on the west coast of Africa. This endangered species is predatory with a particular penchant for the snails that are intermediate hosts for the parasitic flatworms, Schistosoma species. These parasitic worms can live for as long as 30 years inside blood vessels of infected people. Infection with these microscopic parasites causes schistosomiasis, a chronically progressive disease that typically kills its human hosts, either by causing bladder cancer or liver failure or by increasing the risk of AIDS.
By the end of 2015, more than 250 million people were known to be infected with Schistosoma worldwide and more than 800 million are at risk (ref). Currently, the global Schistosoma infection rate rivals that of malaria, another, more familiar, parasite.
These widespread and dire human health consequences inspired the development of a variety of engineering, chemical and pharmaceutical interventions over the last 50 years to interrupt this parasite’s life cycle at each stage and to reduce the likelihood that people come into contact with them (Figure 1; ref).
In 1982, the newly developed drug, praziquantel, was found to be affordable and extremely effective at treating schistosomiasis, so health experts were hopeful that they might finally eradicate this persistent parasite. But praziquantel’s effects are not long-lasting, so when people living in poverty went into the river the next day, they once again were reinfected. Decades after the development of praziquantel, schistosomiasis is still here.
“There’s a constant cycle of treatment and re-exposure and reinfection that we’re not solving with drugs alone,” said disease ecologist and veterinarian Susanne Sokolow, a Senior Research Associate at Stanford University. “So we are trying to think more creatively, think more systemically about the ecology itself and how that can inform ways to truly make a dent in the global burden of schistosomiasis.”
Snails, dams, and prawns
Starting in the 1950s, many huge construction projects were launched around the world. These included dams and irrigation programs that led to a massive rise in water-borne infections, particularly schistosomiasis, which is endemic to tropical and subtropical regions in Africa, Asia and South America. Although precautions had been known that could have minimized infections, they were not implemented because the designers of these projects were unaware of them at the time.
Dams were often constructed in series along the length of particular rivers, severely compromising the integrity of the river’s ecology and biodiversity. These series of dams caused the populations of the Macrobrachium prawns to crash because the females are migratory — adult females periodically walk downstream to spawn in brackish water, probably multiple times during their lives, and walk back upstream to freshwater where they feed, and where the young prawns mature. The resulting collapse of African river prawn populations in dammed rivers where they were historically present was accompanied by a gigantic epidemic of schistosomiasis in the communities living upstream along these rivers.
Research has indicated that African river prawns can be used as an ecologically friendly biological control to reduce the rates of schistosomiasis amongst the people living near rivers because the prawns specifically prey upon Schistosoma’s snail intermediary. The snail provides the prawns with calcium, which is essential for the prawns to build their own exoskeleton.
To demonstrate the relationship between the prawns and the snails and schistosomiasis, ecologist Chelsea Wood, an assistant professor in the School of Aquatic and Fishery Sciences at the University of Washington, Dr. Sokolow, and their collaborators built several prawn hatcheries near villages alongside the Senegal River where the rates of schistosomiasis was highest. These captive-bred African river prawns were re-stocked upstream of the Diama Dam, which is located on the lower reaches of the Senegal River. The results were impressive. The abundance of infected snails dropped by 80% in the villages where the native prawns were re-stocked, and the severity of Schistosoma infection among villagers declined by 50% (ref).
Professor Wood, Dr. Sokolow and their colleagues also found that restocking captive-bred and -raised African river prawns to their native home in the Senegal river, combined with Praziquantel treatment, could potentially eliminate the Schistosoma parasite locally within five years (ref).
Schistosomiasis cannot be caught from eating the prawns, so it is a strategy that has economic as well as human health benefits. Further, these predatory prawns are at their most voracious when they are young (and small), so harvesting the dinner-plate sized adults would not impact the snail-reduction program.
“In this particular case, it turns out that harvest and disease control are a win-win,” Dr. Sokolow said. “You can get both at the same time.”
This conservation strategy is a win for the local environment, too, because it helps restore a food web that supports dozens of other species in addition to humans.
Susanne H. Sokolow, Chelsea L. Wood, Isabel J. Jones, Scott J. Swartz, Melina Lopez, Michael H. Hsieh, Kevin D. Lafferty, Armand M. Kuris, Chloe Rickards, and Giulio A. De Leo (2016). Global Assessment of Schistosomiasis Control Over the Past Century Shows Targeting the Snail Intermediate Host Works Best, PLoS Neglected Tropical Diseases, 10(7):e0004794 | doi:10.1371/journal.pntd.0004794
Susanne H. Sokolow, Elizabeth Huttinger, Nicolas Jouanard, Michael H. Hsieh, Kevin D. Lafferty, Armand M. Kuris, Gilles Riveau, Simon Senghor, Cheikh Thiam, Alassane N’Diaye, Djibril Sarr Faye, and Giulio A. De Leo (2015). Reduced transmission of human schistosomiasis after restoration of a native river prawn that preys on the snail intermediate host, Proceedings of the National Academy of Sciences of the United States of America, 112(31):9650–9655 | doi:10.1073/pnas.1502651112
Originally published at Forbes on 24 March 2019.