Schistosomiasis is the second most common parasitic disease infecting humans. (The first is malaria.) According to The Global Network for Neglected Tropical Diseases, 240 million people in 78 countries are infected by schistosomes at this moment. This disease kills hundreds of thousands of people each year.
Of course, most of the people afflicted by schistosomiasis live in tropical and subtropical regions, especially in poor regions with limited water availability and inadequate water sanitation. Therefore, the average US citizen probably hasn’t even heard of schistosomiasis. So, briefly, schistosomiasis is a disease caused by a parasitic worm, called a trematode. Schistosome trematodes have complex life cycles, as depicted by the CDC diagram here. When infected humans urinate or defecate in bodies of water, the eggs of the parasite are released and hatch in the water. A free-living larval parasite, called a miracidium, swims around until it finds a snail to infect. Later, the infected snail releases more free-living larvae, called cercariae, and these swim through the environment until they find a human to infect. In water bodies with infected snails that are releasing cercariae, humans can get infected whenever they contact water for swimming, bathing, or water-collecting purposes.
The global burden of schistosomiasis is huge, so there is great imperative to figure out a way to control transmission of schistosomes to humans. The most common control strategy is an antihelmentic drug for humans (praziquantel). When entire communities of people are treated, the health of the treated individuals improves, and the chain of transmission from humans to snails is broken. Another control method involves using chemicals (molluscicides) to kill the snails in particular water bodies. However, those chemicals can have unintended adverse effects on non-snail organisms living the same water bodies. Additionally, both control methods tend to be temporary in nature – if the treatments aren’t kept up, snails can be reintroduced into the water bodies and/or humans can become infected with new adult parasites.
This brings me to the topic of this post: biological control of schistosomiasis as a supplement to existing control strategies. As far as I know, biological control of schistosomiasis isn’t being used in a major way in any control programs. But there is a lot of interest in this type of control, and I’ve seen several recent papers on this topic that I wanted to share:
- Introduce snail parasites: Duval et al. (2015) recently discovered a bacterial pathogen of snails (Paenibacillus glabratella) that causes high snail mortality. The pathogen is also transmitted from adult snails to eggs, and infected eggs are less likely to hatch successfully. So, this bacterium might be a promising biocontrol option! However, it’s unclear at this point whether the bacteria are specific to the snails that are intermediate hosts for schistosomes, or if the bacteria would infect many invertebrate species.
- Introduce snail competitors or predators: There has been interest in using competing snail species and predators of snails to control snail populations for a long time. Of course, these biocontrol agents need to have a strong enough effect on the target snail species that they greatly reduce or eliminate populations of the target species. And ideally, the biocontrol agents won’t wreak havoc on any other species, even after the target species has been eliminated. Sokolow et al. (2014) recently showed that river prawns are voracious predators of snails when the two are maintained together in the laboratory. Furthermore, Sokolow et al. (2014) point out that prawns are nutritious, delicious, and sell for high prices, and local people could harvest the larger prawns for food while leaving the small and medium prawns to do their snail terminating. WIN-WIN.
- Introduce parasite predators: The free-living larvae that trematodes use for transmission between hosts are susceptible to predation by all kinds of animals: fish, dragonfly and damselfly larvae, filter-feeding invertebrates, etc. My personal favorite is an oligochaete worm (Chaetogaster limnaei) that lives symbiotically on the snail and eats both miracidia and cercariae. People often suggest that these parasite predators could control trematode transmission, including the transmission of the trematode species that cause schistosomiasis and the related trematodes that cause Swimmer’s Itch. Cool stuff!
(Added to the list of things I cannot draw: motorcycles.)
Duval D, Galinier R, Mouahid G, Toulza E, Allienne JF, et al. (2015) A Novel Bacterial Pathogen of Biomphalaria glabrata: A Potential Weapon for Schistosomiasis Control? PLoS Negl Trop Dis 9(2): e0003489.
Sokolow, S.H., K.D. Lafferty, and A.M. Kuris. 2014. Regulation of laboratory populations of snails (Biomphalaria and Bulinus spp.) by river prawns, Macrobrachium spp. (Decapoda, Palaemonidae): Implications for control of schistosomiasis. Acta Tropica 132: 64–74.