A Creative Approach to Controlling a Deadly Snail
As far as the world’s deadliest creatures go, large predators like sharks and lions tend to get all the credit. But in fact, if we were to point to the animal kingdom’s most frequent killer, it’d actually be the mosquito.
Another creature belonging to the “small but deadly” category is the freshwater snail, which is responsible for more than 200,000 deaths a year — more deaths than sharks, lions and wolves combined.
Freshwater snails carry a parasitic disease called schistosomiasis, which infects nearly 250 million people, mostly in Asia, Africa and South America.
“It’s one of the world’s most deadly parasites,” says Susanne Sokolow, a disease ecologist at Stanford University’s Hopkins Marine Station.
“You do contract it from just wading, swimming, entering the water in any way, and the parasites basically exit the snails into the water and seek you. And they penetrate right through your skin, migrate through your body, end up in your blood vessels where they can live for many years even decades. It’s not the worms that actually cause disease to people, it’s the eggs. And those eggs have sharp barbs because they eventually need to make it back out of the human body and back into the water and find that there are snails that they need to complete their reproduction cycle. And so those eggs can lodge in different tissues and cause severe symptoms ranging from anemia and fatigue, all the way to various severe symptoms, even death in about 10 percent of chronic cases.”
In the 1970s, the drug praziquantel became an affordable option for combating schistosomiasis around the world, and countries abandoned alternative methods of “snail control” in favor of modern medical treatments.
But today, researchers are starting to rethink a drug-only approach to combating schistosomiasis. In a new study, researchers at Stanford University discovered that countries that used a creative ecological approach to snail control — such as introducing a predator to the environment—greatly reduced infection rates in those communities.
“One sort of creative avenue we’re looking at now is reintroduction of snail predators,” Sokolow says. “It hasn’t been used much in the past but in particular in an area in Senegal in West Africa we’re working with a local nonprofit … trying to investigate how the environment has changed by human activities and has driven away a predator — these native prawns — that you know are real voracious predators of the snails, and how we might bring them back through creative engineering — building ladders over dams so that prawns can access these sites that they’re now eradicated from or even using aquaculture.”
Sokolow says other countries have also had success in reducing snail population using an integrated approach.
“Japan eliminated the disease in the late 1970s pretty much exclusively focusing on creative engineering solutions to reduce snail habitat,” Sokolow says. “The snails have not disappeared from Japan even today, but the parasite did because we addressed medical care for the people in combination with creative strategies to reduce the snails. It’s that integrated strategy … that has really worked.”
Some people worry about the environmental impact of reducing a snail population, but Sokolow says an integrated approach to the problem should lead to beneficial results.
“As a team of ecologists and epidemiologist and medical doctors, you know, we’re looking at every angle,” Sokolow says.
“But we’re talking about real human lives here. We’re talking about a lot of suffering. In fact, the vast majority of people suffering from schistosomiasis is children — young children and young adults, and, you know, we have to balance all of those risks with the benefits. And here we’re really talking about restoring a system, studying that system, finding those ecological levers, we like to call them, that have been pushed by human activity and pushing them back so that they’re back in balance. And really, you know, all human activity has some risk, but I think if we do things well and do things smartly we can reap the benefits.”
Susanne Sokolow is a Basic Life Science Research Associate in Biology at Stanford University, Hopkins Marine Station in Pacific Grove, California.
JOHN DANKOSKY: This is Science Friday. I’m John Dankosky. Thanks to popular depictions in Hollywood and television, sharks may be one of the world’s scariest creatures to many of us. But they’re certainly not the most deadly. That title belongs to mosquitoes, which can transmit diseases that cause over 750,000 deaths a year.
But right below mosquitoes on that list– the humble freshwater snail. They can carry a disease that we rarely hear about in the US. It’s called schistosomiasis. It affects nearly 250 million people, mostly in Africa, Asia, and South America. So far, drug treatments have only been able to curb the epidemic by around 40%.
Now a couple of Stanford researchers have discovered that fighting the disease ecologically– that is, by introducing a predator to the environment– can be more effective than modern medicine. Here to talk about the findings is Susanna Sokolow, who is a research associate in biology at Stanford University. Welcome to the program. Thanks for being here.
SUSANNE SOKOLOW: Thanks so much for having me.
JOHN DANKOSKY: So most people will be surprised to hear, first of all, that freshwater snails are one of the world’s deadliest creatures because of this parasite. Talk about schistosomiasis and why we don’t hear about it more in the US.
SUSANNE SOKOLOW: Sure thing, yeah. So schistosomiasis, as you said, is a waterborne parasitic infection carried by snails. It’s particular species of snails that are distributed pretty widely in the world throughout the tropics but mostly in the developing world.
So as you mentioned, these snails are not found in the US– on the continental US, anyway. So of course, we don’t have the disease here. And people may have not heard of it. That doesn’t change the fact that it’s one of the world’s most deadly parasites.
So you do contract it from just wading, swimming, entering the water in any way. And the parasites basically exit the snails into the water and seek you. And they penetrate right through your skin, migrate through your body, end up in your blood vessels, where they can live for many years, even decades.
It’s not the worms that actually cause disease to people. It’s the eggs. So the worms do mature into– they’re about a centimeter long. And they live there and lay hundreds to thousands of eggs a day. And those eggs have sharp barbs, because they eventually need to make it back out of the human body and back into the water and find their snails that they need to complete their reproduction cycle.
And so those eggs can lodge in different tissues and cause severe symptoms all the way ranging from anemia and fatigue all the way to various severe symptoms, even death in about 10% of chronic cases.
JOHN DANKOSKY: Oh, it sounds just horrifying. So how have we treated this terrible disease in the past?
SUSANNE SOKOLOW: Yeah, so about 40 years ago, we discovered– and it hit the market– a brand-new drug that carried a lot of promise. It is a fantastic drug called praziquantel. Kind of a mouthful, but it does a great job of clearing the parasites out of infected people.
The problem is it’s really short acting and short lived. And people can head right back out in the water the next day, especially in developing countries, where they’re really dependent on these freshwater lakes and rivers for their everyday life for washing dishes and bathing their animals and all their daily activities. So they’re back out in the water the very next day and reinfected.
So we suspect that this might be part of the picture why the drugs themselves, while they’re very, very important as access to medical care for individuals who are infected, they really haven’t made a big dent in the global burden yet.
JOHN DANKOSKY: And that’s one of the things that happens, right? If there’s a drug that comes along, maybe we don’t treat the other parts of the problem quite as well. So what are we looking at here now? Just getting rid of the snails altogether– that will help to curb this problem.
SUSANNE SOKOLOW: Sure, yeah. So it’s actually not required to get rid of the snails altogether. You just have to address both sides of this transmission picture. So we have the people who have the worms in their system, are suffering from that. We absolutely need to treat them with these great drugs.
But we also have the environmental component of this cycle, and we can’t lose sight of that. And I think, with the promise of this new drug, maybe as a global health community we did lose sight of that a little bit. And we were very, very creative.
We’ve really been fighting this disease in many countries around the world for at least the last century. In fact, records date back to ancient Egypt. So we’ve been fighting this disease for a very long time. And before the drug was available, we were using really creative strategies.
So for example, Japan eliminated the disease in the late 1970s, pretty much exclusively focusing on creative engineering solutions to reduce snail habitat. The snails have not disappeared from Japan, even to today. But the parasite did, because we addressed medical care for the people in combination with creative strategies to reduce the snails. And it’s that integrated strategy, I think, that is what we’ve seen time and again in our historical look at the past 100 years of control for this parasite that has really worked.
JOHN DANKOSKY: So tell us some specifics of the findings of your story.
SUSANNE SOKOLOW: So we studied 83 countries and semi-autonomous territories around the world for the last century that have been combating this disease, which is a very long history. And we were able to gather data for most of those.
And we essentially found that, of the two main strategies– the one focused on the people and the drug treatments– are usually today administered through what’s called mass drug administration. So just distributing drugs to at-risk populations not through hospitals, necessarily, but through programs. And the other being the environmental component, or the snail control component, and sometimes a combination.
And we found that, among the programs that used the snail control, there was a 90% on-average reduction in the long term of the prevalence of this disease. And in the programs that focused so far solely or primarily on distributing drugs through these mass drug administrations, there was little less than on average a 40% reduction. So a big difference.
JOHN DANKOSKY: So what exactly does snail control look like? What do you do?
SUSANNE SOKOLOW: So that’s a great question and one that we’re really interested in. I mean, so in the past, there’s been a number of strategies. I mentioned Japan with their engineering. So they did things like reduce snail habitat through lining their irrigation canals, which are great snail habitat with cement, and building bridges so people didn’t have to contact the water.
Other countries have used creative, more biological solutions, like introducing snails that compete with the snails that carry schistosomiasis but that are not susceptible. And those drive the schistosomiasis snails’ populations down.
And there’s potentially many other strategies. One that we don’t really favor, even though is used in the past, was chemical molluscicides, or snail poisons. They have a lot of environmental impacts other than killing snails. And they were effective, of course, in the past.
But I think at this point, what we’re advocating is that, you know, we’ve spent a lot of time and energy and money researching in the biomedical realm new drugs and vaccines and those kind of things. They’re fantastic. They’re going to save lots of lives.
But how many lives can we save by innovating and being creative in ways to control and reduce snail populations or other environmental interventions for other diseases that are passed in some way through environmental hosts or reservoirs? One avenue, sort of creative avenue, we’re looking at now is reintroduction of snail predators. Hasn’t been used much in the past. But in particular, in an area in Senegal in West Africa, we’re working with a local nonprofit called EPLS.
And we’ve been working there for about five years trying to investigate how the environment has been changed by human activities and has driven away a predator, these native prawns, that are real voracious predators of the snails and how we might bring them back through creative engineering– building ladders over dams so that prawns can access these sites that they’re now eradicated from or even using aqua culture, you know, growing the prawns on purpose to reduce the snail populations and help synergize with the medical interventions to help curb this disease there. We don’t have results yet. It’ll be a couple years. We’ll check back.
JOHN DANKOSKY: I was going to say, any worries about introducing predators and giving them the upper hand in something like this?
SUSANNE SOKOLOW: Sure, that’s a great question. And of course, as an ecologist and as a team of ecologists and epidemiologists the medical doctors, we’re looking at every angle. But we’re talking about real human lives here. We’re talking about a lot of suffering.
In fact, the vast majority of people suffering from schistosomiasis is children, young children, and young adults. And we have to balance all of those risks with the benefits. And here, we’re really talking about restoring a system, studying that system, finding those ecological levers– we like to call them– that have been pushed by human activity and pushing them back so that they’re back in balance. And really, all human activity has some risk. But I think if we do things well and do things smartly, we can reap the benefits.
JOHN DANKOSKY: Susanna Sokolow is a research assistant in biology at Stanford University. She joined us today from KAZU in Monterey. Thank you so much.
SUSANNE SOKOLOW: Thank you.