PLOS Science Wednesday: Hi Reddit, we’re Susanne and Michael and we recently published a literature review in PLOS NTDs that showed countries that targeted the snail host are more successful at reducing or eliminating schistosomiasis – Ask Us Anything!


Hi Reddit,

My name is Susanne Sokolow and I am a disease ecologist and a Research Associate at Stanford University. My research focuses on feedbacks between health and the natural environment. I have studied a variety of diseases, most recently, human schistosomiasis, a neglected tropical parasite and a disease of poverty.

My name is Michael Hsieh and I am a physician scientist and an Associate Professor at George Washington University. My research focuses on how microbes and the host immune system interact in the urinary tract, including during urinary tract infections and urogenital schistosomiasis, which is infection by the parasitic Schistosoma haematobium worm.

We recently published a paper titled Global Assessment of Schistosomiasis Control Over the Past Century Shows Targeting the Snail Intermediate Host Works Best. This publication reviewed the scientific literature on various strategies to control schistosomiasis (human infection with parasitic Schistosoma worms), and indicated that countries which targeted the snail host for these parasites tended to be most successful at reducing or eliminating this worm.

We will be answering your questions at 1pm EST. Ask Us Anything!

And don’t forget to follow Michael on Twitter @perforin.

Is there any way to treat the snails for schistosomiasis, rather than killing them off? Have you assessed the environmental impact of using molluscicides on a large scale?


Susanne: That is a good question. A few countries in the Caribbean successfully eliminated schistosomiasis using intentional introductions (and sometimes accidental invasions) of competing snail species that simply filled the habitat and pushed the population numbers of schistosome-susceptible snails down. There have been few studies over the decades trying to find creative solutions for reducing schistosome infections in snails, such as introduction of competing parasites that don't affect people that also use the same snail hosts (and once a snail is infected with those competing parasites, then the snail becomes resistant to schistosomiasis infection). This has never, to my knowledge, been implemented in a large-scale control program. One problem is that it is difficult to get funding support for some of these more creative, "experimental" solutions. After our realizations, looking at the data in total for the past century (presented in our paper) is that we hope there will be more investment in research and development of creative ecologically-based solutions to help curb parasitic diseases like schistosomiasis. Our finding that - overall in the past century - snail control worked well in the past whether by molluscicides or other creative methods, we now hope for more attention paid to this. If we can achieve all the technological advances we have achieved in the past century, then surely we may be able to discover and implement creative control methods that are safe and effective at reducing this devastating disease from children in poverty-stricken areas.

Is there any way to treat the snails for schistosomiasis, rather than killing them off? Have you assessed the environmental impact of using molluscicides on a large scale?


Michael: To my knowledge there are no established methods to eradicate schistosome infection from snails without killing the snails. Large scale use of molluscicides such as niclosamide results in snail (and fish) die-off, and stains the water orange. Snail populations can rebound rapidly after niclosamide treatment of bodies of water, and although the orange appearance of niclosamide-treated water is alarming, the water is not toxic to humans.

Thank you for doing this AMA. I understand that there is a theory that some species of Schistosoma, namely S. japonicum, are capable of evading the human immune system by effectively "coating" themselves with non-immune immunoglobulins. My questions are:

  • Do other Schistosoma species utilize similar mechanisms to mask themselves from immune detection?

  • Have there been any advancements to combat this evasion, in any shape or form?


Michael: All three major human-specific species of schistosomes, i.e., Schistosoma haematobium, Schistosoma mansoni, and Schistosoma japonicum, have multiple strategies to evade the human immune system. For instance, all of these species have a "double tegument" that they can shed when the tegument gets coated with complement, antibodies, leukocytes, and other elements of the host immune system. These parasites also produce immunomodulatory molecules which manipulate specific arms of the immune system. It is thought that the genes for these molecules co-evolved with humans (and other schistosome hosts) as part of the ongoing "arms race" between parasites and their hosts.

Having never heard of this illness before, I had to look it up. Apparently it is beaten in proliferation only by malaria. How is it that the general public knows so little of Schistosomiasis, but just about everyone has heard of malaria?


Michael: Schistosomiasis is a classic example of a neglected tropical disease, meaning a disease that is endemic in tropical regions and yet receives very little public attention, research efforts, and public health initiatives despite its importance in said areas. Why neglected tropical diseases have this status is complex, but reasons can include the insidiousness of morbidity (many patients with schistosomiasis don't even know they are infected, even though the infection can greatly hamper school performance and worker productivity).

Could there be some adverse ecological consequences from targeting the host snail?


Susanne: This is a good question, and as an ecologist, one that I have been thinking about for some time. After seeing the data we compiled for this research, that clearly points to snail control as a critical component of successful schistosomiasis reduction programs, our research team has been studying creative solutions, other than chemical toxins (molluscicides) to target the snail hosts. One strategy we've been looking at is restoring natural predators of snails, especially in ecosystems where man-made changes have excluded predators. Check out some of our ongoing work here: This idea is in the research and development phase and one part of the question we're asking is: what is the impact of restoring predators in ecosystems where they have recently been excluded by human activities? Stay tuned over the next couple years at the Upstream Alliance website for more updates as we learn more.

Hi! I thought the info on wealth and access to to clean water was really interesting!

Do you have any data on cost effectiveness of chemotherapy vs. host control, or thoughts on how to scale up programs globally?


Susanne: Thanks for this question. We have given some thought to this, and I think the reality is that any solution that works biologically could still fail if it is not economically feasible. The "optimal" strategy to control schistosomiasis on a large-scale, from both an ecological and economic standpoint, could be different in different situations and there's probably not a "one size fits all solution." We're currently working with mathematicians and economists on investigating the trade-offs in terms of economics and efficacy of several possible "integrated" strategies (i.e. using both snail control and drug treatments for people) in Northern Senegal, one of the hottest "hotspots" of schistosomiasis transmission in the world.

Are there any ethical concerns with wiping out snails? Thank you!


Susanne: Thanks for your question. No past control effort for schistosomiasis, even those that were successful in eliminating the disease using snail control primarily, has ever eradicated the snail hosts. In fact, theory predicts that snails do not need to be eradicated to reduce schistosomiasis disease transmission.

Yet, in places where parasite-hosting snails are increasing, especially where they are increasing as a result of man-made impacts like in irrigation systems and upstream of dams, we need to find creative solutions to control the snail populations and reduce human disease.

As another person interested in NTD research, I'm wary of the funding for the research of diseases like these that don't necessarily get the spotlight that Zika, Ebola, etc. get in the media. How do you guys deal with the relative unpopularity of schistosomiasis and the funding availability for your research?


Michael: the challenges of getting NTD research funded is an ongoing and significant one. I am fortunate that I have an endowed position, e.g. "hard money". My other approach to funding has been to "diversify" my research portfolio to include areas besides schistosomiasis. Finally, I think that studying NTDs such as schistosomiasis has scientific value beyond the intrinsic benefits of understanding the disease itself. I highlighted this point with coauthor Dr. Margaret Mentink-Kane in this article:

As another person interested in NTD research, I'm wary of the funding for the research of diseases like these that don't necessarily get the spotlight that Zika, Ebola, etc. get in the media. How do you guys deal with the relative unpopularity of schistosomiasis and the funding availability for your research?


Susanne: Thanks for your comment and question. I agree with you and with Michael that it is a challenge to work on a disease that does not carry the support of the "big three" infectious diseases with the lion's share of research and funding (malaria, HIV, tuberculosis). But forums like these are helping to raise awareness about the more neglected diseases that still cause a lot of disability and mortality around the world. Keep asking questions!

Hi! Thanks for doing the AMA. I'm currently doing a Biosecurity course so your answers will be fun to read! My question: In dealing with biosecurity, the control of animal hosts (endemic or not) tend to be controversial with the general public (i.e. rabbit control using myxomatosis or RHDV-K5, GMO-mosquitoes). When you speak with the general public, how do explain why snail control is vital in helping eradicate schistosomiasis?


Susanne: Thanks for your comment. Unfortunately there is not consensus among the scientific community, nor the public (evidenced by our conversations here on reddit), about the "optimal" solution for schistosomiasis control and whether this should include a strong focus on reducing the snail intermediate host, or rather should focus on more traditional medical approaches like drug treatment and vaccine discovery. There have been some recent changes in global health policy such as the passing of the World Health Assembly resolution 65.21 that calls for more research and support of snail control and other ecological methods to reduce schistosomiasis.

When thinking about the potential costs and benefits of various approaches to control disease, it's important to recognize that nothing humans do has zero impact. The goal is to find the most beneficial, safe, effective, and sustainable strategy to relieve suffering, improve health, and hopefully, to ensure that our children's and grandchildren's health may also be preserved. For schistosomiasis, this may mean that we have to focus some attention on reducing the snail intermediate host.

No past control effort for schistosomiasis, even those that were successful in eliminating the disease using snail control primarily, has ever eradicated the snail hosts. In fact, theory predicts that snails do not need to be eradicated to reduce schistosomiasis disease transmission.

Lastly, schistosomiasis today is associated with man-made changes to the environment like dams, irrigation schemes, and other water developments for agriculture. Expansion of these man-made impacts have contributed to a rising number of schistosome-bearing snails and a rising number of people infected with schistosomiasis. If we can manage these ecosystems better to help reduce infected-snail infestations and curb schistosmiasis infection in people, I believe we should pursue that option.

I lived in Hawaii where I worked on farms extensively and spent a lot of time in freshwater streams and waterholes. I became very sick, and since have suffered yearly from a terrible rash, urticaria, and generally feel deeply unwell in a cyclical manner. Because it is less common in the United States, I've been treated like I am absolutely crazy by doctors here (I've been to four or five) how can I get the proper testing or be taken seriously by the medical community? Can schistosomiasis affect pregnancy? I am not imagining this, as I've been told by my gps.


Michael: Schistosomiasis is not endemic to Hawaii, but it is certainly possible you have another condition that requires diagnosis and treatment.

Is it possible for the worm to transmit without the use of snails as an intermediate host?


Michael: Schistosome worms are highly co-evolved with their snail hosts and cannot complete their life cycle without them.

Hello, and thank you for helping fight schisto - it's a cause that I feel very strongly about. My question: has there been any progress made on Bayer releasing their stranglehold on the market in South Africa that would allow the sale of cheap, generic praziquantel? The hyper-inflated price that Bayer charges makes wide-scale distribution of this (almost) 50 year old drug cost prohibitive - yet it would help to drastically reduce the number of HIV infections in women and lower numbers across the board.


Michael: There are a number of companies worldwide producing cheap, generic praziquantel. The last time I checked on prices, I know that doses were far less than US$0.37 each, and I am sure they are lower now.

I've been on Reddit for years and have been reading these weekly "PLOS" science posts since they started. But I've still never seen an explanation what "PLOS" is.

What is PLOS?!



Michael: "Public Library of Science"

You've shown how expensive snail control in combination with generic drugs can lower rates of this terrible, highly prevalent disease; however, how will this awareness/research help lower costs of snail control methods? Would any of the authors like to share their experiences (good and bad) with health policy/global health?


Susanne: Thanks for your question. We're currently studying creative strategies for schistosomiasis disease control, other than using chemical molluscicides. We hope others will follow suit with more creative solutions. Public health and global health problems may have many unexplored frontiers for environmental strategies to improve health. We're working here at Stanford University to expand research in this area. Check out more here:

Cancer. I've a friend that traveled to an endemic area, and is now diagnosed with stage four melanoma. Any correlation? Thanks!


Michael: The best epidemiologic data supporting a cancer-schistosomiasis link exists for bladder cancer and Schistosoma haematobium, the cause of the urogenital form of schistosomiasis.

Great reads! My experience with the subject is fairly limited because I was not a student of the class myself, but there is a UMich biological research station in Northern Michigan that has a parasitology couse with a small focus on this cycle. The class' teacher is part of a national educational thing that will go (along with volunteers from the class) to town hall meetings for a town affected by this, and try to educate the locals on the process and control of it. Have you done anything similar? If so, ehat are your experiences?


Michael: I have not personally engaged in schistosomiasis education efforts in endemic areas. Sanna has done field work in Senegal and elsewhere, so perhaps she may have more to say about this (she will be online around 1 PM EST).

I have read a paper where it discussed cryptic species of some trematodes exhibit longer life spans, which resulted in a wide distribution and invasion of distant ecosystems. Are there any possibilities that cryptic speciation plays a role in disease mechanism and transmission of Schistosomes? Thank you.


Susanne: Some schistosome species can readily hybridize. For example, in Senegal cattle and human schistosomes have hybridized in some areas.

What are the symptoms of this disease and how would one catch it? Direct contact with snails?
In which countries would we be in the most Danger of contacting it and how could we best prevent that from happening?


Michael: Infected snails release cercariae, the infective larval stage for humans, into water. Humans get infected by contact with water containing cercariae. Schistosomiasis is primarily based in sub-Saharan Africa, the Middle East, Brazil, China, Southeast Asia, and most recently Corsica. Symptoms can be varied and depend on host genetics, stage and intensity of infection, and what species you have been infected with. In the acute stage, fevers, muscle aches, and rashes can be seen. In chronic stages, it is often asymptomatic, but can also be severely symptomatic, with chronic fatigue, anemia, abdominal pain, liver failure and bladder cancer as examples.

Hi Michael! It sounds like the snails and the water are very critical to schistosomes. Is it possible to maintain the snails in a lab so that you can better study parasites? Is it difficult to maintain the snails in a lab?


Michael: Yes! It is absolutely possible to maintain the snails in the lab, and in fact my lab maintains the life cycle and provides it to the research community as a resource:

The snail hosts for Schistosoma mansoni (Biomphalaria genus) is easiest to maintain in the lab, whereas the hosts for Schistosoma haematobium and Schistosoma japonicum (Bulinus and Oncomelania) are trickier to maintain.

On balance, what do you think is the best means of diagnosis for schisto to focus on going forward (to deploy in large national surveillance programs)? Considering the lower sensitivity and cost of Kato-Katz vs. the higher sensitivity and cost of ELISA techniques, etc.


Michael: This is a tough question. Short answer: I don't know. Long, more nuanced answer: we know that a single Kato-Katz, and even three Kato-Katz's, miss infections (as does urine syringe filtration for S. haematobium infections). Urine CCA is a promising diagnostic, but only works well for S. mansoni, and only in regions with medium or high intensity infections. CAA is very promising, but not ready for field testing. PCR and ELISA are unwieldy in the field. There is a huge need for better schisto diagnostics....

Have you ever eaten snails?


Michael: Not any of the schistosome-bearing species! :)

Thanks for the AMA! Do you think targeting mosquitos to reduce malaria/Zika would be as effective?


Michael: There are very interesting, ongoing efforts to target malaria vectors (i.e., mosquitoes) using genetically modified mosquitoes that are either sterile or render other mosquitoes sterile.

Thanks for doing anothing AMA. Why are we trying so hard to eliminate snails, potentially damaging an economic system when the infection is very rare and very easily treatable? To me it seems like a big risk for little reward.


Michael: I absolutely agree that potential ecological and economic impacts of snail control need to be carefully monitored. However, many countries have conducted snail control efforts successfully and without apparent long-term ecologic detriment. Schistosomiasis affects at least 200 million people worldwide (and possibly at least twice that, depending on which diagnostic test you use), and although praziquantel works for this infection, heavily infected individuals do not necessarily get cured from a single or even multiple doses, and reinfection occurs despite prior praziquantel treatment.

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