What are schistosomes?
Schistosomes are parasitic Platyhelminthes (or flatworms) that infect over 200 million people. Although these parasites infect people in parts of South America and Asia, the overwhelming majority of infections occur in sub-Saharan Africa. The disease these parasites cause is called Schistosomiasis (a.k.a Bilharzia or Snail Fever). Schistosomiasis is just one of a group of diseases known as Neglected Tropical Diseases (NTDs) that disproportionately affect the world’s poorest and most vulnerable people.
How do people get infected with schistosomes?
People get infected with schistosomes when they come into contact with water contaminated with larval parasites, called cercariae. These cercariae are able to sense people in the water and rapidly pierce through their skin. Once inside the skin, the parasites migrate to the blood and begin developing as male or female worms.
Where do these infective cercariae come from?
It’s complicated. Schistosomes have a life-cycle that includes both mammalian and snail hosts. Inside the blood of their human host, male and female worms mate and produce eggs. These eggs are then excreted in the host’s feces or urine. Since many of those infected live in parts of the world with poor sanitation, their waste products that contain schistosome eggs have the opportunity to reach nearby fresh water lakes or streams. Once in the water these eggs hatch, liberating another larval form called miracidia. These miracidia then seek out and infect a suitable snail host. Inside the snail these miracidia that have the ability to make thousands of genetic copies of themselves that develop into cercariae capable of infecting humans.
How do schistosomes make people sick?
Oddly enough, having schistosome worms living in your blood vessels is not what makes you sick. It turns out the main driver of disease is the massive egg output of the female schistosome: she can lay an egg every one to five minutes! Although some of these eggs are excreted via the host urine or feces, many are washed away in the bloodstream and ultimately become trapped in your organs (e.g., the liver or bladder). These eggs induce immune responses that can lead to serious complications including organ failure, internal bleeding, and even cancer. Unfortunately, some of the effects of schistosome infection can persist long after the parasites have been killed (e.g., following drug treatment). Our group is actively trying to understand schistosome reproductive biology with the hope this could lead to new ways to curb the spread of the parasite and also alleviate the pathology of the disease.
If you are infected will the parasites ever go away?
The lifespan of the schistosome inside a human host is a very hard thing to address. However, there are countless cases of people harboring active schistosome infections 20-30 years after moving endemic parts of the world (e.g., Africa) to a non-endemic region (e.g., the United States of Europe). Therefore, schistosomes seem to be capable of surviving for decades inside your body. How these parasites are able to do this is a question our team is actively exploring.
What are the symptoms of schistosome infection?
The symptoms vary depending on the type of schistosome someone is infected with. In Africa, two species dominate: Schistosoma mansoni and Schistosoma haematobium. In Asia, S. japonicum is the most prevalent. Both S. mansoni and S. japonicum live in the blood vessels near the intestine and can cause symptoms including bloody stool, abdominal pain, liver fibrosis, liver cirrhosis, portal hypertension, splenomegaly, and ascites (a swollen belly due to fluid accumulation). In S. haematobium infections the symptoms are associated with the urinary tract and include bloody urine (haematuria), infertility, bladder fibrosis, kidney damage, and bladder cancer.
What happens to people that are infected with schistosomes?
Although schistosomes are thought to kill an estimated 250,000 people every year, perhaps the greatest tragedy is the chronic disability associated with infection. Those with the most serious symptoms are often children who can suffer from malnutrition, anemia, stunted cognitive and physical development, physical pain/discomfort, and chronic inflammation. Diseases like schistosomiasis may also enhance the probability of contracting diseases like HIV/AIDS though sexual contact. The cumulative effect is that those who are chronically infected are effectively robbed of their ability to lead productive lives, condemning themselves (and likely their families) to a life of poverty. Being impoverished then enhances the likelihood that these people are exposed to diseases like schistosomiasis. This perpetuates a seemingly endless cycle of disease and poverty.
Is there a cure?
Treatment of schistosomiasis relies on a single drug called Praziquantel. While Praziquantel is effective at clearing a schistosome infection it is far from a magic bullet. Praziquantel doesn’t prevent people from becoming re-infected next time they step into the water and there is concern that resistance to the drug may arise. It is clear that ridding the world of schistosomes will require the implementation of multiple strategies including (but not limited to): the development of new drugs and/or vaccines, improved sanitation, control of the snail populations, and education in the developing world about how the disease is transmitted.
Are there schistosomes in the United States?
Yes! Fortunately, in the United States these schistosomes are only capable of infecting birds. However, if you were to come into with the cercariae of these bird schistosomes you would get an rash known as swimmers itch or cercarial dermatitis. However, as the earth gets warmer due to climate change there is the possibility that diseases like schistosomiasis could widen their reach. Indeed, there have been recent outbreaks of S. haematobium infections in the French island of Corisica, where schistosomes had not previously been known to exist.
I think I’m infected; can you help or diagnose me?
We are researchers, not licensed physicians, and are not qualified to provide medical advice. I suggest you to contact your local health care provider and have them conduct the proper tests.
How does your lab study this problem?
We firmly believe that understanding the basic biology of this organism is key to developing the next generation of anti-schistosome drugs and/or vaccines. We also contend that by studying the basic biology of these fascinating organisms we can better understand important basic biological processes common to all animals, including humans. Therefore, we are trying to study schistosomes from multiple angles using a variety of modern molecular approaches. Please see our Research page for more information.
How can I help?
Spreading the word to raise awareness about Neglected Tropical Diseases like Schistosomiasis is a great place to start. If you are in the United States, you can also contact your Congressional representatives in the House and Senate and tell them you support basic research and increased funding for the National Institutes of Health and National Science Foundation. Basic scientific and biomedical research absolutely relies on funding from the federal government.
