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​On the hunt for weak spots in tropical parasites

​On the hunt for weak spots in tropical parasites

​The recent discovery of an unknown gene could hold the key to fighting a shape-shifting tropical parasite that feasts on human blood.
December 5, 2018
The fluorescent dots in this snail reveal and provide insight into the growth of a dangerous parasite

The fluorescent dots in this snail reveal and provide insight into the growth of a dangerous parasite. | Image courtesy of the Wang Lab

The parasite lurks in tropical freshwater waiting to find a human host and feast on the victim’s blood. There, it lays 200 to 300 eggs a day, some of which clog the blood system, causing organ damage, blindness, even death. Other eggs exit the host through feces and then leach back into freshwater to perpetuate the species. “All it takes to get infected is stepping barefoot or wading bare-legged into an infested puddle, pond or stream,” says Stanford bioengineer Bo Wang, who for six years has been studying this insidious parasite known as the blood fluke, or Schistosoma, which infests 250 million people mainly in tropical Africa, the Caribbean, Brazil, Venezuela, Indonesia and the Philippines. The World Health Organization has estimated the global annual death rate at 200,000.

Doctors can kill the adult blood fluke with praziquantel, the same drug used to treat its distant cousin, the tapeworm. But the eggs and other immature forms of Schistosoma are immune to praziquantel, which leaves patients vulnerable to reinfection by larval forms of the parasite. So Wang and his colleagues are seeking to understand the full life cycle of the parasite so they can eradicate this scourge once and for all.

In a recent scientific paper, they described how they used many of the advanced techniques of bioengineering to analyze the parasite cell by cell, stage by stage, searching for some common weak spot to attack. They may have found it in a protein called ELEDH, which is made by a gene that had never before been observed in nature. But ELEDH may be central to the creature’s reproductive system, persisting throughout Schistosoma’s life cycle and helping create the adult form of the blood fluke.

The question the researchers are now asking is whether they can find a drug that targets ELEDH, and whether such a medicine could prevent immature forms of the parasite from developing into an adult. “We’re far from a drug or vaccine but we’ve found a promising target,” Wang said.

Wang’s research builds on a legacy of Stanford studies. On his desk sits a copy of the 1991 book on Schistosoma biology by former Stanford Professor Paul F. Basch, whose early death deprived the field of one of its leaders. More recently, Stanford researchers have studied how larval forms of the parasite swim though infected waters; considered how to use prawns to eat the snails that propagate the parasite; and suggested how to expand the distribution of praziquantel to treat more infected people.

“Parasitic diseases debilitate tropical communities,” Wang said. “We have to bring all the tools of science to bear on them.”

Life Cycle of a Parasite

This infographic depicts the life cycle of schistosomes, parasites that infect millions of people in the tropics. | Illustration by Drea Sullivan