Praziquantel has been the main treatment for schistosomiasis since the early 1980s. Schistosomiasis is a parasitic blood-fluke infection that affects well over 200 million people, most of them in sub-Saharan Africa. For nearly four decades, doctors knew what the drug did to the worms: it paralyzed them within seconds and blistered their outer surface. What they did not know was which molecule in the worm the drug actually acted on. This article explains the research, culminating in studies published in 2021, that identified that target as a calcium-permeable ion channel in the parasite. It also explains how that discovery accounts for the drug's two signature effects, and what it does and does not yet tell us about treatment failure and future medicines.

A drug that worked long before anyone knew why

Praziquantel was developed in the 1970s through a collaboration between two German pharmaceutical companies. It is a synthetic compound, a pyrazinoisoquinoline, and was not derived from a plant or soil organism. It proved remarkably effective against schistosomes and many tapeworms. It is also cheap to make and generally well tolerated, which is why it became the backbone of mass treatment programmes across the tropics.

Laboratory work in the late 1970s and 1980s, largely with Schistosoma mansoni kept in culture, established two consistent observations:

Both effects pointed to a sudden flood of calcium into the worm's cells. The difficulty was finding the door the calcium came through. Several candidates were proposed. One was an unusual subunit of the worm's voltage-gated calcium channels, which Robert Greenberg's group at the University of Pennsylvania studied in the early 2000s. Others included effects on adenosine uptake and on structural proteins. None was ever convincingly shown to bind the drug and reproduce its effects. For a medicine given to tens of millions of people each year, this was an unusual gap.

The 2021 identification of TRPMPZQ

The answer came mainly from the laboratory of Jonathan Marchant at the Medical College of Wisconsin. In a 2019 paper in the Journal of Biological Chemistry, Sang-Kyu Park, Marchant and colleagues reported that praziquantel activates a member of the transient receptor potential (TRP) family of ion channels in S. mansoni. Specifically, it belongs to the melastatin, or TRPM, subfamily. They named it TRPMPZQ.

Ion channels are protein pores in cell membranes that open to let charged particles through. TRP channels are found throughout the animal kingdom, including in humans, where they help us sense temperature, pain and taste. TRPMPZQ is a non-selective cation channel, meaning that when it opens it lets in calcium and other positively charged ions.

In a 2021 paper in Science Translational Medicine, the same group showed where and how the drug binds. Using the cloned channel expressed in cultured cells, computer modelling and targeted mutations, they located a binding pocket within a region of the channel called the voltage-sensor-like domain. Changing single amino acids lining that pocket abolished the drug's effect. Several lines of evidence made the case much stronger than earlier hypotheses:

When a biochemical approach and an unbiased genetic approach, from different institutions, arrive at the same gene, the conclusion becomes much harder to dismiss. Most parasitologists now regard TRPMPZQ as the principal target of praziquantel. That is a strong claim, and it rests on convergent evidence rather than a single experiment.

How opening one channel explains paralysis and tegument damage

Once the target is known, the older observations fall into place. When (R)-praziquantel binds, the channel opens and calcium rushes into the worm's cells. In muscle, calcium is the signal that triggers contraction. An uncontrolled influx causes sustained, spastic contraction, which is the paralysis seen within seconds. A paralyzed worm cannot hold its position in the veins around the intestine or bladder. Blood flow carries it toward the liver, where it is exposed to the host's defences.

In the tegument, a sudden calcium overload disrupts membrane integrity and the cell's internal machinery, which produces the vacuoles and blebbing seen under the microscope. This damage matters because the intact tegument is one of the parasite's main ways of hiding from the immune system. Once it is disrupted, surface molecules that are normally concealed are exposed to antibodies and immune cells.

This helps explain an important finding from animal work. In mouse studies, notably by Paul Brindley and Alan Sher at the U.S. National Institutes of Health in the late 1980s, praziquantel worked less well in mice lacking antibody responses. The drug appears to disable and unmask the worm, while the host's immune system does much of the final work of clearing it. The body's own defences, remarkably ordered in how they recognise and remove invaders, are partners in the cure rather than bystanders.

Some honest caveats apply. Much of the channel work was done in cultured cells expressing the worm protein, or in worms outside a host. How strongly each downstream event contributes to cure in a human patient has not been measured directly. Researchers have also noted that praziquantel acts weakly on at least one human serotonin receptor. Whether that has any clinical relevance is unsettled.

What the discovery explains about treatment limits

Praziquantel is highly effective at reducing egg output, which drives disease. However, a single dose does not cure every infection. It is also known to work poorly against immature worms in the first weeks after infection. That is one reason repeat treatment is sometimes recommended. Identifying the target opens a route to understanding these limits, though answers are still preliminary.

What this means for patients and families

For someone prescribed praziquantel, the practical picture has not changed. The drug is approved in the United States for schistosomiasis and for certain liver-fluke infections (clonorchiasis and opisthorchiasis). It is widely used for tapeworm infections and neurocysticercosis, but those uses are off-label in the U.S. Common side effects include abdominal discomfort, headache, dizziness and nausea. Some of these may partly reflect the body's reaction to dying worms. In people who may carry the pork tapeworm larva in the brain, treatment requires specialist care, because inflammation around dying cysts can cause seizures.

Understanding the mechanism also explains a newer development. Because (R)-praziquantel is the active form and (S)-praziquantel is mainly responsible for the bitterness, a paediatric formulation of the active form alone, arpraziquantel, was developed for children aged three months to six years. It received a positive scientific opinion from the European Medicines Agency in 2023. Very young children had long been underserved by tablets designed for adults, and this formulation was developed to meet that need.

Pregnancy deserves particular care, because two lives are involved. The World Health Organization has advised since 2002 that pregnant women with schistosomiasis can be treated rather than excluded. A randomised, placebo-controlled trial in Leyte, the Philippines, published in The Lancet Infectious Diseases in 2016, treated about 370 pregnant women with S. japonicum infection at 12 to 16 weeks' gestation. It found no safety concerns for mother or baby compared with placebo, although it also did not show the hoped-for improvement in birth weight. The evidence is reassuring but not exhaustive. That is exactly the kind of question an expectant mother should weigh openly with her own physician.

Travellers, missionaries and families working in endemic regions do well to learn how schistosomiasis is acquired, through skin contact with fresh water containing infected snails. They should also know that symptoms can appear weeks later. Informed people who raise possible exposures with their doctor are better placed to receive timely testing and treatment. Understanding how a medicine works is part of giving truly informed consent.