Sensitive test brings good news, bad news for schistosomiasis researchers

A new assay for detecting schistosomiasis is forcing international experts to reassess progress toward eliminating this disease, which afflicts people in some of the world’s most impoverished places.

Widespread testing and mass treatment programs have been aimed at two World Health Organization goals: controlling schistosomiasis-related illness by 2020 and stopping transmission of the disease by 2025.

The two are not the same. The disease is caused by parasitic flatworms called schistosomes or bilharzia. These worms live in freshwater snails, and people get infected by having contact with contaminated water. Most of the time, the worms alone don’t have any ill effects on the host, but people get sick when the worms start producing eggs.

For years, researchers have relied on the Kato-Katz technique for detecting one form of schistosomiasis infection, said parasitologist Dan Colley, a professor of microbiology at the University of Georgia. Laboratory workers examine stool samples and if they find worm eggs, the test is positive and the patient is treated with drugs that can kill the worms. This test is also used to monitor progress toward public health goals.

But results of the stool test can mislead by producing falsely negative results. “What if the worm isn’t producing eggs at that particular time?” Colley asked. “What if eggs were in a different part of the stool sample? It is a dirty task, literally.”

Much more sensitive results come from a new assay, which detects CCA, or circulating cathodic antigen. The CCA technique is less messy and more sensitive than the stool test, because it is done on urine. The worms release CCA throughout their life cycle. Field workers collect a small urine sample, add a drop to a pre-treated cassette, and in less than 30 minutes can tell whether the person needs treatment by looking at colored bands that develop on the test strip.

The difference between the two lab tests – and what that might mean for the global battle against schistosomiasis – dominated discussion during the schistosomiasis breakout session at the 2016 COR-NTD conference.

For one thing, CCA testing costs less than the Kato-Katz method. “We don’t need to collect stool or retain it for analysis,” said Fiona Fleming, a schisto expert at Imperial College of London. “We can do the test and read it onsite without a lab.”

She used it recently to map the disease in Uganda and Burundi. The Schistosomiasis Consortium for Operational Research and Evaluation (SCORE), which Colley leads, is also collaborating with Imperial College, other groups and Ministries of Health using the CCA assay in studies in some of the eight countries where it is striving to control and ultimately eliminate the disease.

In Burundi, Colley’s team and their partners compared the Kato-Katz method and the CCA technique as tools for detecting infection in children. The stool test detected eggs, indicating active worms in only 1.5 percent of more than 11,000 children. But in a sample of more than 20,000 children, the urine test yielded positive results for 42 percent of the kids.

This means that preventive chemotherapy programs needed to be ramped up to treat children who aren’t overtly symptomatic, but who are carrying worms.

“We’ve certainly got a ways to go,” said Evan Secor, an investigator affiliated with SCORE and the U.S. Centers for Disease Control and Prevention on the road to global elimination.

Still, there is encouraging news from specific places, including Zanzibar, Colley said. SCORE is collaborating with ZEST, the Zanzibar Elimination of Schistosomiasis Transmission project, to reduce the risk of transmission to less than one percent by treating twice a year and including other means such as snail control and behavioral change. This will hopefully make the disease no longer a public health issue for the people of this archipelago.

Now that the quick and easy CCA test enables researchers who detect infection in people who don’t overtly appear sick, Colley says it’s time to rethink the goal for schistosomiasis elimination by 2025. “This may be realistic for some countries, but not for others,” he said.

A “one size fits all” goal for the world may no longer make sense.