A Better Test for Schisto
Diagnosing intestinal schistosomiasis is tough business.
Identifying an infection of the parasitic worm requires collection, transport and examination of stool that must be immersed in formaldehyde to prevent decomposition in the intense heat common in counties where the parasite is most prevalent. Even then, it is not unknown for a sample bottle full of gases from decomposing feces to explode in a lab researcher’s hand.
The two most common tests for the disease, known as Kato-Katz (KK) and circulating cathodic antigen (CCA), are plagued by inaccuracies and false results. Needless to say, public health officials have yearned for an alternative.
In a July article in the American Journal of Tropical Medicine and Hygiene, a team of Bloomberg School researchers has demonstrated a polymerase chain reaction (PCR) genetic test to detect worm DNA in patient urine. The PCR test has proven not only vastly more accurate than earlier tests, but it makes handling of specimens much easier and safer, as well.
“This is the first time PCR has been used to diagnose parasitic diseases in urine that were previously only detectable in feces,” says postdoc Nilanjan Lodh, PhD, first author of the paper. “PCR allows us to test without collecting stool.”
In comparing all three diagnostic techniques, the researchers discovered that KK and CCA often led to less-than-optimal results, especially in low-level infections. More than half of the individuals who tested negative by KK and CCA were, in fact, found to be positive by PCR, an indication of PCR’s high sensitivity to infections.
A high-tech test proves to be more accurate and reliable while eliminating the risk of exploding samples.
“For health officials, PCR’s sensitivity and specificity are pretty impressive, but as one who has done arduous fecal tests in the field, I can tell you, from personal experience, that the sample stability issue is a big deal,” says Clive Shiff, PhD, an associate professor of Molecular Microbiology and Immunology, who directed the study. “Urine does not decompose like feces and is much easier to collect and transport,” he says.
Current collection methods require long drives to test locations, disseminating sample bottles, returning the next day to retrieve them, immersing each sample in formaldehyde, and delivering them to the lab.
“Collection takes time, and gasoline in Zambia runs about $2 a liter. With urine sampling, you can collect and return in a single trip. If you add in the cost of the KK and CCA tests, PCR starts to look very cost competitive even at a relatively expensive 50 cents per test,” Shiff says.
Next, the researchers are seeking funding to test a less-expensive PCR method known as loop–mediated isothermal amplification (LAMP) that can be powered by a 12-volt car battery. LAMP could be used in rural areas where electric power is absent or intermittent.