Current diagnostic methods include the painful cutting of "skin snips" from patients for microscopic analysis, and an ELISA antibody test for microfilariae, which may yield positive results even for non-active infections. "You can still have circulating antibodies to a nemotode antigen in your blood for a long time after the infection is gone," said Janda.
Looking for a Better Way
A better diagnostic marker would be a metabolite of O. volvulus that appears only during an active, microfilariae-producing infection and that could determine both the presence and the severity of disease. In 2010, Janda's laboratory demonstrated the feasibility of this approach by sifting through the small-molecule metabolites within blood samples from river blindness patients—a technique called "metabolome mining"—and finding a set linked to active onchocerciasis infection. For the new study, the team sought a simpler set of biomarkers—or better yet a single unique biomarker in urine.
Daniel Globisch, a postdoctoral fellow in the Janda laboratory, started with samples of urine from onchocerciasis-infected and non-infected Africans. Using a powerful laboratory technique called liquid chromatography mass spectrometry, he measured the concentrations of hundreds of small-molecule metabolites in the samples. Excitingly, between the infected and non-infected urine samples, one difference stood out clearly: "An unknown small molecule was highly elevated in the samples from infected individuals," said Globisch.In a process akin to looking for the proverbial needle in the haystack, Globisch was able to purify the mysterious metabolite, and, using mass spectrometry, determine the chemical identities of its individual pieces. "The metabolite itself wasn't present in the databases, so I searched the literature for what is known about the biosynthesis and metabolic pathways in these nematodes," Globisch said. Ultimately, he was able to identify the metabolite as N-acetyltyramine-O, beta-glucuronide. Remarkably, this molecule's inception can be traced to O. volvulus as a neurotransmitter molecule that is secreted by young, reproducing worms and then modified by the human body on its way to being excreted in urine. "It's a spectacular find in terms of biomarkers as it does not occur naturally in humans," Globisch said. Levels of the metabolite in a non-infected North American control sample were near zero.