For millions of people in central Africa, being infected with the microscopic worm Loa loa is not a big deal: just a bit of mild itching and swelling. But if they take a dose of ivermectin—a drug that’s being widely distributed around the continent in an attempt to wipe out other parasites—L. loa can cause more severe complications: hemorrhaging, neurological problems, and even death. Now, for the first time, scientists have developed a prototype for a handheld, mobile phone–based platform to screen for L. loa in a matter of minutes that could help health care workers decide who can safely receive the drug.

“This is a remarkable achievement,” says physician and public health researcher Adrian Hopkins, director of the Task Force for Global Health’s Mectizan Donation Program in Decatur, Georgia, who was not involved in the new work. To be used broadly across central Africa, however, he says, the device will have to be commercialized and the cost optimized.

L. loa infections, or loiasis, are spread by mosquitoes and flies that carry L. loa larvae. In human flesh, the larvae mature into worms about as long as a strand of hair is wide. These tiny adult parasites then burrow in people’s skin, lungs, bloodstreams, and eyes. (The disease is also known as African eye worm because of the occasional parasite that wanders across someone’s eye, visible from the outside.) Until now, diagnosing loiasis has been tricky—especially because symptoms can be so subtle.

“There is screening, but it is a long, tedious business,” Hopkins says. The number of miniscule worms in a blood sample must be counted manually by a trained technician, he says, to determine if there are enough worms to cause the serious ivermectin reaction. “There’s no way you could go into a village and do it on everyone.” For programs like his, which oversees the administration of hundreds of millions of Mectizan (ivermectin) doses per year in Africa to treat river blindness and elephantiasis—both caused by parasitic worms—that’s a problem, because of the odd and poorly understood interactions between ivermectin and loiasis.

Researchers had previously tried developing ways to test blood for L. loa antibodies or to stain L. loa parasites for easier identification under a microscope, but the techniques were never fast, cheap, or effective enough.

Bioengineer Daniel Fletcher of the University of California, Berkeley, wondered whether an automated computer program could instead detect the worms in blood samples by sensing their telltale wiggling. “It was really an effort to make it as simple and rapid as possible,” he says.

Fletcher’s group used CellScope, a mobile phone–based microscopy platform they’d already developed, to view samples of L. loa-infected blood. The CellScope device slides onto the outside of an iPhone like an oversized phone case and has a slit on one side for thin slides of blood samples. Once it’s mounted, the iPhone’s light shines onto the blood sample and a microscope aligned against the phone’s camera helps magnify it. For L. loa, Fletcher’s team then created software that, by analyzing a 5-second video recorded through the microscope, could detect that wiggling motion and calculate a concentration of parasites in the blood. Rather than identify and count each worm, the program elucidates their presence by the slight shifting of blood cells when the worms squirm between them.

When the researchers used the new technology to screen 33 patients in Cameroon for L. loa, the so-called CellScope Loa churned out results similar to those from the more tedious manual test, with an estimated false negative rate of less than one in 10 million patients—but in under 2 minutes, the team reports today in Science Translational Medicine. “The great thing about this is that you don’t just get the blood test result,” Hopkins says. “You can also georeference where the patient is and assign them a number, so you get a much more detailed record than we ever could have gotten just going into a village and writing it on a piece of paper.”

Fletcher admits that for the CellScope Loa to be applied to the many millions of people in Africa who need ivermectin treatments, his lab will first have to figure out how to scale up the technology; right now, they’re assembling each scope by hand in the lab. Getting industry help could also be a challenge, he says. “It’s hard to entice companies to make devices whose very goal is to eventually eliminate the need for the device.”