Point-of-care quantification of blood-borne filarial parasites with a mobile phone microscope

Loa loa microfilariae load in blood can be automatically quantified at the point of care using a mobile phone video microscope. Dial “L” for Loa: Answering the call for mass drug administration Filarial nematodes—tiny, parasitic worms that get into the bloodstream and use humans as hosts—are common in certain regions in Africa. One of these filarial nematodes, Loa loa, the causative agent of loiasis, is not compatible with current ivermectin-based mass drug administration (MDA) programs in the region, which are aimed to eliminate other worms that cause onchocerciasis and lymphatic filariasis. MDA causes severe and often fatal neurological side effects for patients co-infected with L. loa; thus, many MDA programs have been suspended. To resume these ivermectin-based campaigns, D’Ambrosio et al. devised a mobile phone–based strategy for quantifying Loa microfilariae in whole blood and, in turn, excluding those individuals from MDA. The authors’ Loa-counting device comprised a mobile phone camera (as the video microscope) and a custom algorithm for tracking the “wriggling” motion of the microfilaria by quantifying the displacement of red blood cells surrounding the Loa. The entire device was packaged for point-of-care use, including its own “app” for smartphones. When tested on samples from 33 potentially Loa-infected subjects in Cameroon, Africa, the device was 94% specific (compared with microscopy results from thick blood smears) and 100% sensitive for patients about the threshold for severe adverse events (30,000 microfilaria per milliliter of blood). With its ease of use and only a fingerprick of blood, this mobile analytical device could be integrated into MDA programs, answering the call for safe and effective programs in Loa-endemic regions. Parasitic helminths cause debilitating diseases that affect millions of people in primarily low-resource settings. Efforts to eliminate onchocerciasis and lymphatic filariasis in Central Africa through mass drug administration have been suspended because of ivermectin-associated serious adverse events, including death, in patients infected with the filarial parasite Loa loa. To safely administer ivermectin for onchocerciasis or lymphatic filariasis in regions co-endemic with L. loa, a strategy termed “test and (not) treat” has been proposed whereby those with high levels of L. loa microfilariae (>30,000/ml) that put them at risk for life-threatening serious adverse events are identified and excluded from mass drug administration. To enable this, we developed a mobile phone–based video microscope that automatically quantifies L. loa microfilariae in whole blood loaded directly into a small glass capillary from a fingerprick without the need for conventional sample preparation or staining. This point-of-care device automatically captures and analyzes videos of microfilarial motion in whole blood using motorized sample scanning and onboard motion detection, minimizing input from health care workers and providing a quantification of microfilariae per milliliter of whole blood in under 2 min. To validate performance and usability of the mobile phone microscope, we tested 33 potentially Loa-infected patients in Cameroon and confirmed that automated counts correlated with manual thick smear counts (94% specificity; 100% sensitivity). Use of this technology to exclude patients from ivermectin-based treatment at the point of care in Loa-endemic regions would allow resumption/expansion of mass drug administration programs for onchocerciasis and lymphatic filariasis in Central Africa.