NIAID scientists and their Central African and French colleagues have developed a tool to detect high blood levels of the parasitic worm

Microfilaria of L. loa in a thick blood smear. Credit: CDC
Microfilaria of L. loa in a thick blood smear. Credit: CDC

Loa loa, a risk factor for adverse reactions to the broad-spectrum antiparasitic drug ivermectin. This portable and sensitive device may help revive community-wide ivermectin treatment campaigns in certain countries by enabling health workers to rapidly identify L. loa-infected people and potentially give them alternative therapies. The scientists report their findings in the Sept. 18, 2014, issue ofPLoS Neglected Tropical Diseases.


Health workers use ivermectin to treat and control the spread of the parasitic diseases onchocerciasis, or river blindness, and lymphatic filariasis, also known as elephantiasis. In many areas of the world where onchocerciasis and lymphatic filariasis are common, entire communities are treated with ivermectin-based therapies, a strategy that has been successful in reducing the burden of these tropical diseases. However, mass drug-administration campaigns have been suspended in certain areas of West and Central Africa where L. loa infections also are common. People with high blood levels of the worm’s larval form, called microfilariae, are at high risk of experiencing severe neurological side effects after ivermectin treatment.

One potential solution is to identify people with high levels of L. loa microfilariae and exclude them from mass treatment campaigns, a strategy termed “test and not treat,” or TNT. The people excluded from ivermectin treatment potentially could be given alternative therapies to treat and prevent onchocerciasis and lymphatic filariasis. The TNT strategy, however, requires the availability of tools to rapidly test for L. loa in rural settings.

Results of Study

NIAID scientists Sasisekhar Bennuru, Ph.D., and Thomas Nutman, M.D., and their colleagues in France and Cameroon, Africa, adapted a laboratory device for use as a tool to measure L. loa microfilariae in the blood. The investigators found that a hand-held automated cell counter, a tool used by researchers to measure the concentration and size of cultured cells, also could be used to count microfilariae and distinguish them from white blood cells. The method requires a very small amount of blood, which is chemically treated to remove red blood cells, and provides results in minutes. Laboratory tests with human blood spiked with microfilariae showed that the device could reliably detect high levels of the parasite, though it was less effective at detecting low counts.

The scientists used the tool in Cameroon to measure microfilariae in 22 L. loa-infected humans and 4 infected baboons. They found that the device provided similar results to those found by microscopic examination of thick blood smears, the current standard for quantifying microfilariae in the blood. However, while thick blood smears took from four hours to several days to process, the cell counter provided results in less than two minutes, suggesting that it could be used in the field to quickly identify people at risk of experiencing severe ivermectin-related side effects.


The scientists developed a portable, sensitive, and rapid tool to identify people with high levels of L. loa that put them at risk for severe side effects from ivermectin treatment. The on-the-spot test may enable health workers to identify at-risk people and potentially give them alternate treatments, while resuming mass ivermectin treatment campaigns to eliminate onchocerciasis and lymphatic filariasis in West and Central Africa.

Next Steps

The expense associated with the device, which includes the cost of single-use sensors and the initial cost of the cell counter, is a potential drawback to its widespread use. The scientists plan to conduct larger studies to further assess the accuracy and reliability of the tool and determine if it is an economically viable option.


Bennuru S, Pion SDS, Kamgno J, Wanji S, Nutman TB. Repurposed automated handheld counter as a point-of-care tool to identify individuals at risk of serious post-ivermectin encephalopathyExternal Web Site Policy. PLoS Neglected Tropical Diseases DOI: 10.1371/journal.pntd.0003180 (2014).