The primary causes of Nodding Syndrome (NS) have stumped scientists for dozens of years, but a new theory based on modeling and disease progression have helped explain a link between Blackfly exposure and this disease.

Image/CIA
Image/CIA

NS is a disease endemic to South Sudan, Northern Uganda, and Tanzania, affecting children aged 5 to 15.  Primarily effecting brain function, NS progresses from an involuntarily nodding of the head to epileptic seizures, cognitive deterioration, and sometimes death.  While the relationship between blackfly exposure and NS has been known, the cause has eluded scientists.

Blackflies in the East African region often carry Onchocerca volvulus, a parasitic worm that causes river blindness.  Only blackflies carrying O. volvulus can cause NS, but O. volvulus, the only known human parasite carried by blackflies, cannot penetrate the blood-brain barrier to cause frank cognitive disease.

Through epidemiological disease modeling based on previous carrier/parasite relationships, researchers from the Institute of Tropical Medicine in Antwerp, Belgium have now shown that a second parasite within the blackfly is causing the disease.  Three factors were crucial in their model: first, insecticide sprays in areas with high blackfly populations led to a significant decrease or eradication of NS; second, disease pathogenesis wanes over time until a new human population arrives in a blackfly area; and third, previous laboratory studies have shown that blood feeding flies can harbor symbiotic relationships with viruses, harboring the organism until the fly feeds on a susceptible host.

The third point is perhaps the most crucial to future research.  “[A]rboviral transmission is enhanced in mosquitoes and other blood feeding flies that concurrently ingest microfilariae, and the same could be true for blackflies,” explains Robert Colebunders, MD, PhD, who is head of the HIV/STD Unit, Department of Clinical Sciences at the Institute of Tropical Medicine, and Professor of Infectious Diseases at the University of Antwerp. “[For example] the biting midge, Culicoides nubeculosus, became infectious after ingesting blue tongue virus and Onchocerca cervicalis microfilariae, but not after ingesting the virus alone.”

While the need for more research is apparent, this approach to disease treatment is the first new step in a long line of research dead ends.  By developing a clinical trial to study the occurrences of epilepsy with and without nodding in the endemic area, and combining the results with molecular biology on the symbiotic relationships possible with the O. volvulus parasite, the research team has one of the best chances to pinpoint the secondary pathogen causing NS.