'Tiny Targets' Aid in Fighting African Sleeping Sickness

NEW ORLEANS -- Deployment of an extremely simple device in the Democratic Republic of the Congo (DRC) appeared to dramatically reduce the tsetse fly population, though whether it cut transmission rates for gambiense-human African trypanosomiasis (gHAT) remains to be seen, a researcher said here.

Called "Tiny Targets," the device consists of insecticide-treated fabric not much bigger than a handkerchief, mounted on a stick and placed along rivers and other water bodies where tsetse flies congregate, explained Inaki Tirados, PhD, of the Liverpool School of Tropical Medicine in England.

The devices were installed in three yearly phases during 2015-2017 as part of a broader effort to combat gHAT (also known as African sleeping sickness), with each phase in a different region ranging from 500 to 2,700 km².

Tsetse fly surveys in those regions found that counts declined 73% to 91% from the pre-intervention baseline, Tirados reported at the American Society of Tropical Medicine and Hygiene's (ASTMH) annual meeting.

But he cautioned that gHAT transmission rates have not yet been measured, and reductions in fly populations of that magnitude by themselves may not have much effect. He noted that, in some of the areas treated, humans themselves are the primary reservoir of Trypanosoma brucei, the parasite responsible for gHAT. In those areas, Tirados said, the surviving 10% of flies may still be enough to maintain transmission.

Still, he argued, vector control including Tiny Targets must be continued if the DRC is to meet goals for eliminating gHAT. Tirados cited previous projections that showed it would take more than 100 years for screening and treatment alone to end gHAT transmission in hyper-endemic DRC regions. In contrast, adding vector control at 90% effectiveness would bring elimination within short-term reach.

As background, Tirados explained that the DRC is essentially ground zero for gHAT. Although conventional screen-and-treat programs have brought case counts down considerably in the DRC and elsewhere in Africa, the country has never accounted for less than half of all cases on the continent. Moreover, other countries have been more successful than the DRC in combating gHAT: in 2014, the last year for which continent-wide case counts are available, more than 90% were in the DRC. (That's not necessarily a fair comparison, though, since the DRC is geographically bigger than most other countries where gHAT has been endemic.)

A goal of gHAT elimination in the DRC by 2030 was set in 2012 at a major international conference on so-called neglected tropical diseases. Vector control was included as part of the recommended strategy.

That's where Tiny Targets came in. The devices have been used successfully in other countries including Chad, Guinea, and Uganda, where fly densities were cut by more than 80%. Their simplicity and low cost (about one-fifth that of conventional traps) is a major advantage in resource-poor countries, allowing them to be deployed in large numbers (40 per linear kilometer along rivers extending hundreds of kilometers in the DRC program, Tirados said) by people with minimal training.

The intervention is planned to continue, Tirados said. "Hotspots" for transmission have been identified, totalling some 12,000 km², and efforts will be directed to "capacity strengthening" so that this area, about triple the total in the first three phases, can be effectively covered in future deployments. Tirados said dedicated "operational units" should be formed to continue the deployments.

As with any insecticide-based vector control effort, development of resistance is a concern. But in response to an ASTMH audience member's question, Tirados said there's been no sign that the Tiny Targets are losing effectiveness.

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