Types of Control
No Good Drugs Available!
It became apparent early on that drug therapy was of limited use in the fight against onchocerciasis. Until the 1970’s, there were two known drugs – diethylcarbamazine (DEC) citrate and suramin, both microfilarical agents, both non-ideal for mass administration because of severe side effects. In the absence of better alternatives, onchocerciasis treatment and control has relied heavily, especially in the Western hemisphere, on denodulization and occasional mass chemotherapy. [2,a,h]
Denodulization campaigns prevent high rates of blindness and eye lesions by removing adult worms but do not reduce parasite levels in the patient’s body. Even used in conjunction with chemotherapy, effectiveness is limited because:
- microfilaria survive in the human body for over a year after removal of adult worms,
- detection and removal of nodules is often difficult and incomplete,
remaining microfilaria can still be transmitted to other hosts,
- neither treatment is prophylactic, and
- reducing the parasite density provides a more favorable environment for the survival of the remaining parasites. [e]
Nodules removed from a patient. Courtesy of WHO/TDR.
Controlling the Blackfly: Is it Possible?
Blackflies are millions in number, widely distributed, live in rough terrain with lots of brush, and exist as multiple species within the genus Simulium with varying habitats, flight patterns, and life cycles. This makes vector eradication nearly impossible, but two French scientists working in Burkina Faso (then Upper Volta) believed that if the blackfly population could just be controlled in endemic and surrounding areas for 20 years -- the supposed life span of the adult worm in the human body -- transmission of the disease could be stopped. (Later studies showed that the adult worm only lives 14 years in the human body.) In 1972, the scientists convinced Robert McNamara, then president of the World Bank, who went on to pull together a massive international public health effort to fight onchocerciasis. [e,h]
Choosing a Method of Vector Control
Given the heterogeneity of vector and terrain, it seemed best to launch an attack on many fronts, though some methods came to be more fruitful than others.
- Environmental modification and organic pollution of blackfly breeding places and habitats are too costly, too difficult, and too dangerous for other species.
- Around 1974, the University of Newfoundland and OCCGE (a West African multinational medical research group) studied the possibility of using predators, parasites, and pathogens of the blackfly. (The authors of this webpage welcome any updates on this study.)
- The primary tool after the 1950s, larvicides easily attack blackflies in their vulnerable larval stage. Larvicides are general sprayed aerially. [e]
Aerial spraying of larvicide over vector
breeding areas. Courtesy of WHO/TDR.
Concerns About Larvicide
- Larvicides are carefully monitored for possible environmental/biological effects and developing biodegradable pesticides. The Ecological Group, an independent watchdog committee composed of international experts, screens the pesticides.
- Larvicide treatments must be at least weekly to catch every new generation of organisms until they are gone. (Slightly less cost is involved at higher elevations, where Simulium species tend to have longer life cycles.)
- Blackflies from surrounding areas may re-invade treated areas, so larvicide campaigns must be extended to areas surrounding endemic zones as well. [e,h]
- Blackflies can develop resistance to larvicides, so more are continually being developed and extant ones must be rotated regularly. Resistance to temephos, the first larvicide, came in 1979-80 in southern Côte d’Ivoire and spread to Mali and most of the OCP area by 1987. As of 1993, six larvicides – temephos, chlorphoxim, bacillus thuringiensis (B.t H-14, a biological agent), permethrin, carosulfan, and pyraclosfos – are used in rotation to “fool the blackfly.” The least toxic are used in dry season, and the most toxic (eg. permethrin) in the rainy season when there are more vectors and greater chances of washing away of the pesticides. “Resistance is no longer a problem because we never use the same pesticide for more than six weeks,” said Dr. Ebrahim M. Samba, Director of the OCP. [h]