Vector Control Strategies

In addition to advances in chemotherapy and vaccination strategies to control the disease agents, a great deal of attention has been applied to control strategies that can reduce populations of freshwater vectors of human diseases. The most commonly used approach is the application of toxicants to control the vectors. These can be done over large spatial scales (e.g., aerial applications of insecticides in 11 West African counties to control black fly vectors of onchocercia-sis) or locally (e.g., the distribution of pesticide impregnated bed nets to control malaria vectors at the village level).

A second approach is the introduction of environmental modifications. These include draining standing or stagnant water, creating ditches for recirculation of water, and diverting flow so streams go dry for part of the year.

A third approach, the introduction of biological control agents such as predatory fish (e.g., Gambusia) and invertebrates, along with a variety of bacteria, fungi, and other organisms, has been widely used with mixed success and, like the above control approaches, sometimes with major undesired ecological consequences. In general, predators do not restrict themselves to mosquitoes, black flies, and other vectors as prey. However, the more specific the control agent (such as the bacterial toxins from Bacillus thuringiensis israelensis), the less likely the chance of such unwanted consequences.

Finally, the addition of competitive organisms that are not vectors (e.g., closely related snails to compete with schistosomiasis vectors) has worked in isolated areas. Swamping populations with sterilized males (as was effective in the screw worm control program) and the development of genetic modifications may have future value but have not thus far been found to be effective.

Black Flies Blood

Plate 2 Top left. Simulium damnosum: An adult black fly female taking a blood meal and possibly vectoring river blindness or onchocerciasis (WHO/TDR/Stammers). Top right. Simulium damnosum: Larva of the black fly; all of the immature stages of black flies occupy aquatic habitats (WHO/TDR/Stammers). Bottom left. Onchocerca volvulus: The adult worms that causes onchocerciasis (WHO/ TDR/OCP). Bottom right. A young girl leads her father, blind from onchocerciasis, through their village, a common scene in West Africa before this disease was controlled (WHO/TDR/Crump).

Plate 2 Top left. Simulium damnosum: An adult black fly female taking a blood meal and possibly vectoring river blindness or onchocerciasis (WHO/TDR/Stammers). Top right. Simulium damnosum: Larva of the black fly; all of the immature stages of black flies occupy aquatic habitats (WHO/TDR/Stammers). Bottom left. Onchocerca volvulus: The adult worms that causes onchocerciasis (WHO/ TDR/OCP). Bottom right. A young girl leads her father, blind from onchocerciasis, through their village, a common scene in West Africa before this disease was controlled (WHO/TDR/Crump).

The large-scale control of aquatic vectors of human disease has had mixed success. Malaria, for which eradication through DDT and other pesticide applications was discussed as a possibility in the 1950s, soon re-emerged as a major source of human mortality. Malaria and other mosquito-vectored diseases continue to be an area of research in which both 'high' and 'low tech' solutions are being tested. Many of the former involve molecular biology and genetic engineering approaches. The latter involve widespread distribution of pesticide-impregnated bed nets to protect people during sleep.

Control of schistosomiasis has achieved somewhat better success than malaria and has involved breaking the life cycle of the parasite or interrupting the chain of infection. Clearly, if all snails are eliminated or all humans treated and remain free of infection, the chain of infection would be interrupted. Snail control is achieved through chemical control and, although biological control (e.g., with predatory snails or birds) has not been generally effective, habitat control has been very effective in China. There, removing snails from canals with chopsticks, sealing latrines, and preventing human wastes from reaching water have been very successful measures. Habitat modification for snail control, such as modifying habitats by reducing vegetation, and altering stream bed and channel conditions to increase water velocity are also effective but these approaches require continuous long-term effort.

Guinea worm control has revolved around providing clean water through filtration, treating contaminated water, and preventing persons who have active infections from contact with water supplies. This approach has been very successful in Africa. However, when political instability and migration of humans from infected to disease-free areas occurs, the threat of re-invasion of this and other diseases with freshwater vectors increases.

The true success story about the control of aquatic vectors of human disease, and really of any human disease, is the control of river blindness in

Control Blindness

Plate 3 Top left. Bulinus globosus: The snail that is the intermediate host for Schistosoma haematobium (WHO/TDR/Stammers). Top right. Schistosoma haematobium: The adult worm that causes schistosomiasis (WHO/TDR/Stammers). Bottom left. Schistosoma haematobium: The cercaria that penetrates the skin of humans and leads to schistosomiasis (WHO/TDR/Stammers). Bottom right. A sample of normal urine and blood-containing urine from a child suffering from schistosomiasis (WHO/TDR/Lengeler).

Plate 3 Top left. Bulinus globosus: The snail that is the intermediate host for Schistosoma haematobium (WHO/TDR/Stammers). Top right. Schistosoma haematobium: The adult worm that causes schistosomiasis (WHO/TDR/Stammers). Bottom left. Schistosoma haematobium: The cercaria that penetrates the skin of humans and leads to schistosomiasis (WHO/TDR/Stammers). Bottom right. A sample of normal urine and blood-containing urine from a child suffering from schistosomiasis (WHO/TDR/Lengeler).

West Africa. Although river blindness is known from 35 different countries, 99% of the cases occur in 26 countries of Africa. Prior to intensive control efforts, as many as 30% of adults living in streamside villages were blind, and blindness was viewed as an inevitable part of a person's life cycle if they lived near the fertile flood plains of West African rivers.

The Onchocerciasis Control Programme in West Africa (OCP) was begun in 1974 and continued through 2002. Covering an operational area of over 1.2 km2, it included 30 million people who were vulnerable to the disease. OCP initially was based entirely on aerial application of insecticides to rivers to control black fly larvae. The extent of the program was enormous; over 50000 km of streams received applications of insecticides at the peak of activity, and some rivers for as long as 20 years.

Beginning in the 1980s, ivermectin, a drug used in veterinary practice such as for treatment of dog heart-worm, was distributed to humans to control the larval worms in their bodies and to complement the aerial application of insecticides in breaking the cycle of transmission of this disease.

OCP has been heralded as both a public health (the elimination of river blindness as a public health concern) and economic development (the opening of the river valleys free of onchocerciasis to settlement has enabled food to be grown for an estimated 17 million Africans) success. It has been expanded to 19 other African countries, primarily based on ivermectin distribution. Likewise, the success of the control program in West Africa prompted efforts to eliminate onchocerciasis from affected areas of Latin America.

A remarkable feature of OCP was the establishment of the first large-scale, long-term monitoring of the fish and benthic macroinvertebrate populations in the areas treated with insecticides. This, coupled with earlier experimental and taxonomic studies, has been the basis for much of what is known today about tropical African streams and rivers.

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