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Candidate Parasitic Diseases
This paper discusses five parasitic diseases: American trypanosomiasis (Chagas disease), dracunculiasis, lymphatic filariasis, onchocerciasis and schistosomiasis. The available technology and health infrastructures in developing countries permit the eradication of dracunculiasis and the elimination of lymphatic filariasis due to Wuchereria bancrofti. Blindness due to onchocerciasis and transmission of this disease will be prevented in eleven West African countries; transmission of Chagas disease will be interrupted. A well-coordinated international effort is required to ensure that scarce resources are not wasted, efforts are not duplicated, and planned national programmes are well supported.
The Division of Control of Tropical Diseases (CTD) in WHO has global responsibility for African trypanosomiasis, Chagas disease, dracunculiasis (guinea-worm disease), foodborne trematode infections, intestinal parasitic infections, leishmaniasis, lymphatic filariasis, malaria, onchocerciasis and schistosomiasis. National programmes to combat these diseases are supported by WHO, in many instances in collaboration with other international agencies, development aid agencies, nongovernmental organizations, and industry. The mission of CTD, working closely with the WHO Regional Offices, is to provide support to country activities, to promote, advocate and coordinate tropical disease control with the aim of improving the health status of individual communities and populations, and to contribute to social and economic development.
This paper discusses five of the diseases listed above: Chagas disease, dracunculiasis, lymphatic filariasis, onchocerciasis and schistosomiasis. Four of these have been targeted for global elimination by the World Health Assembly, the objectives being the interruption of transmission of Chagas disease by the year 2010; the eradication of dracunculiasis by 2008; the elimination of lymphatic filariasis by 2020; and the elimination of onchocerciasis as a public health problem in 11 West African countries by 2002.
Criteria for Establishing Elimination Programmes
Technical feasibility is the criterion for changing from control of infection to an objective of elimination or interruption of transmission. This requires that the disease has been adequately researched in terms of the causative organism, clinical impact, management, treatment and epidemiology. The change is facilitated by a breakthrough in the form of a new strategy and/or tool that can effectively and rapidly reduce the incidence of infection and disease using the infrastructures in place.
Public Health Strategy
Disease elimination or eradication programmes have to fit within the existing public health strategies. The essential public health functions in each country should include major parasitic disease problems as an integral and coherent part of the "Renewal of Health for All" process led by WHO.
Determinants of Success or Failure
The definitions of success and failure are never very clear and these terms tend to be used to promote different points of view. Perhaps the criteria for success or failure should be spelled out from the very beginning. There are, however, many factors that determine success or failure, and the major issues are discussed below.
The objective is the interruption of vectorial and transfusional transmission in the Americas, by the year 2010, of the blood-borne parasite Trypanosoma cruzi which causes Chagas disease. Natural transmission occurs through the bite of triatomine bugs and iatrogenically through blood transfusion. The strategy, therefore, is to eliminate both vectorial and transfusional transmission by the household application of insecticides and through blood bank screening.
There are 16-18 million infected persons in Central and South America and 100 million people at risk. Currently, human infection of young age groups has been reduced by 68% over the last 6 years in the Southern Cone countries (Argentina, Brazil, Bolivia, Chile, Paraguay and Uruguay). In 1997, Uruguay had eliminated the vector Triatoma infestans, demonstrating that elimination of transmission is a feasible goal.
A similar initiative for the Andean countries (Colombia, Ecuador, Peru, and Venezuela) was launched in February 1997 with preparation of detailed plans of action and budget for 1998 to 2001. It is foreseen that interruption of vectorial and transfusional transmission will be achieved in these countries by 2005. Similar efforts for the Central American countries were launched in October 1997; it is foreseen that transmission will be interrupted by 2010.
The disease is caused by a parasitic worm Dracunculus medinensis (guinea worm). The infection is acquired by humans through drinking water containing infected cyclops. This minute crustacean becomes infected by ingestion of the larvae of Dracunculus which are released into water when an infected person steps into it to relieve the pain caused by the emerging worm. The emergence of the adult worm through the skin, usually from the legs and feet, approximately one year after the individual concerned drank unsafe water, is extremely painful, causing fever, nausea and vomiting, and disabling the person for months.
There are 100 million people still at risk of infection. In 1997 alone, approximately 70,000 cases were reported, compared to the estimated 10 million individuals infected per annum before the inception of the eradication programme. This drastic reduction is the result of the efforts made jointly by the countries with WHO, UNICEF, CDC, Global 2000 and a multitude of other NGOs and industry. Although there are no specific drugs to treat or prevent infection, the recommended strategy aims at case containment of infected individuals, community-based surveillance, and provision of safe drinking-water through the distribution and use of cloth filters.
Lymphatic filariasis, often referred to as elephantiasis, causes profound lymphoedema, genital and renal involvement, and secondary bacterial infections, and can result in disfiguring enlargement of the limbs, breasts, and genitalia. It is endemic in 73 countries, where 120 million people are infected. Worldwide it is estimated that there are 25 million cases of genital disease and 15 million cases of lymphoedema/ elephantiasis. The disease is caused by a blood-borne infection with the parasitic worms Wuchereria bancrofti, Brugia malayi, and B. timori, which are transmitted by various mosquito species. Humans are the only definitive host for W. bancrofti, which accounts for 90% of infections. For B. malayi and B. timori, which account for the remaining 10%, a number of other animals may harbour the parasites. However, the epidemiological role of this in relation to transmission is thought to be small.
Epidemiologically it has been shown that, where hygiene and environmental improvements predominate, there can be a reduction in parasite levels to below those necessary to sustain local transmission. Introduction of simple treatment regimens can greatly hasten the interruption of transmission. Largely because of newly available and dramatically effective treatment and diagnostic tools, the outlook for filariasis control/elimination is now so positive that it has been identified as a potentially eradicable disease. WHO has therefore embarked upon the global elimination of lymphatic filariasis as a public health problem globally by the year 2020. To this end, SmithKline Beecham in December 1997 agreed to donate albendazole and support the programme until the disease has been eliminated.
Onchocerciasis is caused by infection with the filarial worm Onchocerca volvulus, which is transmitted by blackflies of the genus Simulium, causing itching and a disfiguring skin disease, serious eye lesions, and blindness among persons in parts of tropical Africa, the Arabian peninsula, and Central and South America. Although the control of onchocerciasis by the Onchocerciasis Control Programme in West Africa has been highly successful, the disease remains endemic in 34 countries, affecting over 17 million people, 99% of whom are in Africa. At least 6.5 million people suffer severe itching or dermatitis and at least 270,000 are blind because of the worm infection.
The strategy that has been shown to be most effective is the annual single-dose treatment of affected populations with the drug ivermectin, and larviciding against the blackfly vector. In 1997, 18 million treatments were given, approximating to 25% coverage. It is possible to sustain this programme due to a drug donation programme by Merck & Co.
The policy aims at prevention of blindness and elimination of onchocerciasis as a public health and socioeconomic problem throughout Africa and the Americas, and interruption of onchocerciasis transmission in selected foci. In the eleven Onchocerciasis Control Programme countries in West Africa, elimination is expected by 2002, and the participating countries are expected to maintain this. In the African Programme for Onchocerciasis Control covering the remaining endemic countries in Africa, the objective is to have established -- by 2005 -- effective, self-sustaining, community-based ivermectin treatment programmes which will lead to the elimination of this disease from the rest of Africa. In the Onchocerciasis Elimination Programme of the Americas, it is expected that -- by 2000 -- morbidity will have been reduced and blindness and other sequelae prevented, leading to the elimination of the pathological manifestations of the disease and interruption of transmission in selected foci.
Schistosomiasis is a parasitic waterborne trematode infection causing chronic ill health and affecting the urinary or intestinal system. Intestinal schistosomiasis is caused by the flatworms or blood flukes, Schistosoma mansoni, S. japonicum, S. mekongi and S. intercalatum, while urinary schistosomiasis is caused by S. haematobium. People are infected by contact with water used in normal daily activities for personal or domestic hygiene and when swimming, or through occupational activities such as fishing, rice cultivation, and irrigation. The intermediate hosts are different species of snail which, when infected, release cercariae into the water which can penetrate the intact skin. In the human, it is not the worm but the eggs which cause damage to the intestine, bladder, and other organs.
The global distribution of schistosomiasis has changed significantly over the past 50 years, as a result of successful control in Asia, the Americas, North Africa, and Middle East. This success has been consistently linked to both political commitment and the implementation of a concerted control strategy. However, schistosomiasis remains endemic in 74 developing countries (600 million people at risk) and infects more that 200 million people (120 million with symptoms and 20 million suffering the severe consequences of the disease). The greatest concern is in sub-Saharan Africa, where over 80% of the cases occur.
The main intervention strategy is an integrated approach using chemotherapy, health education, the installation of wells and safe water sources and latrines, and the control of snails. Today, the global objective remains control, especially in Africa, where transmission continues to be intense. Moreover, recent environmental changes, closely linked to water resources development in previously low or nonendemic areas and increases in population densities, have led to the spread of this disease.
It has long been realized that dracunculiasis can only be contracted by drinking water that contains infected Cyclops. Thus, the source of drinking-water is the crucial link in the cycle. Ever since the United Nations launched the International Drinking Water Supply and Sanitation Decade (1981-1990), the possibility of dracunculiasis eradication became a reality.
In the Southern Cone countries of South America, the vector of Chagas disease is found inside houses in close proximity to humans and control of transmission has proved to be amenable by use of insecticides, house design, and routine blood screening. In the Andean and Central American countries, the habits of the vector species are more extradomiciliary so that vector control will be more difficult and progress slower.
The decision to include lymphatic filariasis as a disease for global elimination was taken, based on advances during the last decade or two in diagnosis, clinical understanding, treatment and control of this disease, as well as the increasing political commitment by Member States. Today, interruption of transmission can be achieved by treating infected persons and by mass treatment of the population at risk. The mainstay of the elimination strategy is the use of simple, safe, inexpensive and conveniently delivered drugs that kill microfilariae and that have some effect on the adult worms.
The very successful Onchocerciasis Control Programme in West Africa was well funded and well managed. The advent of the drug ivermectin, the initiation of the Mectizan Donation Programme, the participation of the ministries of health, nongovernmental organizations, WHO and other collaborating agencies in drug distribution programmes, and rapid epidemiological assessment techniques and mapping methods have given rise to well-founded optimism. Thus, interruption of transmission in selected foci is feasible in these areas in a relatively short space of time with a combination of drug therapy and vector control. In the remaining countries of Africa and in the Americas, the programmes are at the early stage of development and implementation.
Schistosomiasis remains difficult to control because environmental changes which are taking place favour the intermediate host. Even though there has been a major decrease in prevalence and distribution of S. japonicum and S. haematobium, the bulk of transmission remains in sub-Saharan Africa. In Africa the disease is strongly linked with poverty, movement of populations, contamination of water, and agricultural practices. In this continent control remains a difficult task. The poorest countries where schistosomiasis is prevalent do not have the economic potential (national or family level) to organize and coordinate effective and sustainable disease control. Thus, schistosomiasis is not at present among those diseases listed for elimination in the next two decades.
The available technology and health infrastructures in developing countries permit the eradication of dracunculiasis and the elimination of lymphatic filariasis due to W. bancrofti, which will benefit present and future generations. Progress in controlling infections due to B. malayi and B. timori will depend upon future studies on the impact of the epidemiological overlap between the animal and human infection. Persons now suffering from elephantiasis will require special case management. Blindness due to onchocerciasis will be prevented and disease transmission will be interrupted in the eleven West African countries which were in the original Onchocerciasis Control Programme. Transmission of Chagas disease will also be interrupted, leaving a residue of chronic sufferers to be managed by the health services.
The lessons learned from present attempts to eradicate/eliminate/interrupt transmission of the above-mentioned four diseases and the health service systems that are strengthened in the process should contribute to the elimination/eradication of other tropical diseases in the not too distant future. The efficiency and effectiveness of programmes must be strengthened so that the gains achieved can be sustained in the long term. This will require building the capacity of health systems, education, training of health professionals, community mobilization, and information, education, and communication (IEC) activities.
Success calls for a well-coordinated international effort to ensure that scarce resources are not wasted, efforts are not duplicated, and planned national programmes are well supported. Clear priorities and a more equitable distribution of resources should be made by national governments and by international and development aid agencies and nongovernmental organizations.
* Director, Division of Control of Tropical Diseases, World Health Organization, 1211 Geneva 27, Switzerland.
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