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Report of the Workgroup on Parasitic Diseases
The Workgroup reviewed and agreed to work with the definitions of control, elimination and eradication published in the Dahlem Workshop Report. However, it was noted that a number of resolutions of regional and international bodies, including WHO, PAHO, and the World Bank, included the expression "elimination [of a particular disease] as a public health problem".
The criteria for assessing the eradicability of diseases and conditions given in the Dahlem Workshop Report were accepted by the group. It was noted that the development of an effective strategy was part of demonstrating the feasibility of elimination. The economic impact or benefit of disease elimination/eradication may be with respect to intervention factors, including cost-effectiveness, equity (distribution issues), and the impact on the economy. The list of candidate parasitic diseases was reviewed and the group concluded that dracunculiasis was eradicable at present with current tools; separate working subgroups were designated to consider onchocerciasis, lymphatic filariasis, Chagas disease, and "other parasites".
Caution was expressed in relation to the capacity of many developing countries to engage in more than a very limited number of eradication/elimination campaigns at a given time. There is already a global eradication campaign for poliomyelitis and for candidate diseases such as measles, and there are a number of regional disease elimination campaigns. Candidate diseases for elimination will need to be ranked in order of priority on a global and regional basis. In addition, issues of certification of disease elimination and eradication need to be considered. For example, the ability of parasites to survive for long periods in humans makes the certification of elimination even more difficult.
It was agreed that onchocerciasis was a strong candidate for elimination as a public health problem, but not for eradication at the present time. As such, the subgroup endorsed the recommendations and definition used by the 1993 International Task Force for Disease Elimination, where the term "elimination as a public health problem" was used. This is a concept that encompasses both global control and elimination of infection in selected areas.
Essential Facilitating Factors
Considerable achievements have been made towards elimination of onchocerciasis in most of the Americas, all countries within the Onchocerciasis Control Programme in West Africa (OCP), and in several other African countries. Progressive increase in treatment with ivermectin has been achieved, with 500,000 doses of treatment having been distributed in 1988 and 18 million in 1997. This represents near complete coverage in the OCP and the Americas, and about 33% coverage in the APOC (African Programme for Onchocerciasis Control) countries. Extensive partnerships exist which are dedicated to the goal of sustained and complete global ivermectin treatment; the partners include Merck & Co., WHO, the World Bank, Inter-American Development Bank, nongovernmental organizations, research institutes, ministries of health, other donors, and the endemic communities.
An important constraining factor is that ivermectin is not effective in killing the adult worms (macrofilariae). Other factors are the difficulty in achieving and maintaining a sufficiently high coverage and treatment frequency to interrupt transmission, the long life span of the adult worms, and active human and vector migration.
Annual or semiannual mass ivermectin treatment must be sustained through community-based distribution programmes in endemic areas.
Goals and Strategies
Essential Facilitating Factors
Elimination of Triatoma infestans -- the main domestic vector of Chagas disease in the Southern Cone Region -- is an attainable goal, except in some areas of Bolivia, where sylvatic foci of this species exist. From the beginning of national programmes in Uruguay and Brazil in 1980, T. infestans has been eliminated in greater than 95% of the municipalities that were formerly infested. In the places or regions where Chagas disease (CD) programmes were well implemented (as reflected by quality and continuity), there was a dramatic decrease in human CD cases and the interruption of transmission whenever the level of house infestations decreased to 3% or less. In addition, serological surveys showed an impact on schoolchildren: for example, in Brazil (Sao Paulo State) and Uruguay, there were substantial declines in seropositivity in schoolchildren from the 1960s to 1995. Changes also occurred in other groups, including blood donors in Brazil (in 1979, 5% were seropositive versus 0.7% in 1995); pregnant women in Bambui (in 1954, greater than 45% were seropositive, compared with 18% in 1963, 1.5% in 1990, and 0% in 1997). Based on the experience in the Southern Cone countries, the subgroup concluded that domiciliary Chagas disease could probably be eliminated as a human infection in most regions.
Essential Facilitating Factors
Constraining factors include the existence of multiple vectors, some of which are not domiciliary, and of multiple animal reservoirs; the lack of political will in some countries; the absence of an effective vaccine or drug against chronic infection; and a complex strategy requiring six complementary interventions.
Other Parasitic Diseases
The Subgroup on Other Parasitic Diseases considered seven parasitic diseases using the criteria identified by the Dahlem Workshop ( Table 1). The diverse nature of the infectious agents (protozoa and helminths) and their modes of transmission (e.g. vectorborne, soil-transmitted, foodborne and zoonotic) makes comparison of these diseases difficult. Many of these infections, in their natural habitats, are not considered susceptible to elimination using current technologies. However, experience has revealed that they are capable of elimination from certain areas to which they have spread or been introduced.
Previous attempts to eradicate malaria were unsuccessful. However, the extreme burden imposed by this disease warrants that it continue to be considered for elimination. Further research is essential for developing a better understanding of the disease and its effective intervention.
Taenia solium taeniasis/cysticercosis was considered to be potentially eradicable. The two-host life-cycle of this cestode, including humans and domestic pigs makes it vulnerable to a variety of interventions. Historical experiences in western Europe indicate that this infection may even disappear without targeted interventions. Pigs, which rarely are allowed to survive past one year, are an excellent focal point for surveillance of the infection which may be done by local people without expensive equipment or training. There are rapid diagnostic tests for the infective stages in both humans and pigs, and effective and inexpensive drugs for mass treatment of intestinal tapeworm infections in humans. There is a need to demonstrate the cost-effectiveness and sustainability of intervention strategies in a variety of endemic situations.
The leishmaniases are difficult to eliminate because of the existence of reservoirs in domestic and wild animals. However, there are "anthropophilic" strains/species that are vulnerable to elimination by effective vector control and targeted treatment. Current epidemics of these strains are occurring in Sudan, Bangladesh and parts of India. There is a need for demonstration projects to determine the possible effectiveness of such measures.
Schistosomiasis is difficult to control under most situations. However, its public health burden makes it necessary to consider new approaches to elimination. The availability of an inexpensive and highly effective drug, praziquantel, provides a tool for greatly reducing morbidity and rates of transmission in endemic areas.
The geohelminths (ascaris, hookworms and whipworms) currently infect about one-quarter to one-third of the world's population, causing impairment of growth and cognitive development of infected children. Although refractory to elimination in most areas, mass treatment of school-age children is increasingly seen as a cost-effective intervention strategy for reducing the associated morbidity and developmental problems in affected populations. Such interventions are well accepted and form the basis for other community health interventions.
The zoonotic helminth Echinococcus granulosus, which causes human hydatid disease, is widely prevalent in populations involved in raising sheep and some other livestock animals. The disease has been effectively eliminated from island and regional situations by reduction in the number and/or by treatment of dogs, the definitive host of the tapeworm. The existence of sylvatic cycles of Echinococcus spp. precludes eradication of the agent. Similarly, with fascioliasis, the existence of animal reservoirs precludes eradication; however, improved drug therapy provides effective treatment of the disease in humans and animals.
Thanks are due to David Addiss, Joel Breman, Dan Colley, Emanuel Miri, P.R. Narayanan, Peter Ndumbe, Eric Ottesen, Frank Richards, Peter Schantz, and Craig Withers for their special contributions to this report.
* Chief Medical Officer, Ministry of Health, Kingston, Jamaica.
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TABLE 1. Evaluation of additional candidate diseases based on Dahlem criteria*
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