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    Principles of control on Trematodes

    Effective control of most trematode infections is based on strategically applied chemotherapy. Improvements in current farm management can reduce the chances of infections by limiting the contact between intermediate and final hosts. Furthermore, direct action may be taken to reduce or eliminate intermediate host populations. The use of one or more of these measures in an integrated strategy should be based on sound economic assessments of the diseases and the relative merits of control options. Some animal husbandry systems such as zero-grazing (cut and carry) and tethering of animals may minimize the risk of trematode diseases.

    1. ?Fasciola hepatica and Fasciola gigantica

    Efficient control of fascioliasis requires a well planned and executed, integrated control programme designed for each farm, area, country or region. The available strategies which can be used individually or in combination are:

    ??Strategic application of anthelminthics, eliminating the parasites from the host at the most appropriate time for effective prevention of pasture contamination.

    ??Reduction in the number of intermediate host snails by chemical or biological control.

    ??Reduction in the number of snails by drainage, fencing and other management practices.

    ??Reduction in the risk of infection by planned grazing management.

    1.1 ?Strategic chemotherapy of ruminants

    Seasonal strategic application of effective anthelmintics specific for trematodes, as well timed prophylactic and curative treatments, play an important role in the control of liver fluke infections. Strategic treatments have been developed for several regions of the world based on meteorological data. However, it is advisable to supplement meteorological data with sound epidemiological information in order to improve the timing, and thereby the efficiency, of treatments. The basic principles of strategic anthelminthic application (treatment/prophylaxis) are:

    (a) Prophylactic treatment of ruminants towards the end of a period of ecologically reduced activity of the parasites and the intermediate hosts.

    One treatment is therefore recommended towards the end of a period when larval development in the fluke eggs or in the snails has been retarded, and when the reproductive rate of snails is low or their activity is impaired (such as during a prolonged dry season, or extreme cold). At that time, a prophylactic effect can be achieved by reducing the pasture contamination of eggs before favourable climatic conditions for larval development and snail activity resume.

    (b) Curative treatment about one to two months after the expected peak infection of the hosts.

    A curative effect can be achieved by one treatment to remove the residual fluke burden acquired from metacercariae which had survived on the herbage.

    (c) Additional treatment in highly contaminated areas where seasonal variations do not significantly affect the life cycle of the flukes.

    These additional treatments may be required occasionally, when the seasonal climatic conditions are favourable for parasite and snail development, or in areas where high metacercariae intake often occurs as a result of restricted grazing of wet areas during dry seasons.

    The most important prerequisite for efficient chemotherapy and chemoprophylaxis is a prior knowledge of the epidemiology of the disease based mainly on meteorological data and seasonal surveys in hosts.

    The economics of chemotherapy should be evaluated for each farm, area and country, including assessments of the availability of anthelminthics, their price and the economics of the livestock production system in which they are to be used. More treatments are necessary if the drugs available (or selected on the basis of cost) are those that are only effective against mature flukes. Efficient control programmes can be developed based on less frequent treatments with drugs effective against early immature and immature flukes. However, the price of these drugs is considerably higher than those effective only against older flukes and their use may therefore be restricted to the more intensive livestock production systems.

    If animals are grazing communal areas, it is important to achieve a synchronized reduction in pasture contamination of eggs, if possible. Ideally all animals in the area should receive treatment within a short period of time.

    1.2 ?Chemical control of snails

    The use of molluscicides for the control of snail intermediate hosts is a potential tool for the control of fluke infections. Before considering chemical control of snails it should be noted that:

    ??many habitats are topographically unsuitable for the use of molluscicides and it is often very difficult to apply them effectively.

    ??they are toxic to the environment

    ??cooperation between neighbouring properties is required for effective cover

    ??regular (at least yearly) application is required because rapid repopulation of snails may occur

    ??they are not species-specific and may destroy edible snails highly valued as food in some communities.

    ??they are expensive

    1.3 ?Biological methods of snail control

    Reports from several parts of the world indicate that a number of plants have molluscicidal properties. Planting of these trees and shrubs along streams and irrigation channels can reduce the number of snails in a population. The efficacy of this method for control of flukes has not yet been assessed.

    The introduction of large numbers of ducks into rice fields after harvest has been used to reduce the snail population. The ducks eat the snails and the fluke species specific to the ducks compete with the fluke species of ruminants in the infection of snails. It is reported that snails infected with duck flukes will not become infected with flukes of livestock.

    The introduction of edible snail species unsuitable as intermediate hosts into the habitat of the host snails may prevent the flukes from completing their life cycle.

    1.4 ?Managemental methods of snail control

    The important management methods of controlling fluke infections are:

    (a) To prevent snail habitats from developing by regular clearing of drainage channels in vegetation which provides suitable sites for snail development. Good drainage and the building of dams at appropriate sites in marshy and low lying areas may reduce the snail problem.

    (b) To keep livestock away from pastures contaminated with metacercariae. This may only be possible when the number of animals involved is small.

    (c) Establish proper watering facilities to prevent animals from drinking from lakes, ponds and streams.

    2. ?Control of paramphistomes

    Most of the principles described for the control of liver flukes also apply to the control of intestinal and ruminant flukes.

    Outbreaks of clinical paramphistomiasis caused by the immature flukes in calves, sheep and goats often pass undiagnosed, and the importance of this disease may be considerably underestimated.

    3. ?Control of schistosomes

    Control of schistosomes is based on control of the snail intermediate host and treatment of infected animals and humans.

    The proposed control strategies for the control of snails outlined in the section on F. hepatica and F. gigantica are also applicable to the control of the intermediate host snails of schistosomes. The managemental methods recommended in section 7.2.1.4, points a and c, may also contribute to the control of schistosomiasis.

    ?Note: Source of this article: from www.ilri.org

     
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