Various kinds of fish (particularly Tilapia and Gambusia) are natural predators, but they will only reduce snails to a certain level unless they have an alternative source of food. Snails, especially of the Marisa and Helisoma genera, compete for food supplies and Marisa will even prey on eggs and juveniles of Biomphalaria. Another approach to biological control is the sterile male technique, but since many snails are hermaphrodite, it is only suitable with Oncomelania.

Where small and temporary ponds are foci of infection, they can be drained or filled (by controlled tipping of household refuse). Where canals and irrigation systems are responsible, concrete lining, increasing rate of flow and any method to reduce vegetation can discourage snail habitation. Unfortunately, these methods are rarely effective on their own and need to be combined with molluscicides (e.g. niclosamide, Bayluscide), which can be administered as a liquid, suitable for treating moving water, or as granules in lakes and ponds. Continuous application is required to make a sustained effect on the snail population. It has the disadvantage of killing fish and is expensive. Cheaper preparations, such as copper sulphate, are still in limited use and naturally occurring plant preparations, such as Endod (Phytolacca dodecandra), have shown promise. However, the remarkable recovery of snail populations once control methods are removed and the cost of molluscicides make snail reduction a less effective approach in schistosomiasis control.

reduction of water contact Preventing water contact can be highly effective in the individual. Various ways of encouraging this are:

• health education, especially to school children, but this is often ineffective unless an alternative (e.g. swimming pool), is provided;

• providing places to wash have been disappointingly ineffective for the cost involved;

• where areas of absent or minimal transmission occurs in occupational or recreational bodies of water, people can be encouraged to use these, rather than the heavily infected parts;

• wear rubber boots when wading through water, or if accidental exposure occurs, then rub vigorously with a towel and apply 70% alcohol;

• drinking water can be treated with iodine or chlorine or if left to stand for at least 48 h, cercarial die-off will be complete.

reduction of human infection by mass chemotherapy With the discovery of effective preparations, such as praziquantel, singledose MDA is now a good method of control. A suitable target population is school children between 5 and 15 years of age where mass therapy is used. Alternatively, only the positive cases, or those with heavy infections, are treated following a simple diagnostic procedure. Individual treatment, based on worm load estimation, aims at disease control by reducing morbidity. It permits limited resources to be more widely spread and attempts the less ambitious target of disease rather than transmission reduction.

The anti-malarial drug artemethur is also valuable in the control of schistosomia-sis and could be used in areas where there is no malaria, such as China, southern Brazil and Southwest Asia. It can be used in combination with praziquantel.

reduction of the animal reservoir Animal reservoirs are responsible for maintaining S. japonicum. In order of importance they are dogs, cows, pigs, rats and water buffaloes. As most of these are domestic animals, proper animal management can reduce contamination of the environment. Vaccination of domestic animals could be done. Baboons and monkeys have been shown to be reservoirs of S. mansoni and could play a part in maintaining infection. There is little prospect of controlling these animals.

vaccination There are difficulties in preparing a vaccine because the schistosome is able to absorb host antigen and mask its presence, but three vaccines are currently under trial for S. mansoni.

strategies for schistosomiasis control Various approaches for the control of schisto-

somiasis have been tried depending on the resources and nature of the disease as follows:

• Raising of economic standards by the provision of water supplies and sanitation, environmental engineering and water management has been shown to be effective on a long-term basis in countries such as Japan and China.

• In well-controlled irrigation schemes, mollusciciding on its own may be effective. Where discipline and motivation of the population are less certain, a double approach of mass chemotherapy and reduction of water contact is more effective.

• When resources are scarce and greatest benefit for limited finance is required, treatment of high worm load cases is the method of choice.

Surveillance Effectiveness of control strategies can be measured by:

• change in incidence rate;

• a shift in peak prevalence to an older age group;

• reduction in geometric mean egg output;

• greater awareness of socio-economic values (e.g. the use of water supplies and sanitation facilities).

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