Schistosomiasis Control

Control of Schistosomiasis, as with control of any infectious organism, aims to prevent new infections, usually by interruption of the parasites lifecycle. This may be achieved by a number of methods;

1) Acting to eliminate the intermediate host

2) Elimination of the parasite from the definitive host

3 Prevention of infection of the definitive host

4) Prevention of infection of the intermediate host, including;

5) The eventual use of a vaccine

Each of these methods may be effective, but all have different advantages and disadvantages, depending on the parasitic organism.

• Elimination of the Intermediate Host

In terms of schistosomiasis this means elimination of the snail hosts. There are a number of ways of doing this.

By application of molluscicides - This method of control is the main one used under this heading, and may be successful in control of schistosomiasis. But it also has a number of major disadvantages, including the expense of molluscicides, and, very importantly the toxicity of the chemicals used, which may kill other aquatic organisms such as the fish the affected populations may rely on.

Alteration of the aquatic environment - This includes altering the rate of flow of the water, clearance of vegetation and drainage (for example of canals) at certain times of the year. Disadvantages of these methods may include impracticality for economic or environmental reasons, damage to fish populations (e.g. by clearance of vegetation), and altering the environment such that it is then suitable for other disease organisms, for example, increasing rate of water flow may inhibit snail populations, but may then be suitable for colonisation by Simulium larvae, the vectors for river blindness.

Biological control - For example by introduction of competitor snail species. For example the snail Marisa cornuarietis will both compete with Biomphalaria snails for food, and eat its eggs, and has been used in Puerto Rico as a control agent. Other methods have been suggested such as the introduction of snail eating fish. Also growth of certain plants on the sides of waterways has been used to control snail. The berries of endod (Phytolacca dodecandra) have been shown to be naturally molluscicidal when the fall into the water, and its presence on the sides of rivers in Ethiopia has been shown to be associated with a reduction in local snail population.

• Elimination of the Parasite from the definitive Host

This is carried out by chemotherapeutic cure, and is currently the main control method for schistosomiasis. The two main drugs used are Oxamniquine and Praziquantel. These drugs are relatively safe to use, are the development of drug resistance by schistosomes is not a problem. The major disadvantages to the use of drugs to control schistosomiasis are firstly the cost, and second and more importantly the need to repeat drug treatment at relatively short intervals. This control method however is the most widely used with respect to schistosomiasis.

• Prevention of infection of the definitive host

Environmental Infrastructure - Various preventative control methods are used, principally reducing contact between at risk populations and contaminated water, by construction of wash houses and other safe water supplies. These methods have the added advantage in reducing or eliminating other water borne infectious diseases at the same time as reducing schistosomiasis. Disadvantages include the large cost of implementing these schemes.

The use of a Vaccine - An important control measure that has been successfully used with many other infectious diseases (e.g. smallpox) is the use of a vaccine. However, for schistosomiasis, there is as yet no effective vaccine for human use currently available. The identification of schistosome antigens that may form the basis is a major priority of the research carried out in our laboratories. To be an effective vaccine against schistosomiasis, (or indeed any tropical disease), the vaccine must have the following features:

i) It must be cheap - The countries affected by schistosomiasis include some of the poorest in the world, and for them to be able to use the vaccine, they must be able to afford it, especially when this expenditure is in competition with other major health costs.

ii) It must be effective in terms of protection - Protect of a large percentage of those people vaccinated is essential to interrupt transmission of the parasite.

iii) It must be stable - For use in many of the affected countries it must be able to survive storage as it is transported to isolated endemic populations.

iv) It should ideally be effective with a single application - This is important as a means of keeping the costs of the mass vaccination schemes needed to a minimum.

• Prevention of infection of the intermediate host

These control methods at present are principally in this case introduction of latrines, thereby reducing or eliminating contamination of the populations water supplies with human faeces containing eggs. The disadvantages of these methods are therefore also the same as those detailed above for prevention of human infection, namely the large costs involved. Interestingly - infection of the snail with other larval trematodes, particularly those of echinostomes, has been shown to protect the snails from infection by larval schistosomes, and although these might not eliminate schistosomes from endemic areas, they may to some extent limit intensity of snail infections.