Schistosomiasis

An idealized situation is illustrated in Fig. 2.9. Ten people with schistosomiasis are all potential polluters of a body of water. Each gram of faeces might contain 80 eggs, but if only half of them reach the water, then there are still 40 x 200 (an average stool specimen is 200 g) = 8 x 103 eggs per person or 8 x 104 eggs from all ten people, reaching the water every day. The miracidium that hatches from the egg needs to find a host snail to complete its development. Snails can reproduce rapidly so that one snail can produce a colony in 40 days and be infective in 60 days. The numbers of cercariae liberated from a snail are immense, but because they need to find a human host within 24 h (generally less), few are successful. The ten people entering the water at the other side of the picture could all become infected, but in reality, only a proportion are likely to be so.

When control is considered, there is the choice of preventing pollution of the

Prevent all faecal contamination

Kill nearly all snails

Prevent water contact

Prevent all faecal contamination

Kill nearly all snails

Prevent water contact

Very difficult Difficult

Fig. 2.9. Theoretical environmental control of schistosomiasis.

Most feasible water, destroying the snails or preventing water contact. (There is also mass treatment of the population which will reduce the total egg load, but for the present argument, it will not be discussed here.) If latrines were provided and nine out of the ten people used them, there would still be 8 x 103 eggs from the tenth person going into the water, sufficient to maintain almost the same level of snail infections. If all the snails were destroyed except a few, then within 60 days, the situation would return to what it was before. However, if any one of the ten people could be prevented from making contact with the water, then his/ her freedom from infection would be absolute.

Of course, the situation is never as clear-cut as this, but the illustration is made to show that a sanitation or molluscicide programme needs to be virtually perfect, whereas prevention from water contact can provide complete protection to the individual. This is a simplified example, but a more realistic situation can be simulated by the use of mathematical models.

Mathematical models will not be covered in any more detail here, but examples will be found in measles (Fig. 12.1), malaria (Section 15.6 and Fig. 15.7) and lymphatic filariasis (Fig. 15.10). They are especially useful in determining control strategy, which is the subject of the next few chapters.

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