Evaluation of the « Time/Risk » Probability of being Infected and its Evolution Before/After a Long Term Village Scale Malaria Vector Control Programme in Angola

Three main entomological indicators are classically used in the epidemiology of malaria: the inoculation rate (“h”) of Ross, the vectorial capacity (“C”) of Garrett-Jones and the reproduction rate (“z”) of Macdonald. In spite of their undoubtfully usefulness it appeared that their formulae did not integrate the key parameter of “t” i.e. the time of exposure and therefore the probabilities of being infected according to the entomological condition (density, infectivity, longevity of the vectors) but also the time/risk and the reduction of this risk when some village-scale vector control measures are implemented.

To deal with this approach we used the Briley’s formula, elaborated some years ago, to analyze the time/risk of being infected in the framework of a long term village scale vector control programme implemented around Balombo (Benguela Province, Angola) with classical method (inside residual spraying) and newly developed tool s (insecticide treated plastic sheeting) used alone or in association with the classical long lasting insecticide treated nets.

Before vector control the risk was almost 20% in one week, 60% in one month and 100% in 3 months and this explain why plasmodic index are so high in this area without any organized vector control programme.

The 3 methods actually reduced the risks which become of the order of 2% in one week, 10% in one month, 26% in one trimester but 70% in one year; the three methods had the same efficacy in reducing these risks. The fact that the risk reach 70% in one year even with right vector control shows the needs of renewing regularly the operations, the needs of other than entomological methods of prevention but also that immunity could be maintained and feared “rebound” was not observed even during the long term of the programme.