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MALARIA: Obstacles and Opportunities
tions. The equation used to calculate VC is C = ma2pn/−logcp, where C = vectorial capacity, m = density of vectors in relation to humans, a = number of blood meals taken on humans per vector per day, p = daily survival probability of vectors (measured in days), and n = incubation period in the vector (measured in days). The formula expresses the capacity of a vector population to transmit malaria based on the potential number of secondary inoculations originating per day from an infective person. The formula is specific for a given species of vector, because different species vary with respect to m, a, and p. If several vector species coexist, the VC is the sum of the vectorial capacities of each of the individual vector species. Vectorial capacity is an essential component of mathematical models of malaria transmission. There are a number of assumptions that must be taken into account when VC is used to either assess the status of a malarious situation or predict its evolution (Molineaux, 1988).
Theoretically, VC can predict the extent to which mosquito populations must be reduced to affect the intensity of malaria transmission. For example, according to the formula, the mosquito population would have to be reduced by 99 percent in a holoendemic area before any change in transmission would occur. Such predictions are difficult to verify in natural situations, however. Indeed, there is a need to determine how reductions in vectorial capacity affect patterns of disease in human populations.
Malaria surveillance may be the most important first step for endemic countries hoping to understand and manage their malaria problems. Surveillance networks must be able to monitor the disease in human populations, track patterns of parasite drug resistance, and monitor transmission by vector populations.
The geographic variation in the intensity of malaria transmission is of prime importance for development of appropriate control measures. Few endemic countries have useful information on the patterns and intensities of transmission. This is in marked contrast to small, size-limited studies, in which information has been collected over many years. For example, investigators have studied malaria in the Kisumu area of Kenya for 60 years, but relatively little is known about vectors and transmission anywhere else in the country.
Recently developed immunoassays that detect sporozoites in mosquitoes make vector surveillance more feasible, especially in areas where vectors prefer indoor environments and can be easily sampled, as in parts of Africa. The ability to dry and store mosquitoes for later processing by enzyme-linked immunosorbent assay will facilitate the collection of mosquitoes from multiple sites for testing in central facilities. Indices of trans-