identify particular Plasmodium species. This is important since sporozoites of all Plasmodium species that infect humans are morphologically similar, and sporozoites of most animal malarias cannot be distinguished morphologically from those that infect humans.
Species are sometimes referred to as primary and secondary vectors. The incrimination of primary vectors is usually clear-cut; they are often abundant, commonly feed on humans, and have measurable sporozoite rates. Incrimination of secondary vectors is more complicated, because these species may be uncommon and have low sporozoite rates. However, they may be seasonally abundant and, at times, play a major role in transmission. Adult mosquito behavior and larval ecology may be significantly different in primary versus secondary vectors, and measures taken to control primary vectors may not have an impact on secondary vectors.
Gathering site-specific information about vectors is an important first step in planning vector control measures. It is sometimes necessary to extrapolate vector-related data from areas where actual vector identification has been performed. Such an approach is not without problems, since epidemiologically significant shifts in primary vector species can occur due to changes in the environment, such as urbanization, deforestation, and irrigation.
Haworth (1988) provides a detailed review of the global distribution of human malaria and, for each geographic zone, lists the primary and secondary vectors. In general, malaria in each zone is transmitted by a specific set of Anopheles species. Distribution patterns for mosquito species are fairly stable. Vector species rarely completely disappear from a region, and in no case have indigenous vectors been deliberately eradicated. The introduction of malaria vectors into nonindigenous areas is a serious public health concern. For example, the introduction of Anopheles gambiae to Brazil and Egypt in the 1940s caused devastating epidemics and required unparalleled efforts to eliminate the newly arrived vector (Duffy, 1977).
The natural distribution patterns of anophelines are largely determined by environmental conditions. Each species has unique environmental tolerance limits. The same is true for malarial parasites. For example, the distributions of P. vivax and P. falciparum are theoretically limited by summer isotherms of 15°C and 18°C, respectively, the temperatures required for the completion of the sporogonic cycle in the mosquito host (Boyd, 1949).