of its contribution to malaria relapse, is able to persist in areas of wide climatic fluctuation. Plasmodium malariae and P. ovale are less widely distributed. In areas where P. falciparum is prevalent, efforts to limit mortality must be considered first. In areas where P. vivax predominates, debilitating disease rather than mortality is the primary concern. Where more than one species is present, control efforts should be focused on P. falciparum, even when it is not the predominant parasite, because of its potentially fatal consequences.
Mosquito Vectors There are nearly 400 Anopheles species, of which about 60 are proven vectors of human malaria. The existence or creation of situations favorable for particular anopheline vectors is determined largely by the eight basic types of malaria paradigms. As difficult as it is to manage clinical malaria or to prevent infection through prophylaxis given the interventions currently available, controlling populations of anopheline vectors in the long term has proven to be a nearly insurmountable challenge, at least in some of the major paradigms (e.g., African savannah and forest).
To define the risks of infection, to identify methods of reducing human-vector contact, and, where feasible, to reduce vector density, it is essential to understand the population dynamics of the predominant vector(s) in areas where antimalarial activities are being contemplated or implemented. In practice, vector identification is often based on traditional impressions and outdated data rather than on a clear understanding of the current situation.
There are a number of vector-related variables that should be considered before control interventions are selected. Such variables include the mosquito species in the area, their relative importance as vectors of malaria; their distribution, abundance, and competence; their vector feeding and resting habits; and the characteristics of larval development sites.
To determine the potential effectiveness of any antivector measures undertaken, the following factors must be considered: vector susceptibility to insecticides; the cost, safety, and acceptability of effective insecticides; the utility, acceptability, availability, and local cost of bednets; the feasibility of using bednets impregnated with insecticides on a community-wide basis; and the availability, acceptability, and cost of mosquito repellents and fumigant coils (see Chapter 7).
Characteristics of the Human Population The most important determinant of the impact of malaria on a population is the prevailing level of immunity. For example, in the African savannah, where transmission is intense, immunity in those who survive childhood may provide protection from death and even severe clinical illness. Malaria control activities in