Epidemiology and Control

Malaria transmission in any locale depends upon the complex interactions of parasites; vector mosquitoes; physical, socioeconomic, and environmental factors; and human biology, demography, and behavior.

The four species of plasmodia differ in many biological characteristics. Each species, for example, has somewhat different environmental temperature requirements for sporogony; each is different in many features of its schizogonia cycle, and as a consequence, each has distinctive clinical manifestations. P. falciparum differs so much from the other three species (of the subgenus Plasmodium) that it has been assigned to its own subgenus, Laverania (Garnham 1966). Within each species, variation in strains is also important. Differences in strains influence sporogonic development in mosquitoes, virulence and thus the clinical course, and the development of resistance to antimalarial drugs.

The presence of anopheline mosquitoes capable of supporting sporogony is fundamental for natural malaria transmission. Although more than 400 species of Anopheles have been described, natural sporozoite infections have been found in only about 67 species, and only some 30 species are recognized as important vectors (Spencer 1986). Usually, in any endemic area, only one, two, or three vector species are responsible for most of the malaria transmission. Parasite species and strain differences affect anopheline receptivity to infection. Vector competence for transmission is also influenced by mosquito population densities, flight ranges, feeding and resting habits, blood meal preferences (anthropophilic for human blood; zoophilic for animal blood), blood meal frequencies, and the lifespan of the female anopheline. In recent decades, resistance to insecticides has also become a major vector-related determinant of transmission.

Vector, parasite, and host interactions are profoundly affected by factors in the environment. Physical factors, such as temperature, rainfall, and humidity, control mosquito survival and the duration of sporogony. Other factors such as latitude, altitude, and landscape characteristics (natural or human-modified) influence mosquito breeding, vector densities, and vector survival, as well as many kinds of human behavior. In addition, features of the social and economic environment contribute in many ways to patterns of transmission.

Age, gender, and a variety of human demographic variables, together with genetic factors, immune mechanisms, and health conditions (e.g., nutritional status, pregnancy, and concurrent infections), constitute the final host-related set of determinants in the malaria epidemiological complex that must be well understood before fully effective control can be instituted in any setting.

The incidence and prevalence of clinical cases may be useful measures for describing the status of malaria in an area, but parasite and spleen rates are even more helpful for determining the level of endemicity - spleen rates particularly, because enlarged spleens are indicative of past or recent active erythrocytic schizogony. The spleen rate is the proportion of persons, usually children 2 to 9 years old, with enlarged spleens at a particular time, whereas the parasite rate is the proportion of persons in a population with microscopically confirmed parasitemia at a particular time.

Malaria endemicity is classified on the basis of spleen or parasite rates:

1. Hypoendemic malaria: spleen or parasite rates up to 10 percent in 2- to 9-year-old children

2. Mesoendemic malaria: spleen or parasite rates of 11 to 50 percent in 2- to 9-year-olds

3. Hyperendemic malaria: spleen or parasite rates over 50 percent in 2- to 9-year-olds together with a high adult spleen rate

4. Holoendemic malaria: spleen or parasite rates over 75 percent in 2- to 9-year-olds with low adult spleen rates and high infant parasite rates

In a pioneering work, George Macdonald (1957) attempted to fit many of the variables noted above into an epidemiological model. His ideas continue to be influential in contemporary malariology despite many advances in epidemiological and mathematical modeling sophistication. Macdonald's principles in defining stable and unstable malaria, for example, continue to be helpful in studies of epidemiological patterns. Stable malaria is characteristically endemic, often hyper- or holoendemic. There is little seasonal change; transmission continues through most or all of the year; epidemics of malaria are very unlikely to occur; and most of the population, except infants, has some immunity as a result of experience with the disease. Control under these circumstances is likely to be very difficult. Malaria in much of tropical Africa south of the Sahara corresponds to Macdonald's stable extreme.

Unstable malaria may be endemic, but transmission varies from year to year and may be strictly seasonal, reflecting the strong seasonal change typical in unstable areas. Seasonal epidemics may occur, collective immunity varies and may be low, and children as well as infants may be nonimmune. Control is relatively easy, and indeed, many of the countries only recently freed from endemic malaria were in the unstable category.

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