Question

Why does the epidemiologist or Public Health professional need to understand these aspects of malaria?

Answer 1

Epidemiologists will have made great strides in elucidating the complex determinants of malaria, including the risk factors for severe and complicated disease and the role of acquired immunity. It will be clear how chemoprophylaxis and drug therapy reduce morbidity and mortality, and how malaria more severely affects pregnant women and young children. New techniques to measure the impact of various anti-malaria interventions, including strategies to control mosquito populations, will have been developed and will be used in different regions of the world. Advances in understanding local epidemiology will lead to better targeted and more effective malaria control programs. Malaria-related illness and death will decrease as a result. Drug and vaccine testing will be enhanced by epidemiologically based insights into the variations in parasite and human biology that determine the acquisition of immunity and the development of clinical disease.

Epidemiologically, malaria is extremely complex. The nature, duration, and severity of malaria infection depend not only on the species of malaria parasite but also on the level of malaria-specific acquired immunity in the individual. Malaria is a focal disease whose distribution is influenced by literally dozens of factors related both to human, mosquito, and parasite populations and to the environment.

Because epidemiology is a cross-disciplinary science, many of the building blocks of malaria epidemiology are discussed in greater detail elsewhere in this report. No attempt will be made here to provide a comprehensive review of the status of malaria epidemiology. Rather, this chapter focuses on several key issues in this evolving science and introduces a new epidemiologically-based approach to understanding malaria.

Infection and Disease

For many years, epidemiologic studies have focused on the prevalence of malaria infection in populations, relating levels of infection to a variety of parasitologic, climatologic, and entomologic parameters. Beginning in the late 1950s and continuing through the late 1960s, when considerable effort was directed toward the global eradication of malaria, the goal was to halt malaria transmission altogether. Therefore, it was particularly important to detect and eliminate all malaria infections. The incidence of clinical disease and an understanding of risk factors for disease were considered to be of minor significance, since it was believed that malaria would soon disappear. Indeed, little research was conducted on the clinical progression of, or risk factors for, the disease itself.

This distinction between infection and disease is particularly important with respect to malaria. The vast majority of older children and adults living in some endemic areas may be infected with the parasite, but only a small proportion will suffer occasional mild or moderate illness. Knowing that someone is infected with the malaria parasite, then, is of little practical value; being able to determine which infected individuals will become ill and why would be quite useful, however.

Despite the formal abandonment by the World Health Organization (WHO) in 1969 of plans to eradicate malaria in favor of strategies to control the disease, epidemiologic studies and surveillance persisted in tracking malaria infections. Even today, many malaria control programs continue to measure the magnitude of the malaria problem and the relative success of their control efforts by using the annual parasite incidence, a calculation of the number of parasite-infected individuals as a proportion of the total population at risk for becoming infected. Millions of blood films are examined under the microscope each year in malaria-endemic countries, of which less than five percent may be found to contain parasites. The resources expended on such questionably useful surveys are enormous, and given the risk of transmitting the human immunodeficiency virus, hepatitis B virus, and other blood-borne pathogens through nonsterile finger-prick methods, such routine mass screening cannot be recommended.

In countries where malaria is highly endemic, the epidemiology is focal, the burden of disease varies greatly, and surveys that evaluate the prevalence of malaria infection can be particularly deceptive. For example, areas of both Papua New Guinea and the Gambia are highly endemic for malaria and have similar prevalences of malaria infection, yet the levels of malaria-related mortality in the two countries appear to be quite different. On the north coast of Papua New Guinea, few deaths can be traced to malaria, even among very young children (Moir et al., 1989), while in the Gambia, a quarter of all deaths in children one to four years of age are believed to be malaria related (Greenwood et al., 1987). Some of the differences may be explained by the fact that the population around Madang, Papua New Guinea, is relatively advantaged and has better access to antimalarial drugs.

Not all people in malarious areas are at the same risk of becoming sick or dying from the disease. Indeed, the risk of severe and potentially fatal infection with Plasmodium falciparum falls principally on nonimmunes, such as young children, immigrants from malaria-free areas, and pregnant women, in whom immunosuppression during pregnancy appears to be associated with a higher frequency of malaria infection and adverse pregnancy outcomes (Breman and Campbell, 1988). Despite this general pattern, and for reasons not well understood, not all individuals within these groups are at equal risk of becoming seriously ill or dying. Much of the most recent work in malaria epidemiology has thus concentrated on the identification of the variables that place certain groups at greater risk of illness and death. Central to this work has been a better understanding of the acquisition of immunity.

Acquired immunity appears to be relatively short-lived and depends on repeated exposure to the parasite over time. It is directly related to the level of malaria endemicity in a given area, transmission patterns, frequency of human-vector contact, and the length of time a person resides in an endemic area. The species of parasite present, the level of endemicity, and the biologic, behavioral, and socioeconomic characteristics of the human population determine the prevalence of infection and the distribution of disease.

An increasingly important issue in understanding the epidemiology of malaria disease is the availability and use of antimalarial drugs. With the spread of drug-resistant parasites, the effects of drugs on immune status and on drug resistance itself are issues of paramount concern.

By looking at malaria as a disease, epidemiologists are better equipped to assess the short- and long-term impacts of various control strategies, including antimosquito measures (e.g., insecticide-treated bednets) and antimalarial drugs (given as chemoprophylaxis, as therapy, or for both purposes), in reducing malaria-related morbidity and mortality. The shift from studying malaria as an infection to focusing on its importance as a disease is recent, and the questions being asked are a substantial departure from traditional routes of inquiry

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