Malaria risk factors
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [2]; Associate Editor(s)-in-Chief: Rim Halaby, M.D. [3]; Alison Leibowitz [4]
Overview
A risk factor for malaria is travel to endemic areas. For travelers, regions associated with the highest estimated relative risk of infection are West Africa and Oceania. Human behavior, often dictated by socioeconomic situations, can influence the risk of malaria for individuals and communities. In addition, children and pregnant women are at a higher risk of contracting malaria. Certain biologic characteristics can protect against particular types of malaria. Two genetic factors, the sickle cell trait and absence of Duffy blood group, have been shown to be epidemiologically significant.[1]
Risk Factors
Genetic Factors
Sickle Cell Trait
Individuals who have the sickle cell trait (heterozygotes for the abnormal hemoglobin gene HbS) are relatively protected against P. falciparum malaria and thus possess a biologic advantage. Because P. falciparum malaria has historically been a leading cause of death in Africa, the sickle cell trait is found more frequently in Africa and in individuals of African ancestry, than in other population groups.
In general, the prevalence of hemoglobin-related disorders and other blood cell dyscrasias, such as Hemoglobin C, thalassemias, and G6PD deficiency, are more prevalent in malaria endemic areas and are thought to provide protection from malarial disease.[1]
Duffy Blood Group
Individuals who are negative for the Duffy blood group possess red blood cells that are resistant to infection by P. vivax. Since the majority of Africans are Duffy negative, P. vivax is rare in Africa south of the Sahara, especially in West Africa. In that area, the niche of P. vivax has been taken over by P. ovale, a similar parasite that can infest Duffy-negative individuals.[1]
Other Genetic Factors
Other genetic factors related to red blood cells can also influence malaria, but to a lesser extent. Various genetic determinants (such as the "HLA complex," which plays a role in control of immune responses) may equally influence an individual's risk of developing severe malaria.[1]
Children
In areas with high P. falciparum transmission (most of Africa south of the Sahara), newborns will be protected during the first few months of life, by maternal antibodies transferred to them through the placenta. As the transferred antibodies decrease with time, these young children become vulnerable to disease, and subsequent death, by malaria. If they survive repeated infections and reach an older age (2-5 years) they will develop a protective semi-immune status. In malaria-endemic areas, young children are at a high risk of developing malaria. For this reason, these young children are targeted preferentially by malaria control interventions.[1]
In areas with lower transmission (such as Asia and Latin America), infections occur less frequently, resulting in a larger proportion of the older children and adults possesing no protective immunity. In such areas, malarial disease can be present in all age groups, and epidemics can occur.[1]
Travel
Traveling to endemic areas is a risk factor for malaria. For travelers, regions associated with the highest estimated relative risk of infection are West Africa and Oceania. For these areas of intense transmission, exposure for even short time periods can result in transmission. For travelers, regions associated with a moderate estimated relative risk of infection are the other parts of Africa, South Asia, and South America.
First- and second-generation immigrants from malaria-endemic countries, returning to their native countries to visit friends and relatives, tend not to use appropriate malaria prevention measures and subsequently have an increased likelihood of developing malaria .[2]
Pregnancy
Pregnancy decreases immunity against many infectious diseases. Women who have developed protective immunity against P. falciparum tend to lose this protection when they become pregnant (especially during the first and second pregnancies). Malaria during pregnancy is harmful not only to the mothers but also to the unborn children. The latter are at greater risk of being delivered prematurely or with low birth weight, with consequently decreased chances of survival during the early months of life. For this reason pregnant women are also targeted (in addition to young children) for protection by malaria control programs in endemic countries.[1]
Behavioral Factors
Human behavior, often dictated by social and economic reasons, can influence the risk of malaria for individuals and communities. For example:[1]
- Poor rural populations in malaria-endemic areas often cannot afford the housing and bed nets that would protect them from exposure to mosquitoes. These individuals often lack the knowledge to recognize malaria and to treat it promptly and correctly. Often, cultural beliefs result in use of traditional, ineffective methods of treatment.
- Travelers from non-endemic areas may choose not to use insect repellent or medicines to prevent malaria. Reasons may include cost, inconvenience, or a lack of knowledge.
- Human activities can create breeding sites for larvae (standing water in irrigation ditches, burrow pits)
- Agricultural work such as harvesting (also influenced by climate) may force increased nighttime exposure to mosquito bites
- Raising domestic animals near the household may provide alternate sources of blood meals for Anopheles mosquitoes and thus decrease human exposure
- War, migrations (voluntary or forced) and tourism may expose non-immune individuals to an environment with high malaria transmission.