Malaria overview and epidemiology

Malaria: An Epidemiological Overview

Distribution: Environmental risk factors

Because of malaria's mode of transmission (through Anopheles mosquito bites), it is an almost exclusively tropical disease. "Temperature is particularly critical. For example, at temperatures below 20°C (68°F), Plasmodium falciparum (which causes severe malaria) cannot complete its growth cycle in the Anopheles mosquito, and thus cannot be transmitted" ("Where malaria occurs," 2010). But where the parasites can complete their growth cycle, wherever the Anopheles mosquitoes can survive and multiply, so do incidents of the disease. Prevalence and intensity is highest where the mosquito can breed year 'round in regions nearest the equator -- particularly in sub-Saharan Africa ("Where malaria occurs," 2010). In some tropical and subtropical countries, prevalence has been reduced because the disease cannot be transmitted at higher altitudes, during colder seasons, in deserts (mosquitos prefer humid climates), and also because of government mosquito control programs ("Where malaria occurs," 2010).

Distribution: Social factors

The fact that control programs have significantly reduced or eliminated the prevalence of malaria in many tropical and subtropical regions highlights the fact that environmental conditions alone cannot explain the prevalence of outbreaks. In the southern United States, for example, malaria used to be a threat until a public health campaign designed to eliminate the disease through appropriate control measures was successful. With the creation of the U.S. Tennessee Valley Authority (TVA) during the New Deal, however, a program for controlling malaria in the region was established. From a disease that affected 30% of the population when the TVA was incorporated in 1933, thanks to control operations by 1947 the disease was eliminated in the area. "Mosquito breeding sites were reduced by controlling water levels and insecticide applications" ("The history of malaria," 2010). As a result of this success, the National Malaria Eradication Program was established in 1947. "By the end of 1949, over 4,650,000 housespray applications had been made. In 1947, 15,000 malaria cases were reported. By 1950, only 2,000 cases were reported. By 1951, malaria was considered eliminated from the United States" ("The history of malaria," 2010).

Unfortunately, in sub-Saharan Africa, the healthcare infrastructure is not stable enough to support such efforts. Poverty and a lack of organizational resources mean that the types of effective antimalarial campaigns which were so successful in the U.S. And Europe cannot be established. Even simple and inexpensive measures such as providing mosquito nets to residents have proven to be challenging to implement. The World Health Organization (WHO) recommends a three-pronged strategy for malaria reduction in affected areas: environmental management; biological and chemical controls; and personal prevention strategies. Environmental management includes drainage (eliminating the breeding-grounds of mosquitos); biological and chemical controls include insect sprays, larvicides, or introducing fish which eat mosquito larvae; and personal protective strategies include personal use of insecticides and nets. "IVM supports more accessible and affordable disease diagnosis and treatment with effective anti-malarial drugs, within the framework of a multi-disease control approach" ("Malaria control," 2015).

Distribution: Malaria and genetics

"It is now clear that malaria parasites have imposed strong selective forces on the human genome in endemic regions" (Driss et al. 2011).Certain genetic factors can make individuals more or less susceptible to malaria. For example, while sickle cell anemia is a severe illness, merely having a recessive gene for the condition can actually have a protective effect for the individual. Also, "several inter-ethnic comparative studies showed that the Fulani population from West Africa is more resistant to P. falciparum malaria than are other sympatric ethnic groups" versus Asia, where malaria is still present but is less widespread where no such innate resistance has been detected within any specific ethnic group (Driss et al. 2011). According to molecular analysis, "different genes have been identified that are associated with different malaria related phenotypes. Factors that promote severity of malaria include parasitaemia, parasite induced inflammation, anaemia and sequestration of parasitized erythrocytes in brain microvasculature" (Driss et al. 2011). Thus other genetic profiles can actually increase one's susceptibly to malaria. In general, persons with ancestors from regions where malaria has not been present will be less likely to have protective genotypes and phenotypes.

Screening for malaria

Part of the screening process for malaria is based upon a visual assessment of a patient's symptoms. These may include "fever, chills, headache, splenomegaly (enlargement of the spleen), signs of anemia (e.g. pallor), nausea, or vomiting" (Stauffer 2011). However, these symptoms can be characteristic of a wide variety of conditions and other tests are needed to conclusively determine the cause of the patient's condition. "Typical laboratory findings may include one or more of the following: thrombocytopenia (low platelet count), anemia, low white blood cell count, increased liver enzymes, and elevated inflammatory markers such as C-reactive protein" (Stauffer 2011). The usual test for malaria is that of traditional thick and thin blood films and rapid antigen detection testing (RDT). This test is superior to simply visually assessing the individual, given that malaria many manifest itself in different ways, depending upon the person (and some patients are asymptomatic). However, "the RDT should always be used concurrently with a traditional blood film because it has sub-optimal sensitivity for non-falciparum malaria. Additionally, RDT cannot determine the quantity of malaria in the blood and it may not effectively distinguish the species of malaria" (Stauffer 2011). For asymptomatic or sub-clinical malaria, "the most sensitive and specific test is polymerase chain reaction (PCR)" (Stauffer 2011).