Malaria is a life-threatening disease transmitted by Anopheles mosquitoes infected with the parasites of the genus Plasmodium. The species P. falciparum accounts for the majority of the disease's morbidity and mortality. More than 40 percent of the world's population lives in areas where malaria is transmitted. It is a preventable and treatable disease, but nonetheless kills 1-3 million people each year, primarily young children¹.

Disease Transmission

The life cycle of the malaria parasite Plasmodium begins when an infected female Anopheles mosquito bites a human. The Plasmodium sporozoites enter the bloodstream and travel to the liver where they invade liver cells and divide repeatedly. New morphological stages of the parasite called merozoites emerge and infect red blood cells. After invading red blood cells and rapidly multiplying, merozoites are released and some merozoites in the blood will develop into sexual forms called gametocytes. When a female Anopheles mosquito bites the infected person, it ingests these gametocytes. In the mosquito's digestive system, the gametes then combine to form a zygote. The zygote goes through several developmental stages to form more sporozoites. The sporozoites migrate to the salivary glands of the mosquito; when the insect bites another person, the life cycle begins anew. In most cases of Plasmodium infection, the incubation period is from seven to fourteen days, but varies if the individual has taken a preventive medicine.

Disease Characteristics

Malaria is characterized by extreme exhaustion associated with high fever, sweating, shaking, and chills. Due to the fact that the protozoa destroys red blood cells, malaria can also lead to anemia. The disease may present as uncomplicated malaria, characterized by flu-like symptoms, or severe malaria, in which infected red blood cells block vessels to the brain or damage other vital organs, often resulting in death. People without a naturally acquired immunity who contract malaria are prone to the development of severe P. falciparum malaria. Severe malaria usually occurs in persons who have no immunity to malaria or whose immunity has decreased. In some instances, people in highly endemic areas who are infected frequently may develop immunity to the disease and become carriers of malaria, contributing to transmission.

Prevalence

Malaria causes 300-500 million new infections and 1-3 million deaths annually. The global burden of this disease falls heaviest among children under the age of 5. Malaria also poses a risk to pregnant women and newborn babies. Malaria infection in pregnant women is often asymptomatic in the areas of greatest transmission rates.

Geographic Distribution

Malaria is largely a disease of poverty. It is endemic in over 100 countries worldwide, notably in sub-Saharan Africa, where 90 percent of deaths occur, mostly among young children. In other areas of the world, malaria is a less prominent cause of death, but can cause substantial disease and incapacitation, especially in rural areas of some countries in South America and Southeast Asia.

Current Therapies

There are a number of drugs capable of preventing Plasmodium infection. Chronic use of these drugs is not recommended due to their potential side effects. These options are usually only available to occasional travelers, expatriate workers, or military personnel in endemic areas, and can be very expensive.

For many people in malaria-endemic countries, traditional first-line treatments such as chloroquine and sulphadoxine-pyrimethamine are no longer effective due to increasing drug resistance. Today there are reported cases of Plasmodium resistant strains for each available first-line therapy, leading to the spread of malaria to new areas and the re-emergence of the disease in areas where it had previously been eliminated. In response, scientists have developed new treatments using combinations of artemisinin-based drugs with other antimalarials (artemisinin combination therapy, or ACTs). At present, there has not been a documented clinical case of artemisinin drug resistance.

Vaccines are an effective way to prevent disease, and several different malaria vaccine candidates are now in human clinical trials in Africa, Asia, Europe, and the United States. However, it may take years before an effective vaccine is available. Other effective preventive measures, such as insecticides and insecticide-treated bed nets (ITNs), also play an important role in reducing disease incidence in malaria endemic areas.