Malaria in humans is caused by one of four protozoan species of the genus Plasmodium: P. falciparum, P. vivax, P. ovale, or P. malariae. P. vivax and P. falciparum are the most common and falciparum the most deadly type of malaria infection. Plasmodium falciparum malaria is most common in Africa, south of the Sahara, accounting in large part for the extremely high mortality in this region. There are also worrying indications of the spread of P. falciparum malaria into new regions of the world and its reappearance in areas where it had been eliminated.
The malaria parasite enters the human host when an infected Anopheles mosquito takes a blood meal. Inside the human host, the parasite undergoes a series of changes as part of its complex life-cycle. Its various stages allow plasmodia to evade the immune system, infect the liver and red blood cells, and finally develop into a form that is able to infect a mosquito again when it bites an infected person. Inside the mosquito, the parasite matures until it reaches the sexual stage where it can again infect a human host when the mosquito takes her next blood meal, 10 to 14 or more days later.
Occurrence
Today approximately 40% of the world's population, mostly those living in the world's poorest countries, is at risk of malaria. The disease was once more widespread but it was successfully eliminated from many countries with temperate climates during the mid 20th century. Today malaria is found throughout the tropical and sub-tropical regions of the world and causes 300 - 500 million acute illnesses and at least one million deaths annually.
Ninety per cent of deaths due to malaria occur in Africa south of the Sahara, mostly among young children. Malaria kills an African child every 30 seconds and may account for as much as 25% of child mortality. Many children who survive an episode of severe malaria may suffer from learning impairments or brain damage. Pregnant women and their unborn children are also particularly vulnerable to malaria, which is a major cause of perinatal mortality, low birth weight and maternal anemia. Malaria, together with HIV/AIDS and TB, is one of the major public health challenges undermining development in the poorest countries in the world.
Clinical Presentation
Malaria symptoms typically appear about 9 to 14 days after the infectious mosquito bite, although this varies with different plasmodium species and may be delayed up to several months after the initial exposure. Typically, malaria produces fever, headache, vomiting and other flu-like symptoms; these symptoms can occur at intervals.
If drugs are not available for treatment or the parasites are resistant to them, the infection can progress rapidly to become life-threatening. Malaria can infect and destroy red blood cells, causing anemia and jaundice, and it can clog the capillaries that carry blood to the brain or other vital organs, causing seizures, renal failure and death.
Control and Prevention
Malaria parasites are developing unacceptable levels of resistance to one drug after another and many insecticides are no longer useful against mosquitoes transmitting the disease. Years of vaccine research have produced few hopeful candidates and although scientists are redoubling the search, an effective vaccine is at best years away.
Science still has no magic bullet for malaria, and many doubt that such a single solution will ever exist. Nevertheless, effective low-cost strategies are available for its treatment, prevention and control and the Roll Back Malaria global partnership is vigorously promoting them in Africa and other malaria-endemic regions of the world. Mosquito nets treated with insecticide reduce malaria transmission and child deaths. Prevention of malaria in pregnant women, through measures such as Intermittent Preventive Treatment and the use of insecticide-treated nets (ITNs), results in improvement in maternal health, infant health and survival. Prompt access to treatment with effective up-to-date medicines, such as artemisinin-based combination therapies (ACTs), saves lives. If countries can apply these and other measures on a wide scale and monitor them, then the burden of malaria will be significantly reduced.
By Karen Lewis, M.D.
Adapted from WHO and CDC sources: