MicrobiologyBytes: Microbiology Notes: Malaria Updated: April 8, 2009 Search

MALARIA

Approximately 300 million people worldwide are affected by malaria and between 1 and 1.5 million people die from it every year. Previously extremely widespread, the malaria is now mainly confined to Africa, Asia and Latin America. The problems of controlling malaria in these countries are aggravated by inadequate health structures and poor socioeconomic conditions. The situation has become even more complex over the last few years with the increase in resistance to the drugs normally used to combat the parasite that causes the disease.

Malaria is caused by protozoan parasites of the genus Plasmodium. Four species of Plasmodium can produce the disease in its various forms:

  • Plasmodium falciparum
  • Plasmodium vivax
  • Plasmodium ovale
  • Plasmodium malaria

P. falciparum is the most widespread and dangerous of the four: untreated it can lead to fatal cerebral malaria.

Malaria parasites are transmitted from one person to another by the female anopheline mosquito. The males do not transmit the disease as they feed only on plant juices. There are about 380 species of anopheline mosquito, but only 60 or so are able to transmit the parasite. Like all other mosquitos, the anophelines breed in water, each species having its preferred breeding grounds, feeding patterns and resting place. Their sensitivity to insecticides is also highly variable.

Plasmodium develops in the gut of the mosquito and is passed on in the saliva of an infected insect each time it takes a new blood meal. The parasites are then carried by the blood in the victim's liver where they invade the cells and multiply:

MicrobiologyBytes: Malaria
Life cycle

Animation: Lifecycle of a malaria parasite from mosquito to blood stages (needs Macromedia Shockwave Flash Player)

After 9-16 days they return to the blood and penetrate the red cells, where they multiply again, progressively breaking down the red cells. This induces bouts of fever and anaemia in the infected individual. In cerebral malaria, the infected red cells obstruct the blood vessels in the brain. Other vital organs can also be damaged often leading to the death of the patient.

Malaria is diagnosed by the clinical symptoms and microscopic examination of the blood. It can normally be cured by antimalalial drugs. The symptoms, fever, shivering, pain in the joints and headache, quickly disappear once the parasite is killed. In certain regions, however, the parasites have developed resistance to certain antimalarial drugs, particularly chloroquine. Patients in these areas require treatment with other more expensive drugs. Cases of severe disease including cerebral malaria require hospital care.

In endemic regions, where transmission is high, people are continuously infected so that they gradually develop immunity to the disease. Until they have acquired such immunity, children remain highly vulnerable. Pregnant women are also highly susceptible since the natural defence mechanisms are reduced during pregnancy.

Malaria has been known since time immemorial, but it was centuries before the true causes were understood. Previously, it was thought that "miasma" (bad air or gas from swamps - "mal air ia") caused the disease. Surprisingly in view of this, some ancient treatments were remarkably effective. An infusion of qinghao (Artemesia annua ) has been used for at least the last 2000 years in China, its active ingredient (artemisinin) having only recently been scientifically identified. The antifebrile properties of the bitter bark of (Cinchona ledgeriana ) were known in Peru before the 15th century. Quinine, the active ingredient of this potion was first isolated in 1820 by the pharmacists.

Although people were unaware of the origin of malaria and the mode of transmission, protective measures against the mosquito have been used for many hundreds of years. The inhabitants of swampy regions in Egypt were recorded as sleeping in tower-like structures out of the reach of mosquitoes, whereas others slept under nets as early as 450 B.C.

Systematic control of malaria started after the discovery malaria parasite by Laveran in 1889 (for which he received the Nobel Prize for medicine in 1907), and the demonstration by Ross in 1897 that the mosquito was the vector of malaria. These discoveries quickly led to control strategies and with the invention of DDT during the World War II, the notion of global eradication of the disease. Effective and inexpensive drugs of the chloroquine group were also synthesized around this time.

The hope of global eradication of malaria was finally abandoned in 1969 when it was recognised that this was unlikely ever to be achieved. Ongoing control programs remain essential in endemic areas. Malaria is currently endemic in 91 countries with small pockets of transmission occurring in a further eight countries. Plasmodium falciparum is the predominant parasite. More than 120 million clinical cases and over 1 million deaths occur in the world each year.

Eighty per cent of the cases occur in tropical Africa, where malaria accounts for 10% to 30% of all hospital admissions and is responsible for 15% to 25% of all deaths of children under the age of five. Around 800,000 children under the age of five die from malaria every year, making this disease one of the major causes of infant and juvenile mortality. Pregnant women are also at risk since the disease is responsible for a substantial number of miscarriages and low birth weight babies.

Malaria thus has social consequences and is a heavy burden on economic development . It is estimated that a single bout of malaria costs a sum equivalent to over 10 working days in Africa. The cost of treatment is between $US0.08 and $US5.30 according to the type of drugs prescribed as determined by local drug resistance. In 1987, the total "cost" of malaria - health care, treatment, lost production, etc. was estimated to be $US800 million for tropical Africa and this figure is currently estimated to be more than $US1,800 million.

The distribution of malaria varies greatly from country to country and within the countries themselves. In 1990, 75% of all recorded cases outside of Africa were concentrated in nine countries:
Malaria map
  • India
  • Brazil
  • Afghanistan
  • Sri Lanka
  • Thailand
  • Indonesia
  • Vietnam
  • Cambodia
  • China

The significance of malaria as a health problem is increasing in many parts of the world. Epidemics are even occurring around traditionally endemic zones in areas where transmission had been eliminated. These outbreaks are generally associated with deteriorating social and economic conditions, and main victims are underprivileged rural populations. Demographic, economic and political pressures compel entire populations (seasonal workers, nomadic tribes and farmers migrating to newly-developed urban areas or new agricultural and economic developments) to leave malaria free areas and move into endemic zones. People are non-immune are at high risk of severe disease. Unfortunately, these population movements and the intensive urbanization are not always accompanied by adequate development of sanitation and health care. In many areas conflict, economic crises and administrative disorganization can result in the disruption of health services. The absence of adequate health services frequently results in a recourse to self-administration of drugs often with incomplete treatment. This is a major factor in the increase in resistance of the parasites to previously effective drugs.

In all situations, control programmes should be based on four objectives:

Malaria is complex but it is a curable and preventable disease. Lives can be saved if the disease is detected early and adequately treated. It is known what action is necessary to prevent the disease and to avoid or contain epidemics and other critical situations. The technology to prevent, monitor, diagnose and treat malaria exists. It needs to be adapted to local conditions and to be applied through local and national malaria control programmes.

 

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Some of the information in this document is provided by generous permission of the World Health Organization Division of Control of Tropical Diseases


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