Malaria by mosquito
Malaria is a dangerous parasitic disease common in tropical and subtropical areas. It is caused by protozoans called Plasmodia and is transmitted by the bite of the female Anopheles mosquito. Victims of malaria suffer attacks of chills and fever, and 1 million to 3 million people die of the disease yearly.
There are four types of malaria, each of which is caused by a different species of Plasmodium. The four protozoans that cause malaria are P. falciparum, P. vivax, P. ovale, and P. malariae. (The P. stands for Plasmodium. )
Malaria causes periodic chills, with fevers that may reach 106 °F (41.1 °C). P. falciparum, P. vivax, and P. ovale cause attacks of chills and fever that recur about every 48 hours. In P. malariae infections, chills and fever recur about every 72 hours.
A malarial attack lasts two or more hours and is accompanied by headache, muscular pain, and nausea. After each attack, the patient perspires, causing the body temperature to drop to normal. Between attacks, the patient feels better but is weak and anemic.
The most serious type of the disease is caused by P. falciparum. Its victims become weaker with each attack of fever, and most die if untreated. In P. vivax, P. ovale, and P. malariae cases, the attacks get less severe and finally stop, even without treatment. But in P. vivax and P. ovale infections, the symptoms may reappear after a long period of apparent freedom from the disease.
The life cycle of the Plasmodium protozoan includes three basic stages. The first stage occurs in the mosquito's body, and the second and third stages take place in a person's body. The first stage begins when the mosquito bites someone who has malaria. Plasmodia enter the insect's body and reproduce in its stomach. The protozoan young find their way into the mosquito's saliva.
The second stage occurs after the mosquito bites another person. Plasmodia from the mosquito's saliva enter the person's blood. They travel to the liver, where they multiply and form clumps of parasites. After several days, these clumps burst and release new Plasmodia.
During the third stage, each Plasmodium invades a red blood cell, where it multiplies again. The infected blood cells eventually rupture and release large numbers of Plasmodia, which invade additional red blood cells. This invasion, multiplying, and cell rupture by the parasites continues, causing the periodic attacks of fever that are typical of malaria. An attack occurs each time the red blood cells rupture.
Some Plasmodia develop further in human blood and are able to reproduce in a mosquito's body. They enter the insect's body when the mosquito bites a person, and their life cycle begins again.
Treatment and prevention.
Physicians diagnose malaria by identifying Plasmodia in a sample of the patient's blood. Most cases can be cured by using two drugs, chloroquine and primaquine. Some varieties of P. falciparum resist treatment by these drugs. In such cases, physicians prescribe quinine, mefloquine, or halofautrine.
Chloroquine can prevent malaria in addition to curing it. In most cases, people who plan to travel in areas where they could be exposed to malaria should take chloroquine before, during, and after their trip.
Prevention of malaria also involves controlling the Anopheles mosquito. To do so, workers spray people's homes with insecticides. They drain, spray, or fill in bodies of stagnant water where the insects breed. People also use mosquito netting and insect repellents and put screens on windows and doors.
During the 1950's and 1960's, the World Health Organization (WHO) tried to wipe out malaria. At first, the widespread use of insecticides, particularly DDT, eliminated malaria in some areas and greatly reduced the number of cases in others. However, the fight against malaria slackened, and the number of cases increased again. Anopheles mosquitoes became resistant to DDT and other insecticides, and some Plasmodia became resistant to drugs. Also, the cost of fighting malaria increased greatly. These problems prompted researchers to step up efforts to develop a vaccine that could help eliminate the disease.
Contributor: Wasim A. Siddiqui, Ph.D., Former Professor of Tropical Medicine, University of Hawaii School of Medicine.
Source : World Book 2005