Tags: Malaria, Mosquitoes, Parasites
Malaria parasites have been with us since the beginning of time, and fossils of mosquitoes up to thirty million years old show that malaria’s vector has existed for just as long. The parasites causing malaria are highly specific, with man as the only host and mosquitoes as the only vector. Every year, 300,000,000 people are affected by malaria, and while less than one percent of these people die, there are still an estimated 1,500,000 deaths per year. While Malaria was one of the first infectious diseases to be treated successfully with a drug, scientist are still looking for a cure or at least a vaccination today (Cann, 1996). Though many people are aware that malaria is a disease, they are unaware that it is life threatening, kills over a million people each year, and is a very elusive target for antimalarial drugs (Treatment of Malaria, 1996).
Being a very specific disease, malaria is caused by only four protozoal parasites: Plasmodium falciparum, Plasmodium vivax, Plasmodium ovale, and Plasmodium malariae. Not only is the disease specific, but the parasites are too, with only 60 of 380 species of female Anopheles mosquitoes as vectors. With the exception of Plasmodia Malariae which may affect other primates, all parasites of malaria have only one host, Homo sapiens. Because some mosquitoes contain substances toxic to Plasmodium in their cells, not all species of mosquitoes are vectors of Plasmodium. Although very specific, malaria still causes disruption of over three hundred million people worldwide each year (Cann, 1996).
The life cycle of the parasite causing malaria exists between two organisms, humans and the Anopheles mosquito. When a female mosquito bites a human, she injects an anticoagulant saliva which keeps the human bleeding and ensures an even flowing meal for her. When the vector injects her saliva into the human, it also injects ten percent of her sporozoite load. Once in the bloodstream, the Plasmodium travel to the liver and reproduce by asexual reproduction. These liver cells then burst releasing the parasites back into the bloodstream where they then enter red blood cells. Here, the Plasmodium feed on hemoglobin and reproduce again by asexual reproduction. Afterwards, the red blood cells burst and release the parasites. Some of the parasites released from red blood cells may be able to replicate by sexual reproduction. When the host has been bitten by a mosquito again, infected blood inters the mosquito. Here, sexual forms of the parasite develop in the stomach of the Anopheles mosquito completing the parasites life cycle (Herman, 1996).
People infected malaria have several symptoms including fever, chills, headaches, weakness, and an enlarged spleen (Herman, 1996). The amount of time for symptoms to appear differs depending on the form of the parasite. Those infected with Plasmodium falciparum experience symptoms after about twenty-four hours, those infected with Plasmodium vivax and Plasmodium ovale produce symptoms after a forty-eight hour interval, and after seventy-two hours Plasmodium malariae begin causing fever and chills (Cann, 1996).
Most malaria cases seem to cluster in the tropical climate areas extending into the subtropics, and malaria is especially endemic in Africa. In 1990 eighty percent of all reported cases were in Africa, while the remainder of most cases came from nine countries: India, Brazil, Afghanistan, Sri-Lanka, Thailand, Indonesia, Vietnam, Cambodia, and China. Globally, the disease circulates in almost one hundred countries causing up to 1,500,000 deaths annually (Cann, 1996).
Because there is no definite cure for malaria, scientists are trying their hardest to contain the parasite to where it now exists. The range of a vector from a suitable habitat is fortunately limited to a maximum of two miles (Cann, 1996). If this were the only factor, scientist would have no problem containing the disease. Humans migrate, however, and over time the disease has slowly spread throughout the tropics. Major problems also exist when ignorant tourists to Africa transfer the parasite to non malarious areas (Graham, 1996). Biologists are also using control measures, such as spraying DDT to kill mosquitoes, draining stagnant water, and using the widespread use of nets to contain the mosquito itself (Herman, 1996). Because of the worsening situation, the World Health Organization (WHO) declared malaria control to be a global priority (Limited Imagination, 1996).
Although limiting the spread of malaria is not easy, finding a cure has presented several problems in recent years. One main reason finding a cure for malaria is so hard is that different strains in different parts of the world require different drugs, all of which soon lose their effectiveness as the parasite evolves resistance to them (Limited Imagination, 1996). Secondly, once the parasite enters the human bloodstream, it changes form several times inside the body, making it an elusive target for the immune system (Cann, 1996). Last, while research and development is very expensive, Africa’s third world countries don’t have the money to support such research (Graham, 1996).
Research in the field of malaria’s microbiology enables a search for better vaccines and a possible cure for malaria (Atovaquone, 1996). In the past several decades, scientists have developed many drugs that have all fallen victim to the resistance of the Plasmodium parasites. Such drugs include chloroquine, pyrimethamine, chloroguanide, desipramine, halofantrine, mefloquine, and arteether (Herman, 1996). Scientists too often find their drugs effectiveness wearing off as malarial parasites build tolerance to them (Graham, 1996).
Several drugs used to treat the disease have been around for centuries. One such drug is quinine, a compound extracted from the bark of the cinchona tree. This drug was a secret of the locals of the Amazon jungle for centuries until European missionaries learned of its use. The trouble remains that quinine is expensive to harvest, is extremely hard to synthesize, and fails to prevent relapses (Limited Imagination, 1996). Another unique treatment of malaria is the use of the herb Artemisia annua. This herb has been used for centuries in traditional Chinese medicine to treat malaria and fever. Neither of these drugs are one hundred percent effective (Herman, 1996).
While the need for malarial vaccines grows urgent, so does the number of people affected each year. Although it is caused by a highly specific parasite, malaria still seems to kill off between one to two million people annually. As the Plasmodium parasites mutate more and more to resist the effect of antimalarials, it becomes harder for scientist to find a cure (Treatment of Malaria, 1996). Over forty percent of the world’s population still at risk from this deadly disease, is yearning for a cheap, effective vaccine (Cann, 1996).
Dr. Cann, Alan J. PhD., “The Walter and Eliza Hall Institute Malaria Database”, 1996, http://www.wehi.edu.au/biology/malaria/who.html.
Graham, David, “Malaria-Proof Mosquitoes,” Technology Review, October 1996, Vol. 99, Issue 7, p20-22, MAS FullTEXT ELITE, Nancy Guinn Library.
Herman, Robert, “Malaria,” New Groliers Multimedia Encyclopedia, Copywrite 1996.
“Atovaquone and Proguanil for Plasmodium Falciparum Malaria,” Lancet, June 1, 1996, Vol. 347, Issue 9014, p1511-1515, MAS FullTEXT ELITE, Nancy Guinn Library.
“Limited Imagination,” Economist, September 28, 1996, Vol. 340, Issue 7985, p80-82, MAS FullTEXT ELITE, Nance Guinn Library.
“Treatment of Malaria,” New England Journal of Medicine, September 12, 1996, Vol. 335, Issue 11, p800-807, MAS FullTEXT ELITE, Nancy Guinn Library.