Malaria: A Nasty Parasite
A Nasty Parasite
Charles Lavern, a French army surgeon in Algeria in 1880, first described malaria parasites in the red blood cells of humans. Several years later, Sir Ronald Ross observed developing parasites in the intestines of mosquitoes and provided the first major evidence that mosquitoes were acting as vectors, or vehicles, to spread disease. Ross succeeded in demonstrating the life cycle of the parasites of malaria in mosquitoes.
Human malaria is caused by four main species of the Plasmodium parasite:
- P. falciparum: the most important of the malaria parasites because it can be rapidly fatal and is responsible for the majority of malaria-related deaths. It predominates in Africa, New Guinea, and Haiti.
- P. malariae: found occasionally in endemic areas like sub-Saharan Africa.
- P. vivax: more common on the Indian sub-continent and Central America, with the prevalence of these two infections roughly equal in Asia, Oceania, and South America.
- P. ovale: mostly confined to Africa, although sporadic cases occur in Southern India.
A vector is a vehicle for moving a disease-causing organism from one host to another. Mosquitoes were vectors for malaria by helping to spread it from human to human.
A disease is endemic when it is constantly present in a community or among a group of people.
Malaria kills one child every 30 seconds. A total of 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. The disease is also responsible for a substantial number of miscarriages and low-birth-weight babies.
Transmission: Mosquito Marauder and a Deadly Cycle
Malaria parasites are transmitted by the female anopheles mosquito. There are about 380 species of this type of mosquito, but only about 60 species can transmit disease. The parasites can only live within female mosquitoes and can be transferred only to humans. The parasites have a complex life cycle that is split between the human host and the mosquito vector. The process of malaria transmission occurs this way:
- A female mosquito harboring malaria feeds on human blood and transmits threadlike structures, called sporozoites, to the human.
- The sporozoites travel to the liver and multiply. They mature over two to four weeks without causing disease symptoms.
- The mature sporozoites, called merozoites, are released into the bloodstream, where they penetrate red blood cells and multiply and break down hemoglobin, which is essential for oxygen transport.
- The blood cells degrade, and the merozoites escape and infect other blood cells. This induces bouts of fever, chills, sweating, and anemia in the infected individual. The infected red cells can obstruct blood vessels in the brain (which is called cerebral malaria) or other vital organs, leading to the death of the patient.
- A few parasites form a sexual stage, which can be sucked up by another mosquito taking a blood meal, beginning a new transmission cycle.
- Two sexually active parasites meet in the mosquito's gut and produce a new generation.
A sporozoite is a slender, spindle-shaped organism that is the infective stage of the malaria parasite. It is the result of the sexual reproductive cycle of the parasite, which occurs inside the mosquito.
Malaria is not common in the United States, but people who travel to parts of the world where it is endemic need to protect themselves by taking antimalarial medication and trying to avoid mosquito bites by using repellent, netting, and so on.
The parasites must spend about two weeks in the mosquito to undergo further life cycle changes before they can infect humans again. When the mosquito feeds on another human, the parasites are injected into a new host. This mosquito can transmit the infection only if she sucks more blood from an uninfected person before she dies.
Excerpted from The Complete Idiot's Guide to Dangerous Diseases and Epidemics © 2002 by David Perlin, Ph.D., and Ann Cohen. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.