The complex life cycle of the Plasmodium protozoan, the causative agen

The complex life cycle of the Plasmodium protozoan, the causative agent of malaria, has contributed to the difficulty of devising effective public health measures to combat the disease. It took scientists centuries to deconstruct the basic relationship between protozoan, mosquito vector, and human host. Modern physiologists and epidemiologists are still working out the intricacies of malarial infection.

The disease is transmitted by the bite of a female Anopheles mosquito infected with the Plasmodium parasite. Only Anopheles mosquitoes are capable of transmitting the disease, and only females take blood meals from humans. To become infected with Plasmodium, the female mosquito takes a blood meal from a human carrying the parasite in his or her blood. Once ingested, the parasite matures in the mosquito’s gut for approximately a week, after which it migrates to the insect’s salivary glands. By mixing with the mosquito’s saliva, the parasite facilitates its transmission to a human host when the mosquito bites that human.

Once in a human’s bloodstream, the parasite travels to the human’s liver. At this initial stage, the Plasmodium parasite is called a sporozoite. Within the liver, the sporozoite can form 30,000 to 40,000 daughter cells, called merozoites, which are released into the host’s bloodstream at a later date, sometimes within a week of the initial infection and sometimes as much as several months later. The merozoites seek out and attach themselves to red blood cells, in which they incubate 8 to 24 daughter cells over the next two days. When the daughter cells are mature, the red blood cell ruptures and the new parasites are released into the bloodstream to seek out red blood cells of their own.

Some of the new merozoites become male and female gametocytes; if these gametocytes are ingested by a mosquito feeding on the host’s blood, they will fertilize in the mosquito’s gut to produce new sporozoites, and the cycle will continue. The symptoms that we associate with malaria—a high, recurring fever; joint pain; a swollen spleen are caused by toxins released from the red blood cells ruptured by merozoites. The human spleen can destroy these infected blood cells, but the Plasmodium parasite counters this effect by increasing the stickiness of proteins on the blood cells’ surfaces so that the cells stick to the walls of blood vessels. If the sticky surface proteins affect a particularly large number of cells, the malaria can transform into a hemorrhagic fever, the most deadly form of malaria.

A further complicating factor in the natural history of malaria is the many variants of the Plasmodium protozoan. Scientists now recognize that malaria is caused by at least six different species: P. falciparum, P. vivax, P. ovale, P. malariae, P. knowesli, and P. semiovale. Of these species, P. falciparum accounts for the majority of infections and approximately 90 percent of malarial deaths in the world.

1. The passage is primarily concerned with which of the following?

A. Describing the life cycle of the Plasmodium protozoan as it relates to the disease malaria
B. Comparing and contrasting the life cycles of the six variants of the Plasmodium protozoan known to cause malaria
C. Addressing the public health implications of the life cycle of the Plasmodium parasite
D. Providing information on how a person can avoid infection with malaria
E. Describing the life cycle of the Anopheles mosquito as it relates to the transmission of the Plasmodium protozoan to humans

2 . Which of the following most accurately states the role of the first paragraph in relation to the passage as a whole?

A. It summarizes two theories, the relative merits of which are debated in the passage.
B. It puts forth an argument that the rest of the passage is devoted to refuting.
C. It introduces a new concept that the rest of the passage expands upon.
D. It frames the background and relevance of the material to follow.
E. It outlines the major themes of each of the four paragraphs to follow.

3. If a mosquito were to bite a person, and that person were later to develop malaria and die of the disease, it is most likely that the person was infected with which of the following?

A. Anopheles gambiae
B. Anopheles semiovale
C. Plasmodium malariae
D. Plasmodium vivax
E. Plasmodium falciparum

4. The relationship of a merozoite to a sporozoite is most like which of the following?

A. A mother to a daughter
B. A brother to a sister
C. One of several subsidiaries spun off from a large corporation
D. A computer program to a computer
E. Orange juice to an orange tree

5. Based on the information given in the passage, which of the following would be most effective in preventing a person infected with malaria from developing a hemorrhagic fever?

A. Surgical removal of the spleen
B. A medicine that prevents changes to the surface proteins of red blood cells
C. An effective vaccine against malaria
D. A potent pesticide that reliably kills the Anopheles mosquito without producing any negative consequences for the environment or for human health
E. A small infusion of a weaker variant of the Plasmodium protozoan that will then compete with the existing parasitic infection