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Blood (Intracellular) and Other Tissue Protozoa 85

Morphology
Source: Centers for Disease Control and Prevention (CDC) birds. Plasmodium vivax is found in temperate regions
Humans are infected by only four species of malaria, al-
though other species are found in other mammals and

and is the most widely distributed and numerous in the
cases found worldwide where climactic and geographic
features favorable for the breeding of certain species
of mosquitoes are found. P. falciparum is found in the
tropics and subtropics predominantly and is the most se-
rious of the four species affecting humans. P. malariae
infections are disseminated throughout the tropics and
subtropics, whereas P. ovale is mostly found in tropical
FIGURE 4-5 Thin-film Giemsa-stained smear with West Africa, South America, and Asia.
ring-form of Plasmodium falciparum trophozoite
Symptoms

seasons where mosquitoes were less prevalent in cold or Symptoms initially mimic those of minor febrile illnesses
dry weather and that the transmission rates for malaria accompanied by general malaise, muscle aches with fa-
diminishes during these drier periods. But it seems that tigue, headache, abdominal aches, and cycles of fever and
many other inventive explanations arose to rationalize chills. Three stages of fever characteristically occur, first
the spread of the disease in terms of the poisons rising with shaking and chills, followed by the second or hot
from the swamps. The term miasma, which meant in- stage where a small amount of sweating occurs and the
°
fectious particles or germs were floating in the air made body’s temperature may rise to levels as high as 106 F.
noxious by the presence of such particles or germs, were After several hours of the hot stage where brain damage
freely associated with the incidence of malaria at the time may occur, the patient may experience tachycardia (rapid
these discoveries were occurring. heart rate), cough, head and backache with abdominal
Toward the end of the nineteenth century, the germ pain, vomiting, and diarrhea. The third sweating stage
theory was established that led to the birth of the science then begins within a few hours as the fever subsides and
called microbiology. This basic scientific understanding sleep ensues. Hemolytic anemia is associated with the
of invisible life forms that contributed to a number of P. falciparum infection due to destruction of infected
infectious diseases was embraced and advanced by con- red blood cells.
tributions attributed to a considerable number of medi-
cal and scientific pioneers from a number of countries. Life Cycle
Malaria exacted such a toll over several thousand years
on the human population that it became a primary goal The life cycle is a very complex one that begins when
of scientists during the nineteenth century to discover the an infected Anopheles mosquito injects sporozoites, the
cause of the disease that was by then threatening many infectious stages, into the blood of its host (Figure 4-6).
parts of the European empires. Discovery of the origin of Sporozoites enter and multiply in liver cells, and thou-
the malaria parasite and how it was transmitted are among sands of daughter forms, merozoites, are released into the
the most important and historic events in the annals of in- blood. These merozoites invade red blood cells, in which
fectious diseases. In addition to treating the disease that another phase of multiplication occurs; this process is re-
resulted from infection by the Plasmodium parasite, con- peated indefinitely, causing the symptoms of the disease
trol of the vector, a species of mosquito, led to a decrease we call malaria. Some merozoites do not divide but de-
in malaria in the more developed areas of the world, as velop into sexual stages, that of male and female gameto-
is the case in Europe. Malaria still rages in some areas of cytes, which are taken up by another mosquito when it
the world, particularly Southeast Asia and portions of the feeds. Then fertilization and zygote formation occur in
African continent, despite global efforts to control the im- the mosquito’s gut. The zygote develops into an oocyst on
pact of this disease on indigenous populations. the outside of the mosquito’s gut, and within the oocyst