Biological Disorders: Some Parasitic Protozoa of Humans Help

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By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to Protists Creating Biological Disorder

Most of the billions of protist organisms dwelling with us on this planet are harmless, or even helpful, to humankind. Hard-working algae pump zillions of fresh O 2 molecules into our atmosphere every day, and even the lowly slime molds promote the decay of organic debris, so it does not accumulate to excessive levels.

Plasmodium - Infecting Protists

A relatively small number of protist species, however, are extremely dangerous to Homo sapiens, or introduce more Biological Disorder into the external environment. Consider, for example, malaria (mah- LAY -ree-ah) or “bad air.” Malaria is a major disease of the tropics, and it kills over 2 million people every year! The disease gets its name from the humid, sometimes unhealthy, air found in tropical regions. But the real cause of the disorder has nothing to do with poor air quality! Rather, malaria is a deadly infection of human red blood cells (RBCs) with parasitic protozoa injected by the bite of a mosquito carrier. The infecting protists belong to the genus Plasmodium (plaz- MOH -dee-um), which means “mold” (plasm) “shaped.” The Plasmodium organisms are actually parasitic protozoa that look somewhat like slime molds, in that part of their life cycle includes a stage that consists of a naked mass of cytoplasm containing many nuclei, but no cell membranes between them.

Plasmodium Parasites Life Cycle and Infection

Part of the life cycle and infection of human RBCs with Plasmodium parasites is diagrammed in Figure 7.5:

Step 1 - An infected mosquito bites a human, injecting a number of sporozoites ( spor -oh- ZOH -ites) – long, slender, tapered “seed animals” – into the victim’s bloodstream with its saliva.

Step 2 - Within a few hours, the sporozoites travel through the bloodstream and invade cells in the liver. For 1 or 2 weeks, the sporozoites keep growing and dividing and eventually develop into schizonts ( SKIZ -ahnts), literally “dividing” (schiz) “beings” (-onts). It is this schizont stage which gives rise to the name Plasmodium, because each schizont is a fairly large mass of cytoplasm containing many nuclei.

Step 3 - Each large schizont then lives up to its name by splitting or dividing and releasing thousands of smaller parasite cells out of the liver, and into the bloodstream.

Step 4 - Each smaller parasite cell invades a human RBC, where it eventually grows into another large schizont.

Step 5 - The schizont filling the red blood cell eats the RBC from within, finally getting so large that it causes the RBC to rupture and be destroyed.

Step 6 - The rupture of thousands of RBCs releases toxins (poisons) and more small parasite cells into the bloodstream, causing high fever, chills, and other potentially fatal health problems.

The Protists: “First of All” Biological Disorders: Some Parasitic Protozoa of Humans

Fig. 7.5 Malaria: Plasmodium protozoa invade the liver and RBCs.

Practice problems for these concepts can be found at:  The Protists: "First of All" Test

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