Protists And Health

Many protists are parasites. Table 25-3 summarizes the effects of three protist parasites.

Malaria

Apicomplexans from the genus Plasmodium cause malaria, which is characterized by severe chills, headache, fever, and fatigue. Each year, nearly 3 million people die from malaria. The four species of Plasmodium that infect humans all have life cycles involving an Anopheles mosquito. When an infected Anopheles mosquito bites a person, Plasmodium cells called sporozoites enter the bloodstream and infect the liver. New cells called merozoites emerge from the liver and infect red blood cells. At regular intervals, merozoites burst from red blood cells and release toxins, causing malarial symptoms. Without treatment, merozoites may remain in the liver for years, causing recurrent disease.

Some merozoites in the blood develop into the sexual forms of the parasite, gametocytes. When an Anopheles mosquito bites an infected person, the mosquito ingests these gametocytes. The gametocytes form gametes, which combine to form zygotes that develop into more sporozoites. When the insect bites another person, the life cycle begins again. Figure 25-20 shows the life cycle of Plasmodium.

Sexual reproduction Asexual reproduction w

As an infected mosquito bites a human, it injects A ^^ Plasmodium __/

sporozoites. The sporozoites infect the liver cells and develop into merozoites, which are released.

The gametes join inside the mosquito and produce sporozoites

Sexual reproduction Asexual reproduction

The gametes join inside the mosquito and produce sporozoites

Liver

Merozoites hour 0

cycle m\

The merozoites infect red blood cells, multiply, and infect new red blood cells.

Liver

Merozoites

Some of the merozoites develop into gametocytes. When the infected person is bitten by a mosquito, the gametocytes are taken up and form gametes.

hour 0

cycle m\

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