3.6  Malaria and other protist diseases

The pathogenic protists that infect humans are all single-celled organisms, formerly called ‘protozoa’. They are responsible for a range of diseases, including:

• dysentery (bloody diarrhoea) caused by waterborne protists similar to the amoebae [amm-ee-bee] commonly found in freshwater ponds
• sleeping sickness, caused by protists transmitted via the bite of tsetse flies
• leishmaniasis [leesh-man-eye-ah-sis] transmitted by biting sandflies; Leishmania protists cause painful lesions in the skin, affecting around one million people (Figure 10a), or potentially fatal enlargement of the liver and spleen with up to 400 000 cases annually (Figure 10b).

Figure 10  Signs of infection with Leishmania protists: (a) the skin lesion on this woman’s cheek; and (b) this boy’s enlarged abdomen and wasted upper body.

The following activity describes the life cycle, impact and prevention strategies against the most widespread infectious disease caused by a protist – malaria.

Activity 3  Malaria: a vector-borne protist infection

Timing: Allow about 10 minutes for this activity.

The protists that cause malaria belong to the genus Plasmodium [plazz-moh-dee-umm]. Study their life cycle in Figure 35 and then answer the questions below the diagram.

Figure 11  Life cycle of the protists that cause malaria. Details of the developmental stages in humans and mosquitoes have been simplified.

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This is as a pictorial life cycle diagram of the malarial protist with brief descriptions of each stage in the cycle. When an infected mosquito bites a person to take a blood meal, mature protists, present in the mosquito’s saliva, get into that person’s bloodstream. They travel in the bloodstream to the liver, enter the liver cells and divide to produce many immature forms. These immature protists invade red blood cells and reproduce themselves. Protists are shed from the RBC and go on to invade further RBCs so their numbers increase rapidly. A mosquito that takes a blood meal from the infected person therefore picks up immature protists. These mature in the mosquito’s gut, and when the mosquito takes its next blood meal from another person it passes on the mature protists to that person, completing the cycle.

Figure 11  Life cycle of the protists that cause malaria. Details of the developmental stages in humans and mosquitoes have been ...

Question 1

How are the protists that cause malaria transmitted to a new human host?

Answer

When an infected mosquito bites someone to take a blood meal, protists in the mosquito’s saliva get into the person’s bloodstream.

Question 2

How do the protists get into the mosquito’s saliva?

Answer

When the mosquito sucks blood from an infected person or animal, the protists are drawn into the mosquito’s gut from there they migrate to its salivary glands.

Question 3

Which human cells are routinely invaded by malaria protists?

Answer

Liver cells and red blood cells. (The protists are less than 5 micrometres (µm) in diameter at this stage – small enough to get into a red blood cell.)