3.1.5Infection and Response

Protist Diseases

5 min read 9 parts Lesson 6 of 10 Unit quiz

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Section 1 of 9Protists as pathogens

Protists are a varied group of mostly single-celled eukaryotic organisms.

A few are parasites, living in or on a host and harming it.

Vector

Many protist pathogens are carried from one host to another by a vector, an organism that transfers the pathogen but does not usually get the disease itself.

Malaria

The disease you must know for GCSE is malaria.

Section 2 of 9Malaria and the life cycle

Malaria is caused by a protist called Plasmodium. The vector is the female Anopheles mosquito, which needs a blood meal to develop her eggs. The cycle of infection is:

A mosquito bites a person who has malaria and takes in blood that contains Plasmodium.

The protist develops inside the mosquito.

The mosquito bites another person and passes the protist into their blood with her saliva.

The protist infects the liver and then red blood cells, where it multiplies and bursts out of the cells.

The infection of red blood cells causes the symptoms: repeated episodes of fever with shivering and sweating, which happen each time more protists burst out of the cells.

Malaria and the life cycle

Malaria can cause anaemia, damage to the liver and spleen and, in severe cases, death.

It is most common in tropical regions,

where mosquitoes breed all year in warm, wet conditions,
and it is a major cause of death in young children in parts of Africa.

Section 3 of 9Controlling malaria

Controlling malaria means stopping people from being bitten and stopping mosquitoes from breeding, and treating people who are infected. Methods include:

Insecticides

Using insecticides to kill mosquitoes, and spraying inside houses.

Draining or covering stagnant water, or putting oil on it, to prevent the mosquito larvae from developing, because mosquito larvae live in water.

Introducing fish that eat mosquito larvae into ponds.

Mosquito nets

Sleeping under mosquito nets, often treated with insecticide, as the mosquitoes bite at night.

Using insect repellents and wearing long sleeves.

Antimalarial drugs

Taking antimalarial drugs before and during travel to areas with malaria, to prevent infection, and treating patients with drugs.

Section 4 of 9Challenges

Controlling malaria is difficult.

The protist has become resistant to some antimalarial drugs and mosquitoes have become resistant to some insecticides.

A vaccine has been hard to develop because the protist has a complicated life cycle and changes its antigens.

Insecticide and nets are costly for poor communities, and draining water everywhere is difficult.

Climate change may allow mosquitoes to spread to new regions.

A combination of different measures is more effective than any single one.

Section 5 of 9Graph and data interpretation

Data questions may show the number of malaria cases before and after the introduction of nets.

Graph and data interpretation

To interpret them, describe the change in numbers with figures, calculate a percentage change,

and suggest that other factors,
such as rainfall, could also affect the number of mosquitoes.

A fall in cases after the introduction of nets suggests, but does not prove, that nets were responsible.

Compare areas using nets with areas without them to make a stronger case.

Section 6 of 9Why the protist is hard to eliminate

Plasmodium spends part of its life cycle inside human liver cells and red blood cells, where the immune system and many drugs find it hard to reach.

It also exists in the mosquito, so there are two hosts to deal with.

Where mosquitoes are common, people can be bitten again and again, so even if the disease is treated, infection recurs.

For this reason, controlling the mosquito vector is often more effective than treating each patient.

Programmes that combine nets, spraying and drugs have greatly reduced deaths in some countries, showing that a combination of methods can work.

Researchers continue to develop vaccines and new drugs, and also genetic methods to reduce mosquito numbers.

If asked why a control method works, state what it stops: nets stop contact between vector and person, drainage stops breeding, drugs kill the protist in the blood. A mark is usually given for naming the stage in the life cycle that is broken.

Comparing malaria with other pathogen types

Unlike bacterial food poisoning, which spreads through food, or measles, which spreads through the air, malaria needs a living vector and cannot pass directly between people.

Why the protist is hard to eliminate

This makes control quite different: the weakest point in the cycle is the mosquito, not the patient.

Notice also that the pathogen is eukaryotic, so some drugs that attack it also affect human cells, which limits the choice of safe medicines.

Section 7 of 9Exam-style questions

Explain why malaria is described as a disease with a vector. [2]2 marks
Suggest why mosquito nets are used at night to control malaria. [2]2 marks
In a village 400 people had malaria. After using nets for a year this fell to 260. Calculate the percentage decrease. [2]2 marks

Section 8 of 9Key terms

Term	Meaning
Protist	A single-celled eukaryotic organism, some of which are pathogens.
Malaria	A disease caused by Plasmodium and spread by mosquitoes.
Vector	An organism that carries a pathogen from one host to another.
Plasmodium	The protist that causes malaria.
Insecticide	A chemical that kills insects.
Antimalarial drug	A drug used to prevent or treat malaria.
Parasite	An organism that lives on or in another and harms it.

Section 9 of 9Exam technique

Saying that mosquitoes cause malaria

the protist does;
- the mosquito is only the vector.

Believing that antibiotics treat malariaantimalarial drugs are used because the pathogen is a protist.

Saying that all mosquitoes spread malariaonly female Anopheles do.

Writing that malaria spreads directly from person to person by contact.