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B io Factsheet
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Number 206
Malaria
This Factsheet summarises:
• The cause and means of transmission of malaria.
• The worldwide distribution and importance of malaria.
• The social, economic and biological factors significant in the
prevention and control of malaria.
Definitions
Malaria is an infectious disease that threatens over one third of the
world’s population. Up to 124 million people in Africa alone live in
areas at risk of seasonal epidemics. This year, malaria is expected to
kill one million individuals, 90% of them under age five, and the vast
majority living in Africa.
Host: An organism inside which the reproduction of another
organism occurs. Humans act as a host for the malarial parasite.
Vector: An organism responsible for the transmission or spread
of a pathogen. In the case of malaria, the female mosquito is the
vector as it transmits the Plasmodium parasite from human to
human.
Asexual reproduction: Reproduction with just one parent
organism. Offspring are genetically identical, and are produced
rapidly and in large numbers.
What causes malaria?
Sexual reproduction: Requires the fertilization of a male and
female gamete to form a zygote. Offspring contain a combination
of the genes of both gametes.
Malaria is caused by 4 different species of the protoctist parasite
Plasmodium. It is spread from human to human by female Anopheles
mosquitoes as they feed on blood to obtain the protein they need
to produce their eggs.
Transmission of Malaria
1. Mosquito bites usually occur in the early evening. As the infected mosquito feeds, it injects saliva containing an anticoagulant to
prevent the blood from clotting.
2. Malarial parasites known as sporozoites are injected along with the saliva and enter the human bloodstream where they migrate to the
liver. The mosquito is therefore said to act as a vector of the malarial parasite.
3. In the liver cells the sporozoites multiply asexually, very rapidly increasing in number.
4. The liver cells burst open releasing many merozoites, which then invade the red blood cells.
5. These merozoites reproduce asexually again inside the red blood cells, causing the red blood cells to burst, releasing more merozoites
which cause the characteristic fever and other symptoms of the disease.
6. Some of these merozoites develop into gametocytes (the male and female forms of the parasite) which are ingested by the female
mosquito during feeding, and so complete the cycle of transmission between human and mosquito.
7. Once inside the female mosquito’s gut, the male and female gametocytes fuse to form sporozoites which migrate to the salivary glands
ready to be injected into a new human host at the next blood meal.
The diagram below summarizes the life cycle of the malarial parasite and is typical of many exam questions.
Uninfected female mosquito feeds in
the early evening and takes in mature
sex cells with the blood
Infected female mosquito feeds in the
early evening and injetcs malarial
parasites into the blood stream
skin surface
mature sex cells
malarial parasites
After 28 hours sex cells have matured
blood system
of human
liver cell
Immature sex cells
malarial parasite
Malarial parasite rapidly enters liver cell and
changes form. New form ruptures liver cell, escapes
into blood stream and infects red blood cells
red blood cells
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Every 48 hours, infected red blood
cells burst, releasing more parasites
and immature sex cells
Bio Factsheet
206 Malaria
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Controlling the Spread of Malaria
Exam Hint: - Examiners often ask candidates to distinguish
between the stage of sexual reproduction in the female
mosquito, and the asexual stages in the liver and red blood
cells of the human host, and to outline the advantages of
each. Asexual reproduction is much more rapid and produces
large numbers of offspring in a short space of time.
At present, there is no vaccine for malaria, so prevention and control
measures mainly focus on disrupting the breeding cycle of the
mosquito in order to reduce the risk of mosquito bites. The female
mosquito lays her eggs in the water where the larvae and pupae
develop.
• Draining swamps, marshes and other exposed areas of water
removes potential breeding places for mosquitoes.
Typical Exam Questions
1. Explain the importance of the female mosquito in the life
cycle of the malarial parasite.
2. Explain why an infected person may feel feverish every
two days in the early stages of malaria.
3. Describe and explain how Plasmodium is adapted to its
way of life.
Answers
1. Female acts as a vector/ transmits the parasite to humans;
Fertilization of male and female gametocytes occurs in the
female mosquito/ where sexual stage of life cycle occurs;
2. Infected red blood cells burst every 48 hours releasing more
parasites into the bloodstream;
3. Has no locomotory structures because it is transported via
the blood stream in humans and uses two hosts to transfer
it from one stage to the next;
Does not need an attachment mechanism as penetrates
human host using mosquito vector;
Has no need for a mechanism for regulating its water content
because it lives inside cells;
Avoids attack by host immune system by living inside host
cells;
Complex life cycle with several stages aids immune evasion;
Sexual reproduction phase of life cycle allows for antigenic
variation;
Asexual reproduction phase of life cycle allows production
of large numbers of offspring;
Sprays containing the bacterium Bacillus thuringiensis can be
used which kills mosquito larvae but is not toxic to other species.
•
Biological control by stocking permanent bodies of water with
fish which feed on mosquito larvae.
•
Sleeping beneath mosquito nets which have been soaked in
insecticides, and the use of insect repellents on the skin.
•
Keeping skin covered at dusk when mosquitoes are most likely
to bite.
•
Sleeping with a dog or a pig! Local wisdom in New Guinea
advises that mosquitoes much prefer animal blood to human
blood.
Mosquitoes cross national boundaries and so control programs
must be co-coordinated between nations in order to be successful.
The more recent problems of insecticide-resistant mosquitoes and
drug-resistant strains of Plasmodium also hinder prevention and
control.
Many candidates’ knowledge of the malarial parasite’s life cycle
is insufficient
Some candidates do not show that they understand the need
for rapid, asexual reproduction to produce sufficient numbers
of parasites to ensure transmission to another host
Few candidates mention the importance of the anticoagulant
Some candidates are unsure of the sequence of stages within
the human
Exam Question
The maps below show the pattern of annual rainfall in Africa and the incidence of malaria.
Incidence of malaria
Annual rainfall
Describe and explain the relationship
between annual rainfall in Africa and the
incidence of malaria.
/cm
Under 25
25-50
50-100
100-150
Over 150
No malaria
Low incidence
Medium incidence
High incidence
Answer
•
•
•
Correlation between the incidence of malaria and
rainfall;
Malaria is spread by mosquitoes that breed in
water;
Rainfall causes puddles/pools/more breeding
grounds;
•
Spreading oil over the surfaces of bodies of water suffocates
the mosquito larvae and pupae by preventing them from coming
to the surface to obtain oxygen.
Many of these measures, however, are not always successful.
Mosquitoes will breed in the tiniest bodies of water, such as puddles,
making it difficult to treat or eliminate breeding places entirely. Other
measures, such as mosquito nets and insecticides are expensive
and logistically difficult to implement in remote areas. Civil war and
unrest, along with poor infrastructure, prevents access to many
endemic regions.
Where do Students go wrong?
•
•
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Bio Factsheet
206 Malaria
www.curriculum-press.co.uk
Anti-Malarial Drugs and Resistance
In addition to the above measures, anti-malarial drugs such as quinine have been in use for many years. Modern anti-malarial drugs are
often used as prophylaxis by tourists traveling to regions where malaria is endemic, to prevent infection occurring. These drugs can
also be used to improve the condition of individuals who are already infected, reducing the number of parasites within their bodies and
hence reducing the likelihood of the disease being passed on to others.
Anti-malarial drugs, however, have drawbacks:
• They are too expensive to be used in many developing countries where malaria is endemic.
• Malarial parasites have developed resistance to the drugs, especially chloroquine. At present, there are some regions where no antimalarial drugs are effective due to the problem of resistance.
Insecticides, Resistance and the DDT Story
Malaria and Sickle Cell Anaemia
In the 1950s, the World Health Organisation co-coordinated a
worldwide program for the eradication of malaria. The program failed
for two main reasons:
1. The emergence of drug-resistant strains of Plasmodium.
2. The emergence of insecticide-resistant strains of Anopheles
mosquitoes.
Sickle cell anaemia is a lethal condition in which the presence of the
mutant allele HbS in place of the normal allele HbA results in an
amino acid substitution at a critical position in the haemoglobin
molecule. This causes the red blood cells to have a characteristic
sickle shape, which leads to blockage of capillaries and severe
anaemia. The frequency of the HbS allele is correlated with the
distribution of malaria, being particularly prevalent in West Africa.
This is because heterozygotes who are said to show sickle cell
‘trait’, have sufficient amounts of normal haemoglobin to prevent
severe anaemia, however, the small percentage of sickled red blood
cells confers an increased resistance to malaria giving them a
selective advantage over ‘normal’ individuals as they less likely to
die from malaria in childhood.
In addition, the program itself became very unpopular with local
communities. When constantly re-infected with malaria, individuals
acquire resistance to infection that is lost if they are no longer in
contact with the disease. The temporary eradication of malaria in
some areas during the program caused locals to lose their immunity
with many deaths and much suffering when the disease returned.
Vaccination
The diagram below shows the distribution of malaria and the sickle
cell allele and is typical of many exam questions.
Given the problems of drug resistance, the approach most likely to
be successful in controlling malaria is the development of a vaccine.
However, the production of a vaccine against malaria has so far
proved difficult for a number of reasons:
• Plasmodium is a protoctists parasite, therefore much larger and
much more complex than viruses and bacteria. It has many surface
antigens which differ between the several species of parasite
that can cause malaria.
• In order to survive inside the human host, Plasmodium evades
the immune response by quickly entering and remaining
“hidden” inside either red blood cells or liver cells for much of
its time (antigen concealment).
• Once inside host cells, antigens from the parasite are displayed
on the cell surface to aid immune detection. Plasmodium can
mutate and vary these antigens so that the immune system no
longer recognizes them (antigenic drift).
• Different stages in the life cycle display different proteins, and
so a vaccine would ideally target multiple stages in order to be
effective.
Sickle cell allele
Malaria
Typical exam Question
Typical Exam Question
Explain why the sickle cell allele occurs at such high
frequencies in some areas.
Outline the difficulties associated with controlling the spread
of malaria.
Answer
1. Resistance of mosquitoes to insecticides such as DDT.
2. Difficulty in controlling breeding of mosquitoes as they can lay eggs
in very small bodies of water.
3. Mosquitoes can quickly travel long distances making control difficult.
4. Mosquito control programs have been disrupted by war, poor
infrastructure and lack of funds.
5. Resistance of Plasmodium to anti-malarial drugs such as chloroquine.
6. Large number of surface antigens/ antigenic variation and antigenic
concealment within red blood cells and liver cells make it difficult for
immune system to detect.
7. Different stages of the life cycle in the body make it difficult for
immune system to operate.
8. No vaccine.
9. Poor primary health care makes detection and treatment difficult.
10. Lack of education over preventing mosquito bites.
1. Individuals homozygous for sickle cell allele die from sickle
cell anaemia.
2. Individuals homozygous for normal allele are likely to die
from malaria in childhood.
3. Sickle cell allele frequent in malarial areas.
4. Heterozygotes have a selective advantage because they are
resistant to malaria.
5. So are more likely to survive and pass on sickle cell allele.
Acknowledgements:
This Factsheet was researched and written by Katrina Fox.
Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.
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