Bio Factsheet

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MalariaThis 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.

Malaria is an infectious disease that threatens over one third of theworld’s population. Up to 124 million people in Africa alone live inareas at risk of seasonal epidemics. This year, malaria is expected tokill one million individuals, 90% of them under age five, and the vastmajority living in Africa.

What causes malaria?Malaria is caused by 4 different species of the protoctist parasitePlasmodium. It is spread from human to human by female Anophelesmosquitoes as they feed on blood to obtain the protein they needto produce their eggs.

DefinitionsVector: An organism responsible for the transmission or spreadof a pathogen. In the case of malaria, the female mosquito is thevector as it transmits the Plasmodium parasite from human tohuman.

Host: An organism inside which the reproduction of anotherorganism occurs. Humans act as a host for the malarial parasite.

Asexual reproduction: Reproduction with just one parentorganism. Offspring are genetically identical, and are producedrapidly and in large numbers.

Sexual reproduction: Requires the fertilization of a male andfemale gamete to form a zygote. Offspring contain a combinationof the genes of both gametes.

Transmission of Malaria1. 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 inthe early evening and takes in maturesex cells with the blood

After 28 hours sex cells have matured

mature sex cells

Immature sex cells

Every 48 hours, infected red bloodcells burst, releasing more parasitesand immature sex cells

red blood cellsMalarial parasite rapidly enters liver cell andchanges form. New form ruptures liver cell, escapesinto blood stream and infects red blood cells

liver cell

blood systemof human

malarial parasites

skin surface

malarial parasite

Infected female mosquito feeds in theearly evening and injetcs malarialparasites into the blood stream

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Exam Hint: - Examiners often ask candidates to distinguishbetween the stage of sexual reproduction in the femalemosquito, and the asexual stages in the liver and red bloodcells of the human host, and to outline the advantages ofeach. Asexual reproduction is much more rapid and produceslarge numbers of offspring in a short space of time.

Where do Students go wrong?• 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 numbersof 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

Typical Exam Questions1. 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.Answers1. Female acts as a vector/ transmits the parasite to humans;

Fertilization of male and female gametocytes occurs in thefemale mosquito/ where sexual stage of life cycle occurs;

2. Infected red blood cells burst every 48 hours releasing moreparasites into the bloodstream;

3. Has no locomotory structures because it is transported viathe blood stream in humans and uses two hosts to transferit from one stage to the next;Does not need an attachment mechanism as penetrateshuman host using mosquito vector; Has no need for a mechanism for regulating its water contentbecause it lives inside cells;Avoids attack by host immune system by living inside hostcells;Complex life cycle with several stages aids immune evasion;Sexual reproduction phase of life cycle allows for antigenicvariation;Asexual reproduction phase of life cycle allows productionof large numbers of offspring;

Exam QuestionThe maps below show the pattern of annual rainfall in Africa and the incidence of malaria.

AnswerCorrelation between the incidence of malaria andrainfall;Malaria is spread by mosquitoes that breed inwater;Rainfall causes puddles/pools/more breedinggrounds;

Describe and explain the relationshipbetween annual rainfall in Africa and theincidence of malaria.

No malariaLow incidenceMedium incidenceHigh incidence

Incidence of malaria

Under 2525-5050-100100-150Over 150

Annual rainfall

/cm

Controlling the Spread of MalariaAt present, there is no vaccine for malaria, so prevention and controlmeasures mainly focus on disrupting the breeding cycle of themosquito in order to reduce the risk of mosquito bites. The femalemosquito lays her eggs in the water where the larvae and pupaedevelop.• Draining swamps, marshes and other exposed areas of water

removes potential breeding places for mosquitoes.

• Spreading oil over the surfaces of bodies of water suffocatesthe mosquito larvae and pupae by preventing them from comingto the surface to obtain oxygen.

• Sprays containing the bacterium Bacillus thuringiensis can beused which kills mosquito larvae but is not toxic to other species.

• Biological control by stocking permanent bodies of water withfish which feed on mosquito larvae.

• Sleeping beneath mosquito nets which have been soaked ininsecticides, and the use of insect repellents on the skin.

• Keeping skin covered at dusk when mosquitoes are most likelyto bite.

• Sleeping with a dog or a pig! Local wisdom in New Guineaadvises that mosquitoes much prefer animal blood to humanblood.

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. Othermeasures, such as mosquito nets and insecticides are expensiveand logistically difficult to implement in remote areas. Civil war andunrest, along with poor infrastructure, prevents access to manyendemic regions.

Mosquitoes cross national boundaries and so control programsmust be co-coordinated between nations in order to be successful.The more recent problems of insecticide-resistant mosquitoes anddrug-resistant strains of Plasmodium also hinder prevention andcontrol.

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Anti-Malarial Drugs and ResistanceIn addition to the above measures, anti-malarial drugs such as quinine have been in use for many years. Modern anti-malarial drugs areoften used as prophylaxis by tourists traveling to regions where malaria is endemic, to prevent infection occurring. These drugs canalso be used to improve the condition of individuals who are already infected, reducing the number of parasites within their bodies andhence 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 anti-

malarial drugs are effective due to the problem of resistance.

Insecticides, Resistance and the DDT StoryIn the 1950s, the World Health Organisation co-coordinated aworldwide program for the eradication of malaria. The program failedfor two main reasons:1. The emergence of drug-resistant strains of Plasmodium.2. The emergence of insecticide-resistant strains of Anopheles

mosquitoes.

In addition, the program itself became very unpopular with localcommunities. When constantly re-infected with malaria, individualsacquire resistance to infection that is lost if they are no longer incontact with the disease. The temporary eradication of malaria insome areas during the program caused locals to lose their immunitywith many deaths and much suffering when the disease returned.

VaccinationGiven the problems of drug resistance, the approach most likely tobe successful in controlling malaria is the development of a vaccine.However, the production of a vaccine against malaria has so farproved 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 surfaceantigens which differ between the several species of parasitethat can cause malaria.

• In order to survive inside the human host, Plasmodium evadesthe immune response by quickly entering and remaining“hidden” inside either red blood cells or liver cells for much ofits time (antigen concealment).

• Once inside host cells, antigens from the parasite are displayedon the cell surface to aid immune detection. Plasmodium canmutate and vary these antigens so that the immune system nolonger recognizes them (antigenic drift).

• Different stages in the life cycle display different proteins, andso a vaccine would ideally target multiple stages in order to beeffective.

Sickle cell alleleMalaria

Malaria and Sickle Cell AnaemiaSickle cell anaemia is a lethal condition in which the presence of themutant allele HbS in place of the normal allele HbA results in anamino acid substitution at a critical position in the haemoglobinmolecule. This causes the red blood cells to have a characteristicsickle shape, which leads to blockage of capillaries and severeanaemia. The frequency of the HbS allele is correlated with thedistribution 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 preventsevere anaemia, however, the small percentage of sickled red bloodcells confers an increased resistance to malaria giving them aselective advantage over ‘normal’ individuals as they less likely todie from malaria in childhood.

The diagram below shows the distribution of malaria and the sicklecell allele and is typical of many exam questions.

Typical exam QuestionExplain why the sickle cell allele occurs at such highfrequencies in some areas.

Answer1. 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.

Typical Exam QuestionOutline the difficulties associated with controlling the spreadof malaria.

Answer1.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 forimmune system to detect.

7.Different stages of the life cycle in the body make it difficult forimmune system to operate.

8.No vaccine.9.Poor primary health care makes detection and treatment difficult.10.Lack of education over preventing mosquito bites.

Acknowledgements:This Factsheet was researched and written by Katrina Fox.Curriculum Press, Bank House, 105 King Street, Wellington, Shropshire, TF1 1NU.Bio Factsheets may be copied free of charge by teaching staff or students, provided that their school is a registeredsubscriber. No part of these Factsheets may be reproduced, stored in a retrieval system, or transmitted, in any otherform or by any other means, without the prior permission of the publisher. ISSN 1351-5136