# Malaria

> Biology · CIE A-Level 9700
> Source: https://www.owlsprep.com/study/cie-9700-u10-malaria/

This sub-topic covers the causal pathogen, life cycle, transmission route, global public health impact, and evidence-based control strategies for malaria, a high-burden vector-borne infectious disease for CIE A-Level Biology.

**Prerequisites:** [Types of pathogens](https://www.owlsprep.com/study/cie-9700-u10-types-of-pathogens/); [Disease transmission](https://www.owlsprep.com/study/cie-9700-u10-disease-transmission/)

## Learning objectives

- Describe the life cycle of *Plasmodium*, the pathogen that causes malaria
- Explain the transmission route of malaria between hosts
- Outline core control and treatment strategies for malaria
- Describe the global distribution and public health impact of malaria

## Causal Pathogen and Transmission

**Malaria** — A life-threatening febrile infectious disease caused by protist pathogens of the genus *Plasmodium*, transmitted to humans via female *Anopheles* mosquitoes. *Plasmodium falciparum* causes the most severe, life-threatening cases.

*Example:* Over 240 million new cases and 600,000 deaths are reported annually, mostly in sub-Saharan Africa.

Malaria is an anthroponotic disease, meaning humans are the primary reservoir for the pathogen. Only female *Anopheles* mosquitoes transmit malaria: they require protein from human blood meals to develop their eggs, so they bite humans. Male mosquitoes feed only on plant nectar and do not bite.

**Worked example:** Explain why male *Anopheles* mosquitoes cannot transmit malaria to humans.

1. 1. Recall the difference in feeding behavior between male and female mosquitoes:
2. Female mosquitoes need a blood meal high in protein to produce and mature their eggs, so they bite humans and other mammals to obtain blood.
3. Male mosquitoes feed exclusively on plant sap and nectar, and never bite humans to take blood meals.
4. 2. Link feeding behavior to pathogen transmission:
5. Transmission of *Plasmodium* occurs when an infected mosquito injects the pathogen into the human bloodstream during a bite. Since males never bite, they cannot transfer *Plasmodium* to new human hosts.

## Life Cycle of *Plasmodium*

*Plasmodium* has a complex digenetic life cycle, meaning it requires two different host species to complete its sexual and asexual stages of development. The *Anopheles* mosquito is the definitive host (where sexual reproduction occurs), and humans are the intermediate host (where asexual reproduction occurs).

**Definitive host** — The host organism in which a parasite reaches sexual maturity and completes sexual reproduction.

1. An infected female *Anopheles* injects *Plasmodium* sporozoites into the human bloodstream during a bite
2. Sporozoites travel to the liver, invade hepatocytes (liver cells), and multiply asexually to form merozoites
3. Merozoites leave the liver, invade red blood cells, and multiply asexually until the red blood cell lyses
4. Lysis of red blood cells releases toxins, triggering the characteristic cycles of fever and chills
5. Some merozoites develop into gametocytes, which are taken up by a new *Anopheles* mosquito during a blood meal
6. Gametocytes mature into gametes in the mosquito gut, fuse to form a zygote, and develop into new sporozoites that migrate to the salivary glands

**Worked example:** Compare the roles of the mosquito and human host in the *Plasmodium* life cycle.

1. 1. Classify each host by type of reproduction:
2. Sexual reproduction of *Plasmodium* occurs only in the Anopheles mosquito, which is the definitive host. Asexual reproduction (multiple fission) occurs in human liver and red blood cells, so humans are the intermediate host.
3. 2. State the transmission role of each:
4. The mosquito acts as the vector, carrying *Plasmodium* between human hosts. Humans act as the reservoir for the pathogen, allowing asexual stages to persist between bites.

## Global Impact and Distribution

Malaria is endemic in tropical and subtropical regions between the Tropics of Cancer and Capricorn. This distribution matches the habitat requirements of *Anopheles* mosquitoes, which need warm, humid climates and standing water for larval development.

Children under 5 years old and pregnant women are at highest risk of severe disease and death: children have not yet built immunity to *Plasmodium*, and pregnancy suppresses immune function. Malaria traps communities in poverty by reducing work productivity and increasing healthcare costs.

> **info**
>
> The sickle cell anaemia allele is common in malaria-endemic regions because heterozygous carriers have partial resistance to *P. falciparum*, an example of natural selection for the allele.

## Control and Treatment Strategies

Malaria control targets both the mosquito vector and the *Plasmodium* pathogen, using a combination of strategies to reduce transmission and treat infections:

- **Vector control**: Insecticide-treated bed nets (ITNs) prevent bites during peak feeding hours, and indoor residual spraying (IRS) kills resting mosquitoes
- **Chemoprevention**: Anti-malarial drugs are given to high-risk groups in endemic areas to prevent infection
- **Treatment**: Artemisinin combination therapy (ACT) is the first-line treatment for uncomplicated *P. falciparum* malaria
- **Habitat modification**: Draining standing water removes breeding sites for mosquito larvae

**Worked example:** Explain why draining standing water reduces the incidence of malaria.

1. 1. Recall where *Anopheles* mosquitoes develop:
2. *Anopheles* mosquito larvae develop in stagnant, standing water such as puddles, ponds, irrigation ditches and open containers.
3. 2. Link habitat removal to mosquito population size:
4. Draining standing water removes the required habitat for larvae to develop into adult biting mosquitoes, reducing the total *Anopheles* population.
5. 3. Connect to malaria transmission:
6. A smaller mosquito population reduces the frequency of bites and *Plasmodium* transmission between humans, lowering the overall incidence of malaria.

## Common pitfalls

- **Wrong:** Claiming the *Anopheles* mosquito is the only host of *Plasmodium*
  - Why it fails: *Plasmodium* requires two hosts to complete its life cycle, both play essential roles in development
  - Correct: State that the *Anopheles* mosquito is the definitive host/vector, and humans are the intermediate host where asexual reproduction occurs
- **Wrong:** Claiming male *Anopheles* mosquitoes can transmit malaria
  - Why it fails: Only females bite humans to take blood meals required for egg development; males feed only on nectar
  - Correct: Only female *Anopheles* mosquitoes transmit malaria between human hosts
- **Wrong:** Claiming malaria spreads directly between humans without a vector
  - Why it fails: Natural transmission requires the *Anopheles* vector; direct transmission via blood transfusion or congenital spread is extremely rare
  - Correct: Malaria is primarily a vector-borne disease transmitted by the bite of an infected female *Anopheles* mosquito
- **Wrong:** Stating antibiotics are used to treat malaria
  - Why it fails: Malaria is caused by a eukaryotic protist, not a prokaryotic bacterium, so antibiotics have no effect
  - Correct: First-line treatment for uncomplicated malaria uses artemisinin combination therapies (ACTs) that target *Plasmodium* directly

## Cheatsheet

| Key Concept | Core Fact | CIE Exam Note |
| --- | --- | --- |
| Causal pathogen | *Plasmodium* (protist) | Eukaryote → antibiotics do not work |
| Vector | Female *Anopheles* mosquito | Males do not bite/transmit |
| Host roles | Mosquito = definitive (sexual), Human = intermediate (asexual) | Always specify roles for full marks |
| Top control methods | ITNs, IRS, drain standing water, ACT treatment | Name 2+ for 4+ mark questions |
| High risk groups | Under 5s, pregnant women | Common 2-mark exam question |

## What's next

Understanding malaria builds a foundation for exploring other major global infectious diseases, the evolution of drug and insecticide resistance in pathogens, and the design of public health strategies to control communicable disease. This topic also connects to core concepts of evolution by natural selection, seen in selection for the sickle cell allele in endemic regions. Next, you can explore other high-frequency infectious diseases in the CIE 9700 syllabus, or dive deeper into immunity and vaccine development.

- [Tuberculosis](https://www.owlsprep.com/study/cie-9700-u10-tuberculosis/)
- [HIV and AIDS](https://www.owlsprep.com/study/cie-9700-u10-hiv-aids/)
- [Antibiotics and Antibiotic Resistance](https://www.owlsprep.com/study/cie-9700-u10-antibiotics-and-antibiotic-resistance/)

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