How do parasites exploit their hosts and how is their spread controlled?
Parasitism: the spectrum of symbiosis, parasite life cycles and transmission, definitive, intermediate and vector hosts, immune evasion, behavioural manipulation, social parasitism, epidemiology and the control of parasites.
An SQA Advanced Higher Biology answer on parasitism, covering the spectrum of symbiosis from mutualism to parasitism, parasite life cycles and transmission routes, definitive, intermediate and vector hosts, immune evasion, behavioural manipulation, social parasitism, epidemiology and the control of parasites.
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What this key area is asking
The SQA wants you to place parasitism within the spectrum of symbiosis, describe parasite life cycles and the roles of definitive, intermediate and vector hosts, explain how parasites are transmitted and how they evade host defences, and explain the epidemiology and control of parasites, including why control is difficult.
The spectrum of symbiosis
Parasite life cycles and hosts
Transmission and epidemiology
Immune evasion and behavioural manipulation
Social parasitism and control
Examples in context
Example 1. Malaria and its vector. The malarial parasite uses humans as one host and the Anopheles mosquito as the vector and definitive host, and evades immunity by antigenic variation. Control therefore targets the vector (nets, insecticide) as well as the parasite (drugs), showing why attacking several stages is needed.
Example 2. Cuckoo brood parasitism. A cuckoo chick exploits the parental behaviour of its host, which feeds it at the expense of its own young. The example shows social parasitism manipulating host behaviour rather than host physiology.
Try this
Q1. Define a definitive host. [1 mark]
- Cue. The host in which the parasite reaches maturity and reproduces sexually.
Q2. Explain how antigenic variation helps a parasite survive in its host. [2 marks]
- Cue. Changing its surface molecules stops the host immune system recognising and clearing it, so it persists.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA AH style4 marksDistinguish between mutualism, commensalism and parasitism, and define a vector.Show worked answer →
A 4-mark answer needs the three relationships and the vector definition.
Mutualism is a symbiotic relationship in which both partners benefit. Commensalism is one in which one partner benefits and the other is largely unaffected. Parasitism is one in which the parasite benefits at the expense of the host, which is harmed.
A vector is an organism that carries a parasite from one host to another, such as a mosquito transmitting the malarial parasite, without necessarily being harmed itself.
Markers reward (1) mutualism both benefit, (2) commensalism one benefits and the other is unaffected, (3) parasitism the parasite gains at the host's expense, and (4) a vector transmits a parasite between hosts.
SQA AH style3 marksExplain why parasites with complex life cycles are difficult to control.Show worked answer →
A 3-mark answer needs the multiple stages, immune evasion and resistance.
Parasites with indirect life cycles pass through several hosts and stages, so a control measure that targets one stage or host may leave others unaffected and the cycle continues.
Many parasites also evade the host immune system, for example by antigenic variation, so the host gains little lasting immunity and vaccines are hard to develop.
Parasites can also evolve resistance to the drugs used against them.
Markers reward (1) several hosts and stages so one measure is incomplete, (2) immune evasion makes vaccines difficult, and (3) drug resistance can evolve.
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Sources & how we know this
- SQA Advanced Higher Biology Course Specification — SQA (2019)