1. Parasitism (ii): Transmission and virulence
CfE Advanced Higher Biology
Unit 2: Organisms and Evolution

2. SQA mandatory key information
Transmission is the spread of a parasite to a host. Virulence is the potential of a parasite to cause harm to a host. A higher rate of transmission is linked to higher virulence. Factors that increase transmission rates include the overcrowding of hosts at high density, or mechanisms that allow the parasite to spread even when infected hosts are incapacitated. Vectors and waterborne dispersal stages are examples of the latter.
Host behaviour is often exploited and modified by parasites to maximise transmission. Through the alteration of host foraging, movement, sexual behaviour, habitat choice or anti-predator behaviour, the host behaviour becomes part of the extended phenotype of the parasite. Parasites also often suppress the host immune system and modify host size and reproductive rate in ways that benefit the parasite growth reproduction or transmission.
The distribution of parasites is not uniform across hosts. Sexual and asexual phases allow rapid evolution and rapid build-up of parasite population. The most successful parasites have efficient modes of transmission and rapid rates of evolution.

3. Understanding transmission and virulence
To understand parasites we must understand two key aspects of their ecology

4. Understanding transmission and virulence
To understand parasites we must understand two key aspects of their ecology – how they spread (transmission)

5. Understanding transmission and virulence
To understand parasites we must understand two key aspects of their ecology – how they spread (transmission) and how much harm they cause their host (virulence)

6. Understanding transmission and virulence
To understand parasites we must understand two key aspects of their ecology – how they spread (transmission) and how much harm they cause their host (virulence)
The interaction between transmission and virulence must also be understood if we are to predict changes in parasite population densities accurately

7. Understanding transmission and virulence
To understand parasites we must understand two key aspects of their ecology – how they spread (transmission) and how much harm they cause their host (virulence)
The interaction between transmission and virulence must also be understood if we are to predict changes in parasite population densities accurately
Only then will we be able to identify key stages in specific parasite life histories that will allow us to improve public health through disease control and prevention

8. For example, modelling the seasonality of an influenza outbreak
Source: from Kenah E, Chao DL, Matrajt L, Halloran ME, Longini IM Jr (2011)
The Global Transmission and Control of Influenza.
PLoS ONE 6(5): e doi: /journal.pone
For example, modelling the seasonality of an influenza outbreak
(from Kenah E, Chao DL, Matrajt L, Halloran ME, Longini IM Jr (2011)
The Global Transmission and Control of Influenza.
PLoS ONE 6(5): e doi: /journal.pone )

9. For example, modelling transmission clusters for an influenza outbreak
Source: Kenah E, Chao DL, Matrajt L, Halloran ME, Longini IM Jr (2011)
The Global Transmission and Control of Influenza.
PLoS ONE 6(5): e doi: /journal.pone
For example, modelling transmission clusters for an influenza outbreak
(from Kenah E, Chao DL, Matrajt L, Halloran ME, Longini IM Jr (2011)
The Global Transmission and Control of Influenza.
PLoS ONE 6(5): e doi: /journal.pone )

10. Transmission
Transmission is the spread of a parasite to a host

11. Transmission Transmission is the spread of a parasite to a host
Important aspects of transmission include

12. Transmission Transmission is the spread of a parasite to a host
Important aspects of transmission include
The mode of transmission, such as:
direct contact between hosts (e.g. human head lice)
vector (e.g. Anopheles mosquito in plasmodium transmission to humans)
consumption of secondary host (e.g. beef tapeworm)
water-borne infective stage (e.g. free-living stages in Schistosomiasis)

13. Transmission Transmission is the spread of a parasite to a host
Important aspects of transmission include
The mode of transmission, such as:
direct contact between hosts (e.g. human head lice)
vector (e.g. Anopheles mosquito in plasmodium transmission to humans)
consumption of secondary host (e.g. beef tapeworm)
water-borne infective stage (e.g. free-living stages in Schistosomiasis)
The rate of transmission – the rate at which a parasite is successfully transmitted to a new host is obviously of key importance in epidemiology

14. Virulence
Virulence is the harm caused to a host by a parasite

15. Virulence Virulence is the harm caused to a host by a parasite
Virulence can be defined as the loss of fitness in the host as a result of parasite infestation

16. Virulence Virulence is the harm caused to a host by a parasite
Virulence can be defined as the loss of fitness in the host as a result of parasite infestation
Remember that a host may appear healthy but its fitness (e.g. in terms of future offspring) may be compromised

17. Virulence Virulence is the harm caused to a host by a parasite
Virulence can be defined as the loss of fitness in the host as a result of parasite infestation
Remember that a host may appear healthy but its fitness (e.g. in terms of future offspring) may be compromised
Watch the thriller Contagion (2011) which charts the response to a fictional epidemic – it is entertaining but has also been acclaimed for its accurate (for a movie at least) portrayal of a scientific response to a virulent epidemic

18. Traditional understanding of virulence
Science involves the modification of belief in light of evidence

19. Traditional understanding of virulence
Science involves the modification of belief in light of evidence
The traditional scientific view of virulence:
Virulence
Generations

20. Traditional understanding of virulence
Science involves the modification of belief in light of evidence
The traditional scientific view of virulence:
when a host species first comes into contact with a new parasite, virulence is high;
Virulence
Generations

21. Traditional understanding of virulence
Science involves the modification of belief in light of evidence
The traditional scientific view of virulence:
when a host species first comes into contact with a new parasite, virulence is high;
virulence declines over generations of co-evolution until a ‘happy medium’ is reached between parasite and host
Virulence
Generations

22. Traditional understanding of virulence
Science involves the modification of belief in light of evidence
The traditional scientific view of virulence:
when a host species first comes into contact with a new parasite, virulence is high;
virulence declines over generations of co-evolution until a ‘happy medium’ is reached between parasite and host
This hypothesis was generally accepted as fact as it seems to be a logical explanation. However, there are lots of examples of parasites and pathogens that do not follow this trend.
???
???
Virulence
Generations

23. Trade-off between transmission and virulence
More recently it has been suggested that, instead of virulence declining through co-evolution between parasite and host, virulence is linked to the rate of transmission
Virulence
Rate of transmission

24. Trade-off between transmission and virulence
More recently it has been suggested that, instead of virulence declining through co-evolution between parasite and host, virulence is linked to the rate of transmission
A parasite with a high rate of transmission can support a high virulence;
a parasite with a low rate of transmission will tend to have a low virulence;
there is a trade-off between the resources a parasite can take from a host and the need to give sufficient time and opportunity for transmission
These are thought to be evolutionarily stable strategies and are gaining support from models and observational data. Watch this space…
Virulence
Rate of transmission

25. Factors that influence transmission
If a higher rate of transmission can increase the virulence of a parasite, then it is important for us to understand factors that influence transmission

26. Factors that influence transmission
If a higher rate of transmission can increase the virulence of a parasite, then it is important for us to understand factors that influence transmission
Factors that can influence rates of transmission
Overcrowding of hosts at high density will increase transmission
Mechanisms that allow the transmission of even when host incapacitated will increase transmission, such as
Vectors – e.g. mosquitoes, ticks, tsetse flies
Waterborne dispersal – e.g. trematodes, cholera
Host behaviour – some behaviours will increase transmission rates

27. Host behaviour modification
Host behaviour is often exploited and modified by parasites to maximise transmission

28. Host behaviour modification
Host behaviour is often exploited and modified by parasites to maximise transmission
The modified behaviour of the host can be considered to be part of the extended phenotype of the parasite
Extended phenotype – an organism’s phenotype is not restricted to the physical products of its genes but encompasses its own behaviour (such as shoaling), the products of its behaviour (such as a nest) and behaviour modifications in others

29. Examples of host behavioural modification
Movement and habitat choice – horsehair nematodes induce water-entering behaviour in their grasshopper host
(See this observational and experimental study :Thomas, F. , Schmidt-Rhaesa, A. , Martin, G. , Manu, C. , Durand, P. and Renaud, F. (2002), Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts?. Journal of Evolutionary Biology, 15: 356–361. doi:  /j x)
Anti-predator behaviour – lower vigilance in rats infected with Toxoplasma gondii
(read this essay from Scientific American Sapolsky, R. (2003) Bugs in the brain. Sci. Am. 288 (3), LINK:

30. Effect of Toxoplasma infection on rats
Preference for cat urine odor
Time

31. Effect of Toxoplasma infection on rats
Uninfected rats show fear of cat urine compared to rabbit
Preference for cat urine odor
Uninfected rats
Time

32. Effect of Toxoplasma infection on rats
Uninfected rats show fear of cat urine compared to rabbit
Infected rats show preference for cat urine compared to rabbit
Infected rats
Preference for cat urine odor
Uninfected rats
Time

33. Effect of Toxoplasma infection on rats
Uninfected rats show fear of cat urine compared to rabbit
Infected rats show preference for cat urine compared to rabbit
The cat is the primary host of the Toxoplasma parasite
Infected rats
Preference for cat urine odor
Uninfected rats
Time

34. Modification of host physiology
Parasites may also modify
Host immune system
Host size
Host reproductive rate
Host lifespan
These modifications tend to benefit parasite growth, reproduction or transmission.
Host survival may actually increase with some levels of infection

35. Distribution of parasites within a host population
Modelling parasite ecology is very complex
Hosts can be thought of as islands of suitable habitat
The distribution of parasites is not uniform from host to host – some hosts may have high levels of infestation whereas others may be uninfected – you may have seen this for yourself when dissecting whole fish
Hosts interact with one another, secondary hosts, vectors or free-living parasites in order for transmission to occur

36. Parasite reproduction
Successful transmission is only part of the story for a parasite, of course. It must also reproduce
In order to achieve sufficient rate of transmission parasites may use multiple phases of reproduction
Sexual phases allow rapid evolution of parasite genome
Asexual phases allow a rapid build up of population