Transmission and virulence CfE Advanced Higher Biology Unit 2 Organisms and Evolution 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 vi ID: 566237
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Slide1
Parasitism (ii): Transmission and virulence
CfE
Advanced Higher Biology
Unit
2:
Organisms and EvolutionSlide2
SQA mandatory key informationTransmission 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
.
Slide3
Understanding transmission and virulenceTo understand parasites we must understand two key aspects of their ecology Slide4
Understanding transmission and virulenceTo understand parasites we must understand two key aspects of their ecology – how they spread (transmission
)Slide5
Understanding transmission and virulenceTo understand parasites we must understand two key aspects of their ecology – how they spread (transmission
) and how much harm they cause their host (
virulence
)Slide6
Understanding transmission and virulenceTo 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 accuratelySlide7
Understanding transmission and virulenceTo 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 preventionSlide8
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)
: e19515. doi:10.1371/journal.pone.0019515)Slide9
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)
: e19515. doi:10.1371/journal.pone.0019515)Slide10
TransmissionTransmission is the spread of a parasite to a hostSlide11
TransmissionTransmission is the spread of a parasite to a hostImportant aspects of transmission includeSlide12
TransmissionTransmission is the spread of a parasite to a hostImportant aspects of transmission includeThe 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
)Slide13
TransmissionTransmission is the spread of a parasite to a hostImportant 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 epidemiologySlide14
VirulenceVirulence is the harm caused to a host by a parasiteSlide15
VirulenceVirulence is the harm caused to a host by a parasiteVirulence can be defined as the loss of fitness in the host as a result of parasite infestationSlide16
VirulenceVirulence is the harm caused to a host by a parasiteVirulence can be defined as the loss of fitness in the host as a result of parasite infestationRemember that a host may appear healthy but its fitness (e.g. in terms of future offspring) may be compromisedSlide17
VirulenceVirulence is the harm caused to a host by a parasiteVirulence 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 epidemicSlide18
Traditional understanding of virulence Science involves the modification of belief in light of evidenceSlide19
Traditional understanding of virulence Science involves the modification of belief in light of evidence
The traditional scientific view of virulence:
Generations
VirulenceSlide20
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;
Generations
VirulenceSlide21
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
Generations
VirulenceSlide22
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.
Generations
Virulence
???
???Slide23
Trade-off between transmission and virulenceMore 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
Rate of transmission
VirulenceSlide24
Trade-off between transmission and virulenceMore 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…
Rate of transmission
VirulenceSlide25
Factors that influence transmissionIf a higher rate of transmission can increase the virulence of a parasite, then it is important for us to understand factors that influence transmissionSlide26
Factors that influence transmissionIf 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 ratesSlide27
Host behaviour modificationHost
behaviour
is often exploited and modified by parasites to
maximise
transmission Slide28
Host behaviour modificationHost
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 othersSlide29
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
: 10.1046/j.1420-9101.2002.00410.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)
, 94-97
LINK:
http://www.scientificamerican.com/article/fatal-attraction/?
page=2
)Slide30
Effect of Toxoplasma infection on rats
Time
Preference for cat urine odorSlide31
Effect of Toxoplasma infection on rats
Uninfected rats show fear of cat urine compared to rabbit
Time
Preference for cat urine odor
Uninfected ratsSlide32
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
Time
Preference for cat urine odor
Uninfected rats
Infected ratsSlide33
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
Time
Preference for cat urine odor
Uninfected rats
Infected ratsSlide34
Modification of host physiologyParasites may also modifyHost 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 infectionSlide35
Distribution of parasites within a host populationModelling 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 occurSlide36
Parasite reproductionSuccessful transmission is only part of the story for a parasite, of course. It must also reproduceIn 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