BIOS 3010: Ecology
Lecture 10: Processes: Parasitism & Disease
•  1. Lecture summary:
–  Definition.
–  Micro- and
macroparasites.
–  Parasite & disease
diversity.
–  Host diversity.
–  Effects on hosts.
–  Disease dynamics.
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Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 1
2. Parasites:
–  A parasite is an organism that obtains its
nutrients from one or a very few host
individuals, normally causing harm but not
causing death immediately.
–  Probably all organisms on earth have parasites,
so more than half of the species on earth must be
parasites and much more than half of the
individual organisms that exist must be parasites.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 2
3. Micro- and macroparasites:
•  Microparasites
–  Small, very numerous like bacteria & viruses.
–  Multiply directly in the host.
–  Mostly intracellular and so hard to count - therefore counted by
number of infected hosts:
•  e.g. measles or malaria outbreaks are measured by the numbers of
infected people.
–  Direct or vector transmitted.
•  Macroparasites
–
–
–
–
Grow, but do not multiply in hosts.
Produce infective stages that attack new hosts.
Live intercellularly between tissues or on hosts.
Can be counted
•  e.g. tapeworms, ticks and flukes
–  Direct or vector transmitted.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 3
1
4. Taxonomic diversity of parasites & diseases:
•  Diseases, ectoparasites and endoparasites:
–  Invertebrates:
•  viruses, rickettsiae, bacteria, fungi, protozoa
–  Microparasites: multiply within their hosts and often result in
either death or some measure of host immunity.
•  flukes, roundworms, tapeworms, some crustacea
–  e.g. parasitic copepods
•  some arachnids
–  mites, kleptoparasitic spiders & ticks.
•  some insects
–  lice, bedbugs, fleas, mosquitoes, horseflies & parasitic wasps.
–  Macroparasites: multiply outside their hosts and host
response varies according to parasite load.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 4
5. Taxonomic diversity of parasites & diseases:
•  Vertebrates:
–  Fish
•  lampreys, anglerfish and a catfish.
–  Mammals
•  vampire bats.
–  Birds
•  blood-feeding finches in the Galapagos.
•  Plants:
–  some plants
•  E.g. dodders, mistletoes, louseworts, some orchids, bastard
toadflax and Rafflesia which produces the world's largest flower.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 5
6. Parasite diversity:
•  Parasites, especially endoparasites, tend to
live in very stable habitats within their hosts.
•  They show ecological emphases on:
–  Reproduction.
–  Simplified resource exploitation.
–  Defenses against the host immune responses.
–  Reduced movement.
–  Dispersal is achieved through very high
reproductive output.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 6
2
7. Parasite diversity in fish:
•  1) Solitary salmonid (trout) species had lower
parasite diversity than salmonids that school
outside the breeding season (Fig. 12.13).
•  2) Species with wider geographical ranges have
higher parasite diversity.
•  3) Parasite diversity in cyprinids increases with host
size and age (as in Fig. 12.14)
–  So ecological factors such as schooling behavior,
distribution and size influence parasite diversity.
–  Diversity and abundance are also influenced by
interspecific competition among parasites:
•  E.g. possible competitive exclusion among parasites of anole
lizards on Caribbean islands (Figure 12.15).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 7
8. Host diversity:
•  Experimental host diversity mixtures can
influence the spread of disease:
–  For example, late blight of potatoes
(Figure 12.16)
•  Can also limit the spread of disease
infections:
–  Effectively dilutes the density (or contact
rate) of susceptibles in the population and
reduces the effectiveness of vectors.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 8
9. Reduced survivorship, growth
and fecundity of hosts:
•  Host survivorship is reduced with
increasing parasite load
–  See Figures 12.17 and 12.18
•  Also reduction in:
–  Age of maturity.
–  Fecundity.
–  Population rate of increase.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 9
3
10. Dynamics of parasite
populations in hosts:
–  Parasites respond to their own density through:
•  Intraspecific competition and density dependent effects
on birth and death rates (Fig. 12.8).
•  Total weight of parasites may increase and numbers
increase, but individual weights decrease and eggs/
parasite decrease, but eggs/host stay the same.
–  Is this just the effect of intraspecific competition or is the host
immune response also exercising “bottom up” control?
–  See the immune response of a vertebrate host in Fig. 12.9
that also can include a "memory" in an inducible response (as
in plants)(see also Fig. 12.3 in 4th ed.).
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 10
11. Dynamics of parasite
populations in hosts:
•  Distribution in time and space:
–  Distributions of parasites within host populations
also tend to be aggregated (Fig. 12.10) with only
a few hosts carrying many parasites:
•  Prevalence of infection is the proportion of a host
population that is infected
•  Intensity is a measure of the number of parasites in or
on a single host (mean intensity is the mean number
per host in a population of hosts)
•  Prevalence and intensity are related through various
frequency distributions in Fig. 12.11.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 11
12. Disease transmission:
–  The net rate of transmission for directly
transmitted microparasites is directly
proportional to how often infected hosts meet
uninfected (susceptible) hosts, which is also a
reflection of density and distance between
individuals as shown in Fig. 12.6 for a fungal plant
disease.
–  Rates of transmission of microparasites are
dependent upon the basic reproductive rate Rp
(like Ro)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 12
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13. Disease transmission:
–  Microparasites - Rp is the average number of
new cases of the disease (secondary infections)
that arise from a single infectious host introduced
into a population of susceptible hosts.
–  Macroparasites - Rp is the average number of
established, reproductively mature offspring
produced by a mature parasite in a population of
uninfected hosts.
–  Transmission threshold - Rp = 1, an infection
will die out if Rp < 1 and will spread if Rp > 1
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 13
14. Rp and directly transmitted
microparasites:
–  The basic reproductive rate Rp increases with:
•  (1) L = average period of time an infected host
remains infectious.
•  (2) S = density of susceptible individuals in the
population.
•  (3) β = transmission rate of the disease.
–  Also depends on infectiousness or the probability that contact
leads to transmission and the probability of contact.
•  So overall,
•  Rp = SβL and is host density dependent, and the
critical threshold density ST ,where Rp = 1,
is, ST = 1/βL
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 14
15. Epidemic and endemic phases
of disease:
–  When Rp >1 there is a rapid increase in the
incidence of disease with rapid infection of
susceptibles and the disease is epidemic.
–  When Rp ≈ 1 the disease is in an endemic phase
because its rate of spread is much reduced by
increased immunity, mortality and decreased
density of susceptibles
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 15
5
16. Epidemic and endemic phases
of disease:
–  Because Rp = SβL
•  Highly infectious diseases (high β), or those with long
infectiousness periods (high L) will have high Rp values
even in small populations (low ST) and will persist:
–  like endemic Herpes viruses.
•  Conversely, low β and low L diseases will only persist
in large, dense populations:
–  like measles epidemics above a threshold density of 300,000
individuals - see Figure 12.17
–  Fig. 12.17 also shows cycling of host infections due to the
introduction of new susceptible hosts through births and
immigration
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 16
17. Disease immunity and vaccination:
–  Given Rp = SβL, the critical proportion of a population that
needs to be vaccinated (pc) to provide herd immunity and
bring Rp down to 1 or less is defined in terms of the typical
number of susceptibles before immunization (So) and ST is
the number still susceptible and not immunized after
immunization, then pc (proportion immunized) is:
•  pc = 1 - ST / So
(e.g. 1 - 2/10 = 0.8, or 1 - 8/10 = 0.2)
•  so if ST = 1/βL and So = Ro/βL, then
•  pc = 1 - 1/ Ro
•  see Fig. 12.18 for the relationship between pc and Ro, to
bring Ro to <1
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 17
18. Parasites and population
dynamics of hosts:
–  Parasites can dampen cycles of host abundance
and reduce host density (Fig. 12.27a, b).
–  Case of red grouse in Britain and their nematode
parasites:
•  Birds show regular cycles of abundance (Fig. 12.27d)
•  Cycles have been argued to be caused by either
fluctuations in food quality (heather) or the influence of a
parasitic gut nematode, Trichostrongylus tenuis.
–  Fig. 12.31 shows density dependent increase in grouse mortality
with increased parasite load (perhaps through fox predation of
more susceptible individuals) and decreased fecundity with
increased parasite load.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 18
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Figure 12.13 (3rd ed.): Species richness of
parasites in solitary and social salmonids
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 19
Figure 12.14 (3rd ed.): Species richness
of cyprinid fish increases with size
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 20
Figure 12.15 (3rd ed.): Reduced anole lizard
parasite load in mixed infections.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 21
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Figure 12.16 (3rd ed.): Mixing host susceptibility
slows potato blight spread in potatoes.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 22
Figure 12.17 (3rd ed.): Host survivorship and parasite load.
(a) (i) Nematode in mosquitoes, (ii) flukes in sheep; (b) people in LDCs and MDCs
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 23
Figure 12.18 (3rd ed.): Hydrometra fitness
decreases with parasitic mite load.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 24
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Figure 12.8 (3rd ed.): Parasite density
dependent responses
(a) Ascaris in
humans,
(b) tapeworm in
humans,
(c, d) tapeworm
in mice
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 25
Figure 12.9 (3rd ed.): Vertebrate immune
response - antibody memory.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 26
Figure 12.10 (3rd ed.): Aggregated
distributions of parasites
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 27
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Figure 12.11 (3rd ed.): Relationship between
prevalence & intensity of infection.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 28
Figure 12.6 (3rd ed.): Parasite rate of
infection increases with
host density.
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 29
Figure 12.17: Cases of (a) measles and (b)
pertussis in England & Wales
Mass vaccination began
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 30
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Figure 12.18: Vaccination level required for
eradication at different combinations of disease
transmission (pc) and reproductive rate (Ro)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 31
BIOS 3010: Ecology
Lecture 10: slide 32
Figure 12.27
(see Fig 12.23
4th ed):
Host population cycle
dampening by
parasites:
(a) protozoan in flour
beetle,
(b) virus in meal
moth,
(c) virus in larch
budmoth,
(d) nematodes in red
grouse and cycles
without nematode.
Dr. S. Malcolm
Figure 12.31: Density dependent effects of Trichostrongylus
tenuis on red grouse winter mortality and brood size
(See Fig. 12.24 4th ed.)
Dr. S. Malcolm
BIOS 3010: Ecology
Lecture 10: slide 33
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