APS 301' Cooperation

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APS 301. Co-operation Conflict Professor
Francis L. W. Ratnieks Lecture 4 Parasitism
Evolution of Virulence in a Host-Parasite System
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Continuum of Mutualism Parasitism

• The fig plant fig wasp symbiosis is basically
  mutualistic
• Wasps gain a safe place to rear their young
• Plants gain pollination (pollen in pollen
  out)
• But there can still be conflict
• Wasps may parasitize more seeds than optimal
  for the plant
• Wasps male wasps do not pollinate
• Plants gynodioecious figs do not allow wasp to
  lay eggs

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Parasitic Symbioses
In a parasitic symbiosis one species exploits the
other To what degree should the parasite exploit
its host? Should a parasite cause little host
damage? host can survive for longer Should a
parasite cause great host damage? host quickly
produces more parasite propagules Clearly, both
parasite strategies have their advantages. What
determines how much damage a parasite should do
to host?
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Common cold
Smallpox
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Virulence
The harm done to the host by the parasite,
specifically increase in mortality. The relative
capacity of a pathogen to overcome body defenses.
More generally, virulence is the harm done as
measured in increased mortality, lowered
fecundity etc. Do not confuse virulence with
contagiousness. The common cold is contagious but
not virulent. Leprosy, TB, HIV are more virulent
but not as contagious. Foot and mouth disease is
contagious but not especially virulent (most
infected animals would recover if they were not
culled.)
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Virulence Evolution
Medical idea natural selection leads to benign
coexistence between parasite and host. Parasites
that do not harm their hosts have the best chance
of long-term survival. Virulence is seen as
resulting from a recent relationship the
parasite and host have not been together long (in
evolutionary time). Many diseases of humans come
from other animals, such as HIV from chimps.
But a parasite is selected to propagate
itself. This might be achieved by high levels of
virulence even in long established host-parasite
systems.
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Virulence of Fig-Wasp Nematodes
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Parasitic nematode inside fig wasp. Panama.
Photo courtesy Allen Herre
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Fig Wasp Nematode Life Cycle
2. Wasp larvae develop
3. Male (wingless) and female offspring wasps
emerge mate
Immature, nematodes attach to young female fig
wasps. Enter wasp body.
Mature nematodes leave dead or dying wasp and lay
eggs
1. Mated female wasp enters fig lays eggs in
fig flowers inside fig
4. Mated female wasps disperse to fig flowers on
other fig plants.
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Fig-Wasp Nematodes
Fig wasps are often infected with parasitic
nematodes. These nematodes may reduce the
fecundity of their hosts. Herre (1993) has
shown that nematode virulence is zero in fig
wasps that always pollinate figs with just a
single foundress wasp entering the fig. But
nematodes are virulent when they parasitize fig
wasp species which have multiple foundress wasps
entering figs.
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Nematode Life Cycle
In nematode infested figs, immature dispersal
phase nematodes crawl onto newly emerged female
fig wasps and are carried to the next fig. At
some point the nematodes enter the body cavity of
the wasp and begin to consume it. Later, 6-7
adult nematodes emerge from the body of the now
dead wasp, mate and lay eggs within the same fig
that the female wasp has laid her eggs. Nematode
eggs hatch when the next generation of fig wasps
emerge from their pupae inside the fig.
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Fig-Wasp Nematodes

• Herre (1993) determined nematode virulence by
  comparing fecundity of infected and non-infected
  wasps in Panama.
• Measured in figs colonised by a single wasp (
  single foundress broods), with or without
  nematodes.
• Number of wasp corpses in fig number
  pollinating wasps
• Each wasp has one species of fig host and
  nematode parasite
• 11 wasp species studied

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Virulence Results
The table shows the proportion of figs that have
eggs laid in them by only a single female wasp,
and an estimate of virulence for the nematode
that parasitizes that particular species of wasp.
This is the ratio of the fecundity of a
parasitized female wasp to a non-parasitized
female wasp. A value of 1 indicates zero
virulence. As virulence increases this value
reduces. Each number refers to a different
combination of fig plant, fig wasp, and nematode
species. Herre, E. A. 1993. Population structure
and the evolution of virulence in nematode
parasites of fig wasps. Science 259 1442-1445.
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Data on Virulence
The negative relationship below is highly
significant (P lt 0.001) showing that virulence
increases as the proportion of single-foundress
wasp broods decreases (Herre 1993).
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Why Virulence Must be Low with Single Foundress
The diagram below represents four generations of
wasps. Wasps infected with nematodes are black
and uninfected are white. The nematodes are
virulent causing a 20 decrease in reproduction.
Each generation the proportion of infected wasps
decreases. This is because there is no horizontal
nematode transmission among broods of different
wasps.
Vertical transmission
Vertical transmission
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But May be Higher with Multiple Foundresses
Here, nematodes still cause a 20 decrease in
wasp reproduction. But the proportion of infected
wasps does not decrease to zero because figs
contain multiple foundresses. This allows
horizontal transmission. (Because each wasp has
several female offspring, the total number of
wasps stays approximately the same, not
decreasing as shown.)
Horizontal transmission
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Evolutionary Significance

• Herre offers this explanation for increased
  virulence
• In single foundress broods, nematodes infecting
  a particular wasp are likely to be highly related
  to each other
• With multiple foundress broods there is mixing
  of unrelated nematodes
• Consequently, in MFB there is intense selection
  on nematodes to increase their reproduction
  relative to other nematodes infecting the same
  host
• One response eat more of the host sooner
• This increases a particular nematodes
  reproductive output relative to other nematodes,
  but causes harm to the host

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Evolutionary Significance

• Bull (1994) puts it like this
• Higher virulence of nematode infections in hosts
  typically multiply infected is due to selection
  for increased virulence
• Some parasite genotypes are better adapted at
  growing in the host than others
• This faster growth causes increased virulence

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Methods of Transmission and Virulence of Human
Diseases
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• Prevention of Infectious Diseases
• Official Picture 1
• How to prevent cholera
• Get inoculation
• Dont drink unboiled water
• Eat clean food
• Chinese Public Health Posters
• http//clendening.kumc.edu/dc/cp/index.html

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• Prevention of Infectious Diseases
• Official Picture 3
• How to prevent dysentery
• Eat clean food
• Drink boiled water
• Wash hands before eating

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• Prevention of Infectious Diseases
• Official Picture 8
• How to prevent the plague
• Wipe out the rat
• Eliminate the louse

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Prevention of Infectious Diseases Official
Picture 6 How to prevent typhus Take showers
freqently Change clothes frequently
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Cholera
Cholera is an acute intestinal infection caused
by the bacterium Vibrio cholerae. It has a short
incubation period, from less than one day to five
days, and produces an enterotoxin that causes a
copious, painless, watery diarrhoea that can
quickly lead to severe dehydration and death if
treatment is not promptly given. Vomiting also
occurs in most patients. Most persons infected
with cholera do not become ill, although the
bacterium is present in their faeces for 7-14
days. When illness does occur, more than 90 of
episodes are of mild or moderate severity and are
difficult to distinguish clinically from other
types of acute diarrhoea. Less than 10 of ill
persons develop typical cholera with signs of
moderate or severe dehydration.When cholera
occurs in an unprepared community, fatality rates
may be as high as 50, usually because there are
no facilities for treatment, or because treatment
is given too late. In contrast, a well-organized
response in a country with a well established
diarrhoeal disease control programme can limit
the fatality rate to lt 1. Cholera is spread by
contaminated water and food. Sudden large
outbreaks are usually caused by a contaminated
water supply. It can be transmitted by direct
person to person contact, but such transmission
is rare. In highly endemic areas, cholera is
mainly a disease of young children, although
breastfeeding infants are rarely infected. World
Health Organization, 1994
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Insect Vectors of Pathogens
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Direct Human to Human Transmission
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Virulence of Human Diseases
Disease causing micro-organisms need to be
transmitted from host to host. In many cases, one
host can infect another directly (e.g., by
sneezing, coughing, sexual contact etc.). In many
others, transmission may be indirect, such as by
blood-sucking insects or by water. Ewald
surveyed the virulence of human diseases and
found evidence that insect-vectored diseases are
more virulent than those transmitted directly,
and that the more a disease is vectored by
waterversus by direct contact the more virulent
it is. If a disease is directly transmitted, the
host has to be well enough to get up, go to work
etc., in order to infect others. If the disease
is indirectly transmitted, the host can be sick
in bed and the disease-causing micro-organism can
still spread via insects drinking the hosts
blood or via feces-contaminated water.
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Water Transmission Virulence
The data below shows increasing virulence in
human diseases as the proportion of transmission
that is via water increases. Ewald P. 1993. The
evolution of virulence. Scientific American 268
(April) 56-62
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Insect Vector v. Direct Transmission
The data below (Ewald 1993) show that
insect-transmitted pathogens are more virulent
than those directly transmitted by infected
humans.
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American Foulbrood in the Honey Bee, Apis
mellifera
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Extreme Virulence Parasites that Kill their Host
Some diseases may even spread more effectively
from a dead host. The next slide shows some
brood cells of the honey bee Apis mellifera that
have been infected by the bacterium Paenibacillus
larvae, which causes American foulbrood. This
is a very virulent honey bee disease that can
kill infected colonies. The bacteria kill the
larva which turns into a brown liquid. The liquid
dries into a scale which adheres to the cell
wall and cannot easily be removed by worker bees.
The scale is made up of billions of P. larvae
spores which can live for decades. Dead honey
bee colonies are attractive to bees from other
colonies who visit to rob out any remaining honey
and to scout out new nest sites. As a result, P.
larvae can easily be spread from a dead host
colony. Transmission from a dead host colony is
probably easier than from a living host colony
in which the entry of bees from other colonies is
normally prevented by guard bees. P. larvae
benefits from killing its host because this
enhances its horizontal transmission.
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American Foulbrood
Scales formed from the dried reamins of
AFB-killed larvae.
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Guard Bees at Nest Entrance Repelling Non-Nestmate
Non-nestmate