Biology 3332A- Parasitology
Slides One: Intro/ ecological concepts
• CDC (www.dpd.cdc.gov/dpdx) diagnostic images, life cycles, notes
• University of South Carolina (http://pathmicro.med.sc.edu/book/parasit-sta.htm) Images and notes
• Karolinka Institutet (www.mic.ki.se?Diseases/CO3.html
1. Introduction, parasites and life cycles
2. Ecological/behavioral concepts
3. Biochemistry and drugs
4. Host-parasite interface-membrane biology
5. Host-parasite interface-cell invasion
6. Host-parasite interface-signal transduction
7. Immunology-host responses
8. Immunology-parasite evasion
9. Molecular biology-antigens
10. Molecular biology-vaccines
What is parasitology?
What parasites will be studied in this course?
Unicellular and multicellular parasites
Human parasites Can’t culture parasites in vitro very well
Why study parasites?
Parasites have unusual genomes so vaccination is difficult—unique to parasite
“Parasitism is one of the most successful modes of life. In fact, the diversity of parasitic life in the oceans and
on land exceeds that of non-parasitic life, and all organisms are susceptible to parasitic infection during at least
one phase of their life”
“Furthermore, the bottom of the food
chain is perhaps no longer the most
dangerous place to be”
Disability-Adjusted Life Year— DALYs
2.STD minus HIV
5. maternal/ perinatal
6.Other infectious diseases
7. Tropical diseases
Major increase in deaths and disability
related to parasites.
Most parasiets are picked up through feces
There is a bias living in non-tropics, won’t
give funding for parasites because it
doesn’t affect us
Global warming has made tropical
diseases start to move north Global burdens for parasites
Disease Population Cases x10 6 Mortality x106 DALYs x10 6 small amounts of
at risk x10 research $ spent on
Malaria >2100 270-400 1120 42080 (0.3 cents per case)
African Tryp >60 0.3-0.5 49 1570 compared to
Chagas D 120 17 13 642 diseases in
Leishmania 350 12 57 2277 developed countries
Schistosomiasis 600 >200 15 1711 (>1000$ per case of
Onchocerciasis 120 18 0 960 cancer, heart
Filariasis 1000 120 0 5534 disease, etc.)
Intest. Proto’s3500 450 65 ND Science 264, June
Geohelminths 4500 ~3000 17 4811 1994.
Widespread Amphibian Extinction—driven by global warming
Increase in temperature leads to increase in Chytrid fungus—shown by increasing elevation which increases
temperature. The same applies to parasitic infections.
Shows how tropical diseases will become an issue soon and we should start funding them.
Also people going to tropics and bringing them back here.
What is a parasite?
Free living becomes symbiotic (living off another organism)
Three types: Mutualistic (both benefit—like micro flora in intestine), Commensalistic (One benefits, other
not very affected), Parasitic (One benefits, other is being harmed)
Hymenolepis diminuta— “rat parasite”, attaches to intestinal wall and moves around. Does no harm to host.
Alters intestinal environment—is this harm? Therefore—we cannot define parasites based on harm
Shistosomas: mansoni—first to effect humans—Central/South Africa,
haematobium—migration to Nile region, causes urinary tract infection
japonicum—migrate to Asia, most harmful
This makes the claim that more evolved parasites cause more harm.
Daphnia: Has two parasites that affect it. If you put it in a pond with a parasite it has 20% mortality. If you put it
in a pong with BOTH parasites you expect to see an increase in mortality… did not happen. Therefore the most
THEREFORE most recent parasite is NOT most pathogenic
Taxonomy, host specificity, and evolution of parasites
Species concepts: PAST: Only morphology was used to determine a species of parasite. (didn’t have access to
in vitro samples)
Also had the concept of ONE host—ONE parasite species
PRESENT: claudistic analysis and genomics has improved our definitions of species
It has also altered our ONE host—ONE parasite concept
Example: When working with coccidian parasites (Eimeria) Prof found that 98% of oocysts had 4
sporocysts (normal), but some had 1,2,3,5,6 sporocysts. He concluded that this was natural variation
within a species… However, this is not a popular belief…
Host Specificity: Parasites usually only have one host they chose to infect. And usually only one intermediate
host they chose to infect. If infected in the WRONG host, it won’t reach sexually maturity and will be
But Why Specificty?: Based on ecological factors—look at later
Example: Amino acid sequence binding to latch onto host
Parasites and hosts speciate in synchrony—coevolution Is specificity useful? —Use it to track host
migration—track salmon migration
Used to see which routes humans
took to cross the Pacific during Ice
Age to North America.
Only pinworms could survive route a
Hookworms and Whipworms are
subtropical parasites. Could not have
survived the Beringia route (a).
Therefore route b and c are more
likely. C is most likely.
Parasite Populations and Communities
Intensity: number of parasites in a host
Prevalence: number of hosts infected
• Intensity is inversely proportional to prevalence. (Many parasites in few hosts/ few parasites in many hosts)
• Prevalence is inversely proportional to average age of host. (High prevalence in young hosts)
Is there age related resistance?
Models of Parasite populations:
Most of the time hosts are birds—reduce clutch size (lay fewer eggs)
Parasites control rapidly increasing populations in tropics
Domestic animals—important to treat food supply animals with drugs
Example: cattle in Sub Sahara Africa are treated with drugs to reduce trypanosome brucei (tsetse fly)
Wild animals—frequent crashes in wild populations. However, it could also be due to other factors…
Example: Red grouse every three years—correlation between high prevalence of intestinal parasites and
crashes. Example: Big Horn Sheep—due to parasites or harsh winters?
Transmission: Requires actual interaction between parasite and host. Either free living or host-host.
In the same space at the same time. 2. BEHAVIOURAL/ ECOLOGICAL CONCEPTS
PARASITES AND HOST BEHAVIOUR: THREE INTERACTIONS
1. Parasites exploit natural patterns of host behaviour to maximise transmission (ex preferred prey)
2. Hosts use behavioural adaptations to defend against or reduce parasite infections (ex habitat selection)
3. Parasites manipulate the behaviour of hosts, which can have ecological consequences
1. Parasites exploit natural patterns of host behaviour to maximize transmission
Use the host’s behaviour to increase transmission
Use predator-prey interactions. Also use feeding/ foraging habits
Examples of Predator-prey interactions:
Example 1: The Schistocephalus solidus system
First Intermediate Host: Copepod: tiny little free swimming crustaceans (coracidium ingested)
Second Intermediate Host: Gasterosteus: Stickleback fish (becomes pleurocercoid)
Host: Birds ingest fish
Example 2: Diphyllobothrium—but infects humans. Causes vitamin B12 deficiency.
Example 3: Halibut vs Sole
Atlantic halibut (Hippoglossus hippoglossus)
• Diurnal forager
• Rests during night (sand)
• Infected by: Entobdella hippoglossus
Common sole (Solea solea)
• Nocturnal forager
• Rests during day (sand)
• Infected by: Entobdella soleae
Parasites are closely related, but cannot successfully infect the ‘wrong’ host
Parasites lay sticky eggs that adhere to sand particles, near their potential hosts.
Parasites have biological clocks.
Eggs of E. hippoglossus and E. soleae exhibit opposite hatching periodicity, which match host activity patterns
Example 4: Hymenolepis diminuta—rat tapeworm
Will move to the duodenum at the same time each day when rat eats. It will move down the intestine as
food moves down.
Example 5: The Guinea worm (Dracunculus medinensis )
Contaminated water consumed (poor areas)
Copepod releases larvae into intestine
Larvae migrate to connective tissue and develop into adults
“Little dragon”—Adults migrate to limbs create ulcers. Toxic substance breaks down skin. Released into water.
Long parasite that people used to pull out little by little each day. 2. Hosts use behavioural adaptations to defend against or reduce parasite infections
Behavioural resistance: the first line of defence
• Avoiding infections
1. Food selection (cattle)
2. Nest use (great tits)
3. Mate choice (various)
Example 1: Selective Foraging by cattle. Sampled pasture for lungworm larvae. Foraged areas contained only
25% the density of infective larvae found in the random sample. Cattle were grazing selectively on ‘clean’
pasture to reduce the chance of infection.
Example 2: Nest box use by the great tit, Parus major. Nests provide ideal accommodation for ectoparasites.
These birds know when parasites are present.
1. Clean nest box vs. old, parasite free nest—No preference; both used equally
2. Parasite free nest vs. parasite infested nest—Strong preference for parasite free nest
3. Parasite infested nest box vs. nothing—Strong preference for nothing: birds did not use nest boxes
Example 3: Mate choice. Sex is a highly risky, but necessary, behaviour. Transmission of ectoparasites and
1. Reduced exposure to directly transmitted parasites.
2. Reduced chance of passing parasites to offspring
3. Possibly selecting parasite resistance genes
Is there any evidence of parasite mediated mate choice in nature?
Parasites reduce the sex traits in male—like colour or tail length etc. Females select males that are not
reduced by parasites showing selective mate choice in regards to parasites.
Some species don’t have any markers to identify—no reduction in sex traits. Therefore, females can’t
select non-parasite infected males.
• Reducing parasite loads
1. Self grooming (chickens)
2. Allogrooming (impala)
3. Fumigation (starlings)
Example 1: Self preening in chickens (Gallus gallus)
Self-preening is an effective anti-parasite behaviour. Compared de-beaked chickens to normal.
Birds prevented from preening show poor growth—weight loss… although birds without beaks would
have difficulty eating.
Example 2: Reciprocal grooming in Macaroni penguins and impala.
Unpaired Macaroni penguins harbour 2-3x the number of ticks harboured by paired individuals.
Territorial male Impala do not perform reciprocal grooming. They harbour 6x the number of
ticks that females do.
Example 3: Nest fumigation by starlings (Sturnus vulgaris)
Starlings (& all passerine birds) weave fresh green plant material into nest—contain biocidal substances.
Are birds using plant compounds as antiparasite ‘fumigants’? Evidence:
1. Preferred plants (fleabane and wild carrot) retard hatching of louse eggs, non-preferred plants do not
2. Removal of plant material from nests increased nest parasites
3. Species that re-use old nests are more likely to use anti-parasitic plants than those that re-build.
3. Parasites manipulate the behaviour of hosts, which can have ecological consequences
How have parasites adapted to maximize transmission:
1. Exploit pre-existing host behaviour
2. Manipulate host behaviour
The ‘mixed phenotype’ idea: ‘a parasite’s genes find phenotypic expression in the host behaviour’
Host behaviour is a mixed phenotype of host and parasite genomes.
Parasite phenotypes that alter the fate of the host to the benefit of the parasite should therefore be selected. What behaviours are changed?
1. Altered Habitat Selection: Dicrocoelium dendriticum, the lancet fluke
Sheep feces containing eggs consumed by snail—forms sporocysts in snail lung—secreted as slime balls
Ants consume slime balls on the grass—containing cercariae. Becomes metacercariae in ant. Ants
normally occupy the ground. If infected they migrate to the tips of grass in synchrony with sheep
grazing patterns. Cercariae invade the brain of the ant inducing a behavioural change. However,
parasites in brain can no longer develop, but do it so the other parasites in the ant can—altruism (self
sacrifice)? All the cercariae are genetically identical—not altruism because it’s helping its clones.
2. Altered Anti-predator behaviour: Toxoplasma gondii
First Intermediate: Bettle/worm
Second Intermediate: Rat
Infected rats are: less cautious of novel stimuli, more active = more easily seen, more likely to be
caught in traps. In an enclosure experiment, uninfected rats avoided nesting areas that had been sprayed
with cat urine / scent. BUT Toxoplasma infected rats readily approached areas laced with cat scent.
Cats can transmit to humans cause loss of fetus in pregnancy and lowers IQ
How do parasites change host behaviour?
1. Indirect manipulation
Physical presence / sensory disruption.
Example: Fish infected with Diplostomum (Digenea flatworm trematodes)
Intermediate Host 1: Snails
Intermediate Host 2: Fish
Invades the eyes or brains of fish—causes behavioural changes.
More likely prey for birds—spend more time foraging at the water surface, spend less time
hiding from predators, less compact groupings, closer to surface
Suffer from ‘parasitic cataract disease’ as a result of heavy infections. Lenses become clouded
from large amounts of parasites. Less likely caught by humans—can’t see bait
2. Direct manipulation
a. Anaesthetic effects
Example: Anisakis nematodes in cod (Gadus morhua)
Anisakis nematodes require fish intermediate hosts (cods) to be eaten by marine mammals (seals).
Encyst in the musculature of the body wall of cod. Wastes (alcohols and ketones) effect muscle and
impair swimming through anaesthetic effect.
Can also be transmitted to humans—raw fish. It can be deadly if it goes to other regions of body
besides muscles. This is why we freeze fish.
b. Neuro-endocrine disruption
Example: Gammarids infected with Polymorphus paradoxus
Intermediate: Gammarus ↔ Host: Ducks
Gammarus live in vegetation at edge of lakes and streams.
Transmitted to ducks when they feed on submerged vegetation.
Uninfected Gammarus avoid light and dive when disturbed
Gammarus infected with P. paradoxus do not dive, and do not avoid light. Instead they skim around
the surface and cling to plants when disturbed.
BUT, is this relevant to transmission?
Set up experiment to prove clinging Gammarus are more likely to be eaten.
WHAT DOES THE PARASITE DO TO MAKE THIS HAPPEN?
Serotonin levels cause behaviour. 5-hydroxytryptamine neurotransmitter secretes serotonin.
Does parasite secrete or influence secretion of serotonin? Parasite does not have this
neurotransmitter… therefore it induces increased serotonin secretion in Gammarus. Not sure how. Serotonin associated with emotion, behaviour, and thought.
Lack of Serotonin is thought to cause depression in humans. Sources:
– Red wine