ZOOL 313: Parasites

what are four physiological disruptions parasites can cause?

1. tissue damage

2. resource and energy drain

3. immune activation

4. endocrine disruption

what are the five key physiological functions parasites impact?

1. metabolic rate

2. osmoregulation

3. growth rate

4. locomotion

5. reproduction

give an example of how parasites effect metabolic rate?

in arctic charr, encysted trematodes in their eyes lead to a 12% decrease in resting metabolic rate

in the european shag, the more gut nematodes present the higher the metabolic rate (but only in female birds)

give an example of how parasites effect osmoregulation?

in californian krillfish, encysted trematodes in the brain lead to a >50% reduction in Na+/K+ ATPase enzyme (a key enzyme for maintaining ion balance)

however, in atlantic salmon acanthocephalan gut worms had no detectable effect on osmoregulation

give an example of how parasites effect growth rate?

trematode lifecycle (snail - clam - oystercatcher):

snails infected by trematodes were castrated, allowing energy to be rerouted towards growth (gigantism) - improving growth rate but harming the snail.

in the clam stage, the parasites lowered the growth rate.

give an example of how parasites effect locomotion?

parasites either slow speed or have no effect - they are not known to increase speed.

the european eel has swim bladder nematodes - the more parasites, the greater the reduction in swimming speed.

the wall lizard has haemogregarine blood parasites - the more parasites the slower the sprint speed.

give an example of how parasites effect reproduction?

in the pacific hake, fish infected with the intramuscular myxozoan parasite had lower fecundity

in the mountain hair, removing nematodes led to a 50% increase in offspring production

what are the four major environmental stressors to which host responses are affected by parasites?

1. food shortage

2. temperature stress

3. ocean acidification

4. toxic pollution

give an example of how parasites effect food shortage stress?

in Gammarus pulex (amphipod), infection with acanthocephalans increases metabolic rate, but only when food is abundant.

under food shortage, the metabolic boost disappears.

give an example of how parasites effect temperature stress?

in Gammarus pulex (amphipod), infected individuals suffer higher mortality under high temperatures than either stressor alone

in mud snails, Maritrema trematodes make the snails more heat tolerant, while Philophthalmus trematodes make them less heat tolerant. infection with both cancels out the effects.

give an example of how parasites effect ocean acidification (pH) stress?

in mud snails, trematode infection usually raises metabolic rate, but under acidification it lowers it.

snails also grow more under acidification (growth advantage) and have a 10% higher survival rate than uninfected snails (survival advantage).

give an example of how parasites effect toxic pollution stress?

in Antarctic rockcod infected with acanthocephalans, parasites accumulate more heavy metals than host tissues, so parasites act as pollutant sinks.

brine shrimp infected with tapeworms survive arsenic exposure better because parasites absorb the toxin.

why are parasites important to global biodiversity?

• parasites represent 40% of all described species

• they are at least twice as species-rich as their vertebrate hosts

• they regulate host population and influence ecological interactions

how do parasites impact humans, livestock and wildlife?

human health: many parasites cause disease and disability

livestock: parasites reduce productivity, costing industries large sums

wildlife conservation: parasites can drive population declines

what shapes parasite distribution?

(3 things)

1. host availability

2. environmental conditions

3. parasite physiology

give an example of how temperature and elevation can influence parasite distribution?

bird blood parasite prevalence in the Australian wet tropics increases with temperature and decreases with elevation.

• this is likely driven by vector distribution (mosquitos) and because plasmodium development stops below 16C

what are the two key aspects of parasite physiology? briefly explain adaptations to each.

1. water balance and osmoregulation

• protective adaptions include exoskeletons (arthropods) and sclerotised eggs (nematodes)

• species lacking these must live in wet environments to avoid dessication

2. thermoregulation (temperature is important for development and growth)

• adaptations include behavioural thermoregulation and anhydrobiosis (survive loss of all water via suspended animation - metabolism stops)

what are some environmental challenges parasites face?

• environmental limits at any stage of a parasites lifecycle can restrict its distribution

• switching between environments requires physiological adaptations

what are the four areas of the host a parasite can inhabit?

1. skin

2. respiratory tract

3. gastrointestinal tract

4. tissues and blood

what are some of the challenges/benefits a parasite living on the skin will face?

(compare terrestrial and aquatic hosts)

• most exposed to external abiotic conditions

• terrestrial hosts may provide a stable microclimate (especially endotherms)

• aquatic hosts provide little buffering - parasites experience the ambient water conditions

give an example of an aquatic and a terrestrial parasite adaptation to surviving on the skin

(osmoregulatory)

AQUATIC: salmon lice

• in dilute seawater lice osmoregulate independently

• in freshwater the lice rely on host fluids to maintain ion balance (they die quickly in freshwater if not attached to a host)

TERRESTRIAL: tick feeding

• blood is hypoosmotic to the ticks tissues, so they excrete large amounts of hypoosmotic fluid to achieve osmotic balance while feeding

• ixodid ticks use salivary glands to excrete excess water

what are some of the challenges/benefits a parasite living in the respiratory tract will face?

• less exposed to external extremes than skin parasites

• deeper regions are host regulated (presents challenges e.g. immune system) however they are moist enough to allow essentially aquatic taxa to survive

give an example of a parasite who has adapted to living in the respiratory tract?

• pentastomids in reptile lungs produce host-like lung surfactants to avoid immune detection

• they long lived and often cause minimal pathology, feeding on blood, with eggs exiting via the respiratory passages (e.g. sneezing)

• it is a stable environment but requires immune evasion

what are some of the challenges/benefits a parasite living in the gastrointestinal tract will face?

• it is a very nutrient-rich environment and gut-lining is richly vascularised so great for parasites who feed on blood

• the lower gut is anoxic, requiring anaerobic respiration

• the stomach is harsh (acidic, high levels of enzyme activity and peristalsis) so few parasites can live here

• must survive host defences - physical expulsiom via vomiting or diarrhoea; blood feeders face immune responses

give an example of a parasite who has adapted to living in the gastrointestinal tract?

roundworms - hatch in the intestine, migrate to lungs, then return to intestine

• oxygen availability changes dramatically across life stages

• larvae have aerobic respiration while adults need to respire anaerobically to survive the intestine (use rhodoquinone as a co-factor)

• roundworms modulate respiration according to life-stage environment

what are some of the challenges/benefits a parasite living in the tissues/blood will face?

• stable nutrients and oxygen supply, waste/carbon removed

• BUT highly regulated so face immune exposure

• entry and exit often requires complex lifecycles (vectors, intermediate hosts etc)

whats an adaptation parasites evolve to survive in the tissues/blood?

forming cysts help avoid immune detection

whats the general pattern as you move deeper into the host environment?

as you move deeper into the host the environment becomes more stable but more immunologically dangerous

how do hosts recognise parasites?

(3 methods)

1. primitive recognition - proteins, lectins, agglutinins in body fluids (invertebrates and vertebrates)

2. cell-surface antigen detection - receptors on white blood cells detect foreign antigens (invertebrates and vertebrates)

3. MHC-based recognition - highly specific system for identifying self vs non-self (vertebrates only)

what are the two types of immune responses?

1. cell-mediated immunity - immune cells attack pathogens

2. humoral immunity - antibodies and soluble factors in blood/tissues

describe innate immunity and whos involved (4 things)

innate immunity is broad, rapid and has no memory.

• mononuclear phagocytes - phagocytose pathogens, release toxic compounds, secrete cytokines

• cytokines - signalling proteins coordinating immune responses

• neutrophils - first responders, phagocytic, release enzymes

• eosinophils - specialise in attacking multicellular parasites

describe adaptive immunity and whos involved (3 things)

highly specific, long lasting, rapid-response to reinfection

• dendritic cells - detect PAMPs, capture antigens, activate T-cells

• T-cells - kill infected cells (intracellular parasites) and activate other immune cells

• B-cells - produce large quantities of antibodies

what are the five ways parasites can evade the immune system?

1. physical protection

2. immunologically privileged sites

3. hiding and molecular mimicry

4. changing the antigens presented to the host

5. active immune suppression

explain the parasite physical protection and immunologically privileged sites strategies for evading the immune system

physical protection: cysts seal parasites off from immune attack

immunologically privileged sites: parasites hide in regions where the immune system is reduced, for example the eye lens

explain the parasite hiding and molecular mimicry strategy for evading the immune system

• intracellular parasites hide inside host cells (mainly protozoa)

• coating with host antigens - e.g. Schistosoma mansoni absorbs host antigens via LDL and becomes invisible to immune system

explain the parasite changing antigens strategy for evading the immune system

• moulting - changes surface antigens

• rapid antigen turnover (e.g. Trypanosoma cruzi)

• plasmodium - var genes allow switching between ~60 antigens types to avoid immune evasion

give examples for the parasite active immune suppression strategy

• Schistosomes secrete neuropeptides that suppress snail haemocytes

• Filarial nematodes induce high IgG levels to suppress protective IgE responses

how does parasitic infection alter the immune system for other parasites? give examples

graham (2008) found that helminths can weaken host defences against other infections (increased microparasite density aka intracellular protozoa)

what do parasites use the off-host environment for?

(4 things)

1. transmission between hosts

2. development (e.g. moulting, metamorphosis)

3. egg/oocyst survival

4. free-living host finding stages

what are the four major challenges parasites face in the off-host environment?

1. desiccation

2. thermal stress

3. energy conservation

4. locating a host

what adaptions to parasite eggs/oocysts have to protect against water loss when in the off host environment?

• lipid layers (water-impermeable)

• sclerotisation (quinone-tanned hardening)

• keratin-like proteins (tough and water insoluble)

• lipoportein layers (hydrophobic core + hydrophilic exterior)

how does humidity and temperature effect parasite egg/oocyst survival?

survival decreases at low humidity and is best at 100% relative humidity

high temperatures also reduce viability

explain the water balance challenges ticks experience in the off host environment

after feeding, ticks are hyperosmotic (full of concentrated blood nutrients)

this makes them vulnerable to water loss (desiccation) via integument permeability and respiration through spiracles.

what adaptations do ticks have for minimising water loss?

(3 strategies)

1. epicuticular wax lipids: increase during off-host phase to reduce water lose

2. spiracle control: ticks close spiracles to reduce evaporative loss

3. low movement: to conserve water and energy

how do ticks drink in the off-host environment?

ticks absorb water from humid air by secreting a hydrophilic solution onto their mouthparts - this solution dries into crystals.

when humidity rises the crystals re-dissolve and water is ingested. this humidity threshold is known as the critical equilibrium humidity

what are the two behavioural strategies ticks exhibit when searching for a new host in the off-host environment? explain them

1. sit and wait (reptile hosts)

2. questing (mammal/bird hosts)

• move up vegetation when humidity is safe

• retreat to moist microclimates when air becomes too dry

• balance host-seeking and water conservation

what is anhydrobiosis and when is it beneficial?

anhydrobiosis is a suspended-animation state where an organism expels almost all its water and shuts down its metabolism in response to extreme dehydration.

benefits:

• survive extreme dessication

• resume activity when rehydrated

• saves long term energy

what is diapause and what are the benefits for ticks?

diapause is behavioural dormancy triggered by changes in day length. it reduces desiccation during environmental extremes (e.g. freezing) when water availability is low.

benefits:

• immobile ticks lose less water and conserve energy

• synchronises life cycle with host availability

how does environmental variation influence parasite dynamics? what are the constraints?

environmental variation influences:

• transmission between hosts

• infection risk locally and globally

• temporal patterns in parasite abundance

• spatial distribution across landscapes

parasites are constrained by abiotic limits (temperature, humidity) and host behaviour

what are growing degree days (GDD)?

a measure of temperature dependent development

• if GDD is less or equal to 0 then the lifecycle cannot be completes

• if GDD > 0 then the lifecycle can be completed

how does temperature effect parasite-host interactions?

temperature effects:

• parasite development rates

• host immune function (especially ectotherms)

• seasonal infection patterns

what are the thermal effects on host immunity in ectotherms and endotherms?

ECTOTHERMS:

• immune function increases with temperature

• reproduction reduces immune investment (seasonal vulnerability)

ENDOTHERMS:

• immune function if often higher in winter, likely because of seasonal disease risk, reproductive costs, reduced food availability in cold conditions etc

• seasonal immune changes often correlate with seasonal parasite prevalence

how will climate change effect parasite-host dynamics?

(8 things)

• transmission

• infection risk at global and local scales

• temporal dynamics (seasonality, outbreaks etc)

• spatial distribution

• desiccation/moisture

• temperature (development rates, immune function, thermal limits)

• habitat structure (change parasite microclimates)

• nutrition via vegetation changes

how does temperature influence parasitic nematode infection in muskoxen?

• temperature influences larval development in gastropods - under warmer temperatures life cycles can be completed in one year rather than two

• warmer climates mean the worms are infectious for longer

how will malaria development and distribution be effected by climate change?

• malaria development is temperature dependent

• distribution depends on temperature, rainfall (vector breeding sites) and humidity

• climate models predict shifts in distribution (not universal increases)

how will tick/lyme disease distribution be effected by climate change?

as seasonal temperature patterns and vapour pressure deficit (VPD) change with climate changes, the suitability of habitats for ticks will change

what are the four predictors of a parasites vulnerability to extinction?

1. metabolic ecology (thermal developmental requirements and thermal limits)

2. host body size

3. host-specificity and switching

4. transmission and persistence

why does parasite extinction matter?

parasites are ecological regulators. like predators. losing them would disrupt food webs, host population dynamics and co-infection networks

using the example of freshwater snails infected by trematodes, how do parasite impacts intensify at high temperatures?

increased temperatures reduce snail immune function, so there will be more castration, gigantism and reduced reproduction