Flashcards in Viney Deck (53):
describe a GI nematode with direct life cycle
v fecund, females large,30cm
eggs infective and resistant in environment
doesnt kill but causes morbidity
spread by fecal-oral transmission
infects 1500million people
concenrated in young, poor ppl in developing countries.
describe ascaris spp lifecycle
infective eggs in environment, ingested
larvae hatch in gut and penetrate gut wall, migrate to lungs.
coughed up and swallowed.
L4 return to gut and moult into adult worms.
are Ascaris lumbicoides and ascaris suum closely related species in different hosts, or the same species with 2 hosts in their transmission cycle?
4 experimental methods
1. there is no morphological differentiation
2. cross infection experiment: infect pig with human ascaris and vice versa. however, can't experimentally infect humans. evidence that human worm can infect pigs. but doent mean it would happen naturally.
3. Epidemiological intervention - eg clear infection from
one host species and measure reinfection.
describe a genetic investigation of ascaris species
nuclear and mitochondrial DNA analyses of sympatric populations. using allozymes, RFLP, and microsatellites for analysis.
sampled houses in guatemala where ascaris found in pigs, humans and both.
how is it possible to differentiate between allozymes?
allelic versions of enzymes.
different electrophoretic mobility.
what is RFLP?
Restriction fragment length polymorphism
can use restriction endonucleases (eg EcoR1) to measure DNA seq difference, by producing fragments of different sizes. separate on agarose gel and compare 2 peices of DNA based on fragments.
What has RFLP analysis shown from populations in china and guatemala?
in China - freq of RFLP patterns differ in human and pig ascaris populations.
suggests ascaris from diff host species are rep isolated.
allele freq significantly different at most loci, although not all sites.
mit loci give slightly less clear answer.
guatemala: nuclear DNA shows no fixed alleles to species from different hosts, Globally: there was enough significance to draw 3 clades which share mit haplotypes. 1. pig, 2. human, 3. pig and human.
this gives an intermittent answer, there is some difference but not that much between sp in diff hosts.
what are two possible processes for speciation?
Criscione et a;l 2007
1. ancestral pop gave pig and human species, then spread to locations 1 and 2, so are sympatric. would result in species being more closely related within the host than to other host sp.
2. ancestral pop split into locations 1 and 2, then infected humans and pigs within each pop. would result in human 1 and pig 1 being more closely related and human 2 and pig 2.
what did criscione 2007 conclude was the pattern of speciation?
used microsatellite markers, cluster analyses and identified putative hybrid worms (rare, 5/129).
concluded that geography is the first level of separation.
cross transmission and hybridisation is v recent.
probably other limits to gene flow, not just allopatry.
What is th general message about hosts specifity?
parasite host specifity is variable and continually evolving, relies on interplay between host and parasite biology.
not always obvious.
what is a GI helminth parasite of sheep?
(used to be Ostertagia c)
direct life cycle,
cause significant economic loss.
enormous use of antibiotics so resistance now is a problem.
overdispersed distribution (variance>mean)
life cycle of Teladorsagia circumcincta
eggs passed in faeces; larvae develop in the
environment resulting in L3s sheathed in the L2 cuticle. These are
grazed by sheep, the larvae moult to L4 and adults in the abomasum.
What did Stear et al 1996 conclude were the sources of T. circumcincta infection in sheep?
Host sex- 2%
Early egg count - 4%
Date of Birth - 1%
Type of birth (single/twin) - 2%
MCE (maternal common environment) - 14%
error in measurement - 28%
Individual - 29%
Additive genetic - 20%
heritability was 0.36-0.43, therfore may be realistic to select for sheep w low FEC
Large error in measurement can be reduced by taking more measurements which would increase effect of other factors.
how is FEC calculated and what does it depend on?
FEC - no. worms x per capita fecundity
- no. worms depends on how many are ingested, establish and survive. in turn this is dependent on behavioural and pysiological effects
- per capita fecundity usually scaled with length of worm. there are density dependent effects on fecundity.
describe density dependent effects on female worms and FEC
as no female worms increases:
- decrease in no. eggs per worm
- FEC increases with worm density.
when is the main source of variation in FEC in sheep?
at 3-6 months old.
due to per capita fecundity, not worm number.
how is worm length/number associated with the host?
significant association with IgA.
igA is secreted from cells in mucus membranes eg in gut where worms are.
worm number sig associated with discharged mast cells, as they discharge histamine which interacts with the worms.
what is a working hypothesis regarding variation in heritability of worm length (therefore fecundity)?
Heritability in host anti- T.cricumcincta IgA responses.
observations consistent with this are:
- heritable effects on FEC
- variation in FEC due to variation on worm length
- variation in worm length due to host IgA.
St. Kilda and soay sheep
Hirta sheep population founded 1932 from 107 sheep. varies between 600 and 2000 sheep.
periodic population crashes, mortality due to starvation, worsened by nutrient and protein def, worsened by T.circumcincta parasitism.
what was demonstrated by experimental nematode clearance?
greater probability of survival in a 'crash' year.
the probability of over winter survival is neg associated with FEC previous august.
how was sheep immune response to helminths investigated in NZ and australia
Sheep selected for high and low FEC of parasites: Haemonchus contortus and Trichostrongylus colubriformis.
Acheived a response to selection, mapped genetic loci to explain genetic
resistance of sheep.
FOUND: Chr 3 quantiative trait locus was of large effect.
likely that Interferon gamma (IFN g) coding gene is involved (cytokine, produced by T and NK cells).
IFNY upregulates the Th1 type immune responses and down regulates Th2 responses.
What are Th1 and Th2 type responses?
T -helper type 1 (directed against microparasites) and T - helper type 2.
How does IgA affect IFN gamma?
FEC selection acts on IFN gamma QTL
FEC depends on worm length, and Host IgA affects worm length
describe the IFN locus and how was this used in the Hirta experiment?
IFN locus inbetween BL4 and OarVH34
on Hirta experiment, genotyped the IFN gene using these as flanking control loci.
Also measured FEC and IgA.
Flanking loci have no effect, so effect specific to IFNY. alleles are defined by size. alleles are 126 or 130. allele 126 associated with IgA. Greatest effect at 16 months.
therefore, it is accepted that IFNY controls hostcontrol of worm burden, thereby affecting fitness although not actually supported so far.
what is Strongyloides ratti?
nematode in rat gut
20 species, 2 infect humans
life cycle of strongyloides ratti
adult female lays eggs as it crawls through intestines.
DEV SWITCH 1: sex determination by mitotic parthenogenesis - no. X chr, females have 2X, males X. choice by females, affected by temp and humidity. Affected by host immunity.
DEV SWITCH 2: affected by temperature, and host immunity. female only: choose between dauer and non dauer L3 larvae development.
If direct/homogenic development, go straight to being infective L3 and into new host.
if non direct/heterogenic, are free living male/females in the environment for a while. Morphology changes, mate and lay eggs.
how does temp affect development of S. ratti?
eggs kept in faeces at 19and 25 degrees C.
at 25, more free living females, no sig change in free living males, and less directly dev L3.
how is the proportion of free living females and males calculated?
prop of free living males = freeliving males / (males+females+iL3s)
prop of free living females = free living females / (Females + 1L3s)
what happens after a host is infected with S. ratti?
develops an immune response after a few days.
in a normal immune response, proportion of eggs that develop into free living males and females increases with time post infection
post infection with S. ratti, what happens w/o immune suppression?
when treated with gamma radiation, host is immuno suppressed.
there is no sig increase in number of worms that become free living.
shows the first developmental switch is affected by host immunity.
what questions does this raise about S. ratti and immune response?
how does S. ratti sense the immune response/immuno suppression? is it sensing specific molecules or sensing its own damage?
what is an allele conferring resistance in S ratti in rats?
nude rats homozygous Nu/Nu.
WT heterozygous Nu/+
post infection, nude rats stay fecund for much longer than WT.
in WT, proportion of free living females increases post infection but not in Nu/Nu. no effect for the proportion of free living males.
developmental switch 2.
how does developing into a free living adult benefit the worm?
increased sexual reproduction
how can density dependent effects on S. ratti be investigated?
manipulate density by initiated infections with a range of infective doses.
throughout infection,determine total rep output and number of worms in gut. derive establishment, suvivorship, per capita fecundity.
Found no effect of density on establishment. sig effect of density on survivorship, slightly positive density dep effect on fecundity but soon turns very negative.
define establishment, survivorship and per capita fecundity.
probability that an infective iL3 administered to a host establishes as a parasitic female in the gut.
Survivorship - probability that a parasitic female is alive at time t post infection.
per capita fecundity - average number of eggs produced by a parasitic female.
how can immune dependence be investigated in rats with S, ratti?
Compare reproductive output of S.ratti in WT and nude (Nu/Nu) rats, in early and late infection. if there is no immune dependence expect to see no difference between nude and wt. if it is immune dependent, expect to see a difference.
Both: rep output increases linearly with infective dose.
Nude - no effect until about 200 days PI - perhaps old age of worms.
WT - about 20 days after, density dependent effects seen, rep output massively decreases.
concluded S. ratti experience density dependent effects, but it is due to the immune system, not crowding or comp for resources.
when are density dependent effects enhanced for S. ratti?
after host has had prior exposure.
how can hosts be manipulated to be immuno supressed?
corticosteroids (delay between administered and effect)
how does immune response affect worm length?
if 'long immunised' - infection of 100 iL3s for 14 days, before being removed with drugs. 'short immunisation' - same but for 2 days.
worm length shortens when immune response starts
long immunisation has much more of an effect.
how does immune response affect distribution of worm in gut?
with an immune response, spreadout over the gut
when immuno suppressed, worms gather in the anterior section of gut.
what 2 consequences does waiting longer before reproducing have.
what equation did Gemmill et al 1999 come up with?
the period of growth and development is important in deciding when to start reproducing.
- longer growth period = bigger worm and greater lifetime fecundity (high fitness)
- negative consequence - increased chance of harm/death before rep. (fitness =0)
Daily fecundity = constant x (prepatent time)^B
B is >1, so delaying rep is very beneficial. cross taxa studies have determined b to be 2.66.
what model gives the optimal time to reproduce (a*)?
3 assumptions of this.
a* = B/Mi
Mi = prematurational mortality rate
B = fecundity/maturation time relationship
means that optimal maturation time is smaller as prematurational mortality rate increases.
- Lifetime reproductive output is fitness
-when parasites enter a host they have a constant prematurational mortality rate
- per unit time fecundity is determined by body size at first reproduction
how was the optimality model was tested ?
using phylogenetically independent comparisons.
samples entire phyloogeny of nematodes, so v representative.
however, v messy and quite complex, didnt control doses.
FOUND: 50% of among taxa variation in observed prepatent period can be explained by this optimality model. diversity driven by optimising maturation time with respect to the different mortality rates experienced by diff nematodes.
how would life history plasticity benefit parasitic nematodes?
rep earlier in harsher environments.
describe a parasite which causes eosinophilia
- filarial nematode, transmitted by mites to rodents.
- eosinophils promoted by IL 5 cause parasite death.
study of link between eosinophils and L. sigmodontis maturation
used mice that are deficient in IL 5 and hence deficient in eosinophils. observed consequences for measures of parasite maturation.
FOUND: in WT, worms grow bigger, in IL 5 def, grow slower, as in soft environment so can take their time.
however at 30 days PI, no difference.
why would you do the same experiment on a different species?
to see if the theory can be generalised to the genera.
what was found by enhancing IL5?
artificially added IL5 (rIL-5), in normal mice to assess the effect on maturity in L. sigmodontis.
used bovine serum albumin as a control.
measure of patency was presence of microfilaria released into host circulation.
rIL 5 was microfilaraemic compared to BSA mice, suggesting IL-5 hastens parasite development and patency.
what was concluded about the host immunity and L. sigmodontis maturation rate?
adjusts its growth rate, maturation rate, time until patency and fecundity based on host immune environment (measure of within host mortality rate.
must be a trade off as not all worms accelerate development.
what experiment predicted delay in maturation of 2 GI parasites?
S. ratti and Nippostrongylus brasiliensis
manipulation of immune environment using immune def rats (nude).
S. ratti - delayed gravidity.
no diff in worm size between species.
both support and not.
overall not supporting, but shows contrast between L. sigmodontis results and these species.
study of a fluke/flatworm maturity and immune environment.
infected WT and RAG -/- mice (severely immune def).
recovered worms at diff times post infection.
normal growth in WT, and underdeveloped in RAG-/-.
shows in absense of immune function, worms delay maturation. however, there was a reduction in fecundity, so maybe not adaptive?
suggestion: immune response could have a growth factor for S. mansoni. worms maybe delaying reproduction until suitable immune environment develops.
what do you expect to see with chloroquine treatment?
Chloroquine - malaria treatment, enhance parasite mortality. acts against asexual stages.
should therefore increase investment in gametocytes of plasmodium.
treatment, gametocytes fast peak and then decline.