Focus
Snail-host-finding
by Miracidia and Cercariae:
Chemical Host Cues
W. Haas,B.Haberl,M. KalbeandM. Kömer
Snoilsoct os intermediotehosts in tremotode life cydes, ond mony sudres hove
dealt with tfre quesuonof how mirocidio,
moinly of schistosoms, fnd ond recognize their snoi/ hosts. However,the published results on the chemicolsnoil host
cues t/rot owact miracidia of Schistosoma mansoni hove been convodiaory.
Here, Witfned Hoos, Bemhord Hoberl,
Martin Kalbe and Manina Kömer review
doto that indicote that mocromoleculor
gJycoproteinsore the attroctonts, and
thot schistosomemirocidiacon disttnguish
beween snoil svoinsduring their chemo'
oientotion towards the hosts, ln echinostome life cycles,gastropodso/so oct os
second intermediote hosts and are
agtivelyinvaded by cercaioe. However,
their snoilhosts
these cerconoeopproocJ'r
with types of chemo-orientotionthot dif'
fer from those of mirocidio, ond they
respondto smo/l moleculorhost cues.
{68
Gastropod molluscs serve as obligate
intermediate hosts in most trematode
life cycles.The importance of snailhosts
for trematode tr:ansmissionhas stimulated extensivework on the compatibility
between trematode and snail species
and strains.In addrtion to studies on
parasite-host interactions,such as snail
intemal defense systemsand the parasite-mediatedinterfertnce with them,
many studieshave dealt with the question of how miracidiafind and penetrate
their hosts.Saladinr,in an efort to review this work found publishedtnals
with 176 miracidiumsnailcombinations
of 30 trematode speciesto determine
whether the larvae were sensitive to
their snail hosts. Desprte some initial
controversy, it is now broadly accepted
that miracidiarespondto chemicalhost
cues when they approach and invade
their host snails,and much work has
@ 1995,Ebevicrkicnce Ltd 0169-4758/95/$09,50
attempted to characterizethe chemical
nature of these host signals(for reviews
see Refs l-4). This host-finding work
was also spurred by the ideathat'miracidial attractants could be used to interrupt trematode transmission,eg. by
attracting miracidiato particles containing poisons,.
Contradictory
Results
Most studieson miracidiaattractants
were performedin the 1950sand 1970s
with the human blood fluke Schistosomo
monsoni(Box l),butthe resuhsobtained
by the diferent researcherswere contradictory. For example, four researchers
identifiedfour very diferent comPonents
of snail-conditionedwater as the exclusive chemical stimuli (Table l).
Atthough this contradiction warrants an
explanation,the topic has receivedlittle
Today,volsI l, no. | 2, 1995
Parositotogy
ocus
Sn.lls'playa kcf role In mon ücmatode llfe *,*
* offi'r,*nf#?3ai"t"
rrorc. xow the snit holt is found bt miracidh ha5
brcn mo$-crrcnsh,cly rtudi.d in Sd?irtosorno
montori (hft). In lomc cchinostomclifc c,rchs(right), $rib also rcrve as seconü"inürflrcdiatc hosts.nd dtc are acively innded by cerc.riac. CG.c.rirl inrll-ho*-finding hr: bGGnstudiedin thrce echino$omc speciesThasc cGrcrrire find rDd rccotnLe thelr snailhorts cmployinggrienatlon mechanismsard host cuesas:tjmuli tfiet difier conrider.
ebly from öorc usedby lchi$oromc miräcidb.
Schistosomamansoni
Adult worms in human hosls
Echinostomarevolutum
Pseudechinoparyphium echinatum
Hypoderaeumconoideum
Miracidium
invados lirst
intermediate
sneil host
W
Sporocyst genoralions
research in the past decade.Therefore,
our group reinvestigatedschistosome
miracidialhost-findingand realizedthat
at least some of the disagreements
among the publishedchemicalcharacteristics of miracidial attrzctants have
their origin in the use of inappropriate
methods, For example, although 5.
mansonimiracidia respond very sensitively'to minor changesin the environmental pH, the pH of the attractants
oflered to the miracidiawas controlled
only in very few of the studies(only in
two of the publicationsof Table l). In
addition,most studiesdid not take into
account that the orientation behaviour
of 5. monsoni miracidia consists of
highly sensitive responsesto minimal
Redia gensrations
changesin the water by the snail hosts;
an incubationof water wrth a snailfor a
few secondsmakesthe spot of inoculation attractive.Therefore h is not surprisingthat artificiallyhigh concentrations
'attnactants'may release
responses
of
that are not related to host-finding,
Snail-conditionedwater as a substrate
was usuallyobtained by snail crowding
over long time periods, concentrates
up to 12800 snail-hoursml-rwere
used in the experiments (Ref, l), and
pure chemicals were offered to the
miracidiausuallyin concentrationsseveral
orders of magnitudehigherthan those
at which they occur in snail excretorysecretory products. Erroneous conclusionsalso arose from the type of mira-
Table l. Contrastingresults on Schistosomomonsörii miracidial snail-host-finding and
recognition
l.l i raci dial responses
water
that stimulated full responses
lsolates of rnail-conditioned
Entries into zone of inoculation,
Aminoacids
Stimuleting molecules
ions
Magnesium
ratio
Decreasing[Mgz+]/[Ca2+J
Peptides
contafi whh return
Aggregation in zone of inoculaüon
Contact with return, aFtregation
Increaseof rurning
Refs
5,6
7
8
9
Addltional stimulating components
Short-chain fatty acids
Sialic acid
Ammonia
Glutathione
lnorganic ions
o-glucose
p-ecdysterone
Contact with return
Contact with return
Increase of turning, contact with recurn
Seeking behaviour, aggregation
Contact with return
Aggregation
Aggregation, contact with return
Today,
val.I I , no. 12, 1995
Parositology
5
5
l0,ll
t2
rl
t3
t4
Thus,a'concidialresponseconsidered.
tact with retum', in the sense that the
miracidiumretums after a contact with
a substr:ateby swimminga loop, is seen
when miracidiaswim againstthe surface
of artificial substrata,but not when they
encounter living host snails.Caution is
alsonecessaryin the identificationof snail
attractantsby consideringonly miracidial
accumulationsor only an increase of
random tuming of the swimmingoryanisms,as both these responsesalso occur
as a resutt of toxic effects.
'
Snail Host Cues are
Macromolecules
In the effcrt to reinvestigate the
chemicalsnailhost cuesfor schistosome
miracidialhost-finding,seven researchers
registeredas many as 150450 individual responsesof 5. monsoni miracidia
Ql47 replicates)to 23 I diferent fractions of snail-conditionedwater. (Addrtional wor4< deah with miracidia of
other species.)These studieshave contributed to the 5. monsoni miracidial
host-findingenigmsts-ta.
It proved useful to study the effect
of snail excretory-secretory products
on each of the successivephases of
host-findingof the miracidiaseParately.
o monsonimiracidiaare known
Schistosom
to approachtheir host snailsby an increase of random tuming in increasing
attractant gradients and by a tumback
swimmingin decreasinggrzdientsle.This
chemokinesisas an orientation strategy
469
Focus
ffirofChemo.oricntationofMirac|dlaandEchinogtome
Cercariae towardr their Snail Hosts
It was also not exPected that the
samechemicalhost cue stimulateseach
of the three miracidialhost-findingbehaviour pattems studied (Table 2)' esoeciallväs cercariaerespond in each of
ih*i. Ruu host-findingphasesto different host signalszr'zl'However, there
remainsthe lossibility that each of.the
host-finding pattems is stimulated by
different .ä.bohydt t" signals of the
glycoproteins.
SPecificitY in Miracidial
Host'flnding?
Responses to comPlex macromoleculesin their host-findingmay enable miracidiato avoid costly resPonses
to the numerous small molecularcomotn"nts of mud in the microhabitatsof
ih"ir hott snails.The detection of macromoleculesmay requirehighlydeveloped
receptor Processes,and that raisesthe
ouestionlis the complex nature of gastropod glycoconjugatesalso used by the
miä.iaü'to attäin a certain degree of
of miracidia and echinostrome cerThe choice-chambers used to study the resPonses
specificityin host recognition?Schistoare,shoyl
cartae to concent ati-oi gÄa"no of ctremicalcompounds.
*":::lr^r:l
(spotted)
ring
closure
ubt" miracidiaare believedto be not
the
opening
by
rites were released from the Parasitechambers
of
gradients
loncentration
veny specificin their host-finding'howand their swimming paths in increasin! and deäreasing
B;
and
turning'
of
increase
Ä;
(chemotaxis,
ever, and there are reports that they
artrectanrs r"corde? it"" f"Uf" belowi
even penetratetadPoles.
C).
$rnback swimming
exPenments
Nonetheless, first
Turnback
Increase
of an
miracidia
monsoni
5.
that
showed
ewlmmlng Ref.
of turning
more
Chemotaxis
significantly
were
strain
J'liracidia'
Eerptian
l7
yes
yes
no
Schistosomomansoni
"ä.äa"o to snail-conditionedwater of
l7
yes
yes
no
Schistosomohaematobium
r5
their compatiblehost snailspecies.than
yes
no
yes
Schisosomo PPonicum
b
to that of bther gastropods(Fig' la) and
yes
yes
no
Trlüobilharz'n ocellou
b
yes
miracidia of two Brazilian5' monsoni
yes
no
FosciolohePotko
strainsdid not show that preference of
Echlnostome cercariae'
the compatiblehostrs' This shows that
26
yes
no
no
Eülnostonro revoluum
differentttt int of 5.monsonimay achieve
26
no
)res
no
ßeudechinoPotYPhium
a different host specificity even during
echinoum
26
no
chemo-orientation'The snail-host speoyes
llypoderaeum conoideum
ficity of the attractantswas achieved at
b
M. Kalbe, unpublished.
t Two orientadon Patterns pcr species'
similarglycoconjugateconcentrattonstn
c One orienadon Pattern per species'
snail-conditionedwater, which suggestsäifi"r"n."s in the chemicd nature of
cues.In fact,they show snail-strainthe
linked via^senne
be O-glycosidically
also occurs in the miracidia of fricho- to
glycoconjugate-spectrals'but
specific
The
2)'
(Table
N-acelilgalactosamine
aimeringchemical
bilhazio ocellato and Fosciola hepotico and
is sensitiveto lysozyme' informatiän on ihä
even higher
(Box. 2), Only the miracidiao( S' iopon- glycoconjugate
lacking'An
is
iuf i.ft may suggestthat muramic acid characteristics
may be attained
ürrn tho* a chemotactic orientation'
specificity
host
of
J.gr""
is
a gastropodipecific comPonent2o
they are capable of directed swimming as
via"the attachment and penetration rein the recognitionprocess'
involved
their
aloÄgchemicalgradientstowards
,fonr"t. Thus, S' io.ponicum.miracidia
At first sight'it is surprisingthat Tirasnail- hostsl5. When schistosome
could not distinguishbetween their host
with their snail cidia of 5. monsoniand 5' hoemotobium snail Oncomelontohupenss and Biommiracidia have contact
'repeated. investi- find their snail hosts in responseto a
hosts, they show
phaloia globrataduring the. phases of
'attachment' as typical hostsimilar class of chemical host signals'
gation' and
tnuto-irientation' but attachedonly to
ahhoughthey survive in di{Ierent snail
recognition Pattemss'
ö. ftrp.^"|s. However, information on
is especiallystriking,as the
This
n"n"o]
support
strongly
The data obtained
nature of the host cues
of both species,which invade the chemical
Zercariae
host
snail
rnacromolecular
that
the view
that stimulate miracidialattachment and
humans as the same final hosts' show
I'
cues tnd not the smallmoleculeslisted
penetrationis still scarce'
iitr".*nt host-finding strate$esz Further
in Table I attract schistosome miraThese few availabledata suggestthat
now elucidate whether
should
studies
5'
of
miracidia
the
host-finding may contnbute
li6;3rs-rz.At least
carüohydratemoieties of the mira- miracidial
the
to
be
respond
haematobium
S.
monsoniand
to their host compatibility'This may
glycoproteinsdiffer bewhere
studies'
with a molecular mass cidia stimulating
laboratory
in
glycoconjugates
ouedooted
these sPecies'
iior.O", whose saccharidechainsseem tween
vol' | | ' no'| 2' 1995
Today'
Parasitology
170
Focus
schistosome mlracidia and echinostome cercariae
Table 2. characteristics of the chemical snail-host cues that attracr
of chernical attractants
Arce rteined characteritticr
Treetment of SCW
Schi*osomc miracidia'
Molecular filration
Pronase digestion
Precipitationwith Alcian Blue, or oxidation
with NalOa
Mild alkaline treatmenq molecular filtration;
and carbohydrate and protein analysis
Lysozyme digesdon
Molecularmass>30kDa
Not proteins
Glycoconiugates
O-glycosidicJinked carbohydrate chains (<30 kDa)
Core protein (<30kDa)
Muramic acid Presentl
Echlnortome cercariaeb
Molecular filtration
Ashing, desalting
HydrJlysis of pipüdes, separation of peptides
Deamination (HNOJ
Amino acids (tonceigztions as determined in SCW)
Preudechfnoporyphium
echinotumc
<500Da
Organicsubstances
Not peptides
Aminocompounds
Aminoacidsinvolved
Echlnoetomo
rcvofuturnc
<500Da
Organicsubstances
Not peptides
Aminocompounds
Hypodcrocum
conoideumd
<5(DDa
Organicsubstances
Peptides
t
rüdo'n o,mkE In h'rc'dq end
.nd s to.motol*mt ch.r.ct rh.lct of d,€ rnol'cuh' ors '*nd*r lß,..,' ot .ubrut"r'rt 't dclctmin'd bt
r--r,
., **..rffi
ct
wi$
th.
co.rt
.ftcr
twrct6nron'
;'*pcrcd
unb|rl ,wrn nt{ tn arcrrelq conccntnäää;i;;;i
ttoe''rr of ftdt*oidftlo'r1ffiLffi;t",
. chcmoori. ln in .arlnt.tir'ulq t.dlent!'
" dec*elrrusrinürrur
tr.d.n..
the miracidiaare exposedto the snails
"*"ffv i^ " few miililiten of water, a
srt attn $"t gt€"tly difrersfrom that in
the field,wt€re S mdnsonimiracidia;
infecttheir snailhostsin distancesuiü
g7m (Ref.24).
can
andone spect€s
attractantdecreases,
along ins,vim direct€dchernotaEticalb/
oeasingattracuntgradierts'furtherrnore
üe gui1edtg\ /an*
*ttin"ftottt."toi*
.oi.pto -".-Oy
not
Gi;-h651,
molecutes;frey resioru to snull rnoteculesipeptideirnone+ecbq artdanürc
Wht not lu|ü Otlent'tion
bt
by miracidia and
SnaiFhost-findingcercariaeconsistsof severalPhasesincluding leavingthe habitat of the first
intermediatehost in echinostomq,cer-
snen-hortdndrnlrnccrcerrec
ffiä;:#r'R,"iiä1#il"'l*
llm*l"nr.:m"rr:
water in Only whenthe host-conditionedsection
of snail<onditioned
I
I
!
I
The data on miracidialhost-findir8 ä..t^ö
Uit trey.do not accountfor
ore species,
ä?
$ggestürat it may Ue aOvantageous
complete"tfity' tt is ttt.}€t ryry
parasites
',".p#ü to kJeflüryaq,ratrcsrut nostsüy its
or not these vnall molecular\^,fiethe;
"t
h"i
io -ä.nJr*eunr
resuttin a higherdegreeof
sigals
host
inGrestbe
it
would
conter,t
-ä ln ürE
f'ort t'pu.n.itr. fn"o.aii.4y, Ä-.. u
ir* to kno/v trov.vsru1-irvadngparasi;
other than miracidiafind and recognize speciesgecifrity corld be attäinearz,
Takentogether,the Presentlyavailtheh trostssrnilsarealsoima.dedby cerinformaiion on the cherno-orienable
t*r".f
,ue"
ä;-t"r*l
ü;
ä;
fre äif"i of echho- ütion to*".d, the snail host sussests
;'-*,*
äj;
arettre better.host-fii-der:
ä-lndä tg* fl, .nA i"t" .r" *uit"Ui" ttrat miraciOia
with the view that the
harmonizes
ihis
EdlrMornoi
d
on the host-finding
-tftro
ar€ phy'opecies listedin miraciiium-snaitretationshiPs
lDfrr:t.nA * ti*
the
than
older
genedcally
ofth;
ik; , ß"f tö. Th" t ost-tinOing
.iercarii-jnait
ielationships.foyevel tfre degree.of
Jm"- n"ä u"r"r
äil;d^8-$eri;
noJ-ip"iirantryin nort-findingis Jbo destrciea so ar *rai
t .".rij
S**
that r€guby'the Processestermined
ano
amptriuia
ur*,
;;Jdr,
ir*.i;
host
andthe
to
th€
;r1.,;ä, ;-ö"t {19, *ro,v chemo-orien- latethe attachment
attachment
During
steps.
p";J;i.;
tatrontowardstheir hosts Thiss.rffi
the vieyvthat chemo-odendionin sMm- ächinostomecercariaecan distincuish
animals-and
aquatic.
ait."J
'sPecies,
ning cercariaenuy be limiteOto paä- Üä*"
and the
snail
even boween
sites of slqMy ,.*nrg r'or,., *.Ä
macrounknown
are
that
cues
for
host
goOiät nr"yäf.Ufi*t.
&.iä
(Fig'
l)'
.o1"tubEr
echrpstotne
and
miracidia
Although
The ch€;ic-al host cuesf].lat stimucercariaeare similar in that . they af
miracidialättachmentsand .pentate
proactrrheir srat hosrsI chemo-orie'n.* Poot/ studied. When
äti""i
ilnir
Ue
profounay
tation,they difer
oh tifLt"nt qualitiesand
pecu6te
-'.t"Uiü^
we
t"
ir't
."4
;pf.yÄ,f
;ä;il
ä;'
in
"f snail-host-findingiÄi
natue of *imulatingchemicalnost ä
we
need-furcercariae,
and
mincidia
-tj-of
\Mrereas niracirlia show two t/pes
on the host-R$ing
p". .p*'",
.'t'äf"'nftÄ"ti"^
orienurrm rn€chanrsrns
'lffiffitrt1ni-äJ
st'.ututing''hitl
tne
-"oÄpf.,
one
onry
witrr
'i;itä,"
areprwided
ffi
cariae
in echinistome
i.i
species
(Box
2):
two
f"r,irü
orientati:n
bäck when the concentrauonor üle mincidia)'
To@,vo!.t t,no. 12'1995
Pot/a,ttlJt'ogl
of tire environmentis readred2canthe
Then it would
resPonses
3F {-TL
most effectivestrateg/
Ppeaj.lhat llte
is to follow directedconcentration8radientsof the chemicalhost cues However, such a chemotaxisseemsto be
adoPted.in only .one.of the minci<Jial
and one of the
speiies.(S kryYP
cercarialspecies(Hypderaewn conoi&
eum) studied The other speciesshow
chemokinetic.resPor]ses(increase of
random tuminS, and tumback swimming)' in which informationis requir€d
increaseor decreaseof the
oltr
9n 1
attractantconcentration.ratherthan on
the,direaion oJ the attraclantconcentration gradient'
The particularadvantagesof these
differ€nthon-findingstrategiesare not
yet understood Miracidiaseem to be
iaapteOto achievea wide dispersalby
a high swimmingspeed(lhree times as
fastasthe ciliatePrstozoonPuotnq)um
whictr is twice as long) and
coudoiü)m'
by a rotation on. their long axis that
suPPortsa stmightPath of movement'
Howerar, both htghsPeedand rotation
may interferewith: chemotacticalorieniation that has to make use of the
sensoryPapillaewhichate locatedapical
airotiteraion the rotatingmiracidial
body Similarproblems.occurin echino
no;t€ cercarialoriemaion The pansites
'71
Focus
Lynnaca Lynnace Radlx Eionph, Rane
(larval
palwtris atagnatic awicu- glabrata
laria
b
Firh
Lccch
Gontrol
too
80
ul
0t,
.+t
8 6 0
o
o
q
E
4 0
CL
a
o
E
20
Biomphelarh Biatpheteria Eiamphelarie Lymtaca
stegnalia
glabreta
eltbrata
alaxantine
(pigmcntcdt
{albino)
Control
and echinostome
Fig. l. specificity of host-finding responscs in schistosome miracidia
miracldia (Egypllan
mdnsoni
Schistosomct
of
responses
Chemo-orientation
cercarlac.
snail host Sldomphah*t
,r."i"l towards snair,condrdonedwater (scw) of iscompatible
was adiusted to simiSCW
of
concentratlon
(b).
Th;
olexon&imand othergastropods
closed bars:
swimming
turnback
bars;
lar hcxose conten1 licreasä of tuining, open
aftcr concercariae
xhinotum
Pseudahinoparyphium
of
Behaviour
SE,standard crror.
closed
contact'
enduring
tact with living aquattc inimats (a). Aaachrnlnt,'oP"n ^b1;i
(a).
Z0
and
(b)
18
Refs
fto*
Adapt"d
bars; SE,sandärd'eiror.
adjust their position in the water column by downstrokes of their uPward
directed tails, and it can be suggested
that this sruimmingmode is adapted to
minimize energy costs. However, the
swimming creates water currents that
may interfere with the detection of a
chemicalgradient by the numerou.ssensory papillae on the cercarialbody and
tail.
For an understanding of the adaPtive benefrts of the diverse chemoorientation strategies, much more information is needed on function and
172
Press
4 Sukhdeo,M.V.K and Mettrick D'F. (1987)
Adv.Porositol.26,73- )44
51,731-746
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7 Stibbs, H,H. et ol. (|'976l. Noture 260.
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8 Sponhottz,G.M. and Short, RB. (1976)
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eorasitology
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Haas,
23
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Wltfrlcd Hoat, Bemhar.d Hoberl, Martin
Kohe ond Mortina Kömer ore at tfre lnsutut
pr Zootogie l, l)nivenitöt Erlongen-Ntlmberg'
5, D-91058 Erlangen,Ctermony'
Stoudtsträsse
Teh +19 9131 85 t051, Fox: +19 ?l9l
85 8OlO, c-moft whoat@bloloilc'unl'
crlongcn.dc
energetics of miracidial and cercarial
swimming modes,
Acknowlcdgements
The work of {he authors was funded by the
Deutsche Forschungsgemeinschaft,
Refcrenccs
I Saladin, K.S. (1979) Z. Parositenkd60'
t97-?tO
of
Aspe-cts
(1976) in Ecologicol
2 Maclnnis,AJ'
-(Kennedy,
C'R, ed'), pp 3-20'
Parasitology
North-Holland
3 Smyth.J.D. and Halton,D.W' (1983) Ihe
CambridgeUniversity
of Tremondes.
mysiotogy
Today,vol. ! l, no' | 2, 1995
Porositology