Van Leverparasiet Tot Zombieslak - Een parasietenexpeditie in Afrika
- TineHuyse
- MaxwellBarson
(Zie bijlage voor volledig artikel)
Het ’s werelds grootste door de mens gemaakt meer is blank terrein voor wetenschappers. Voor het eerst trokken biologen met moderne moleculaire technieken naar het Karibameer om de diversiteit aan trematode wormparasieten te bestuderen. Al ooit gehoord van schistosomiase, leverbot of een ‘zombieslak’?
‘Vergeten’ parasieten
Sinds de constructie van de Karibadam in de Zambezi rivier, vormt het Karibameer de grens tussen Zambia en Zimbabwe. Vermits het karibameer een ondergelopen vallei is, is al het leven in en rond het meer hier nieuw en zeer interessant voor biologen. Het meer is, zoals alle Afrikaanse meren, een broeikas voor heel wat parasieten, waaronder de trematoden. Trematoden zijn een grote groep parasitaire wormen die leven in het bloed, de lever of het spijsverteringstelsel van mens en dier. Infectie gebeurt via de huid of via consumptie van met larven besmet voedsel of drinkwater. De eieren van de volwassen wormen komen via excreta terug in het water terecht. Hier, ontluiken de eitjes tot larven die zoetwaterslakken infecteren en zich hierin vermenidgvuldigen, waarna ze de slak opnieuw verlaten. De meest gekende trematodesoort is Schistosoma, hij veroorzaakt de ziekte schistosomiase of bilharzia, wat op malaria na de meest problematische parasitaire ziekte ter wereld is. Er zijn echter meer dan 18000 verschillende soorten trematoden gekend en nieuwe soorten worden jaarlijks beschreven. De ziekten die trematoden teweegbrengen zijn wetenschappelijk erg onderbelicht, daarom worden ze ook 'verwaarloosde ziekten' genoemd.
Een alledaags zicht in Kariba: op de voorgrond waadvogels, op de achtergrond dode bomen, overblijfselen van de beboste vallei die onderliep in de jaren 60’.
Een DNA expeditie
Voor mijn thesis reisde ik samen met parasitologe Tine Huyse van het Belgisch Koninklijk Museum voor Centraal Afrika naar Kariba, een dorpje aan de rand van het Karibameer te Zimbabwe. Hier verzamelden we gedurende zes weken zoetwaterslakken om de trematoden die ze verspreiden te onderzoeken in het lab. Kennis over het voorkomen van zowel de trematoden als hun slakkengastheer is essentieel om te begrijpen hoe ze zich verspreiden en hoe we deze transmissie kunnen controleren en voorspellen. Daarvoor zijn efficiënte identificatiemiddelen noodzakelijk. De meest moderne methoden zijn gebasseerd op DNA analyse en worden moleculaire methoden genoemd. Landen waar tropische parasieten het meest talrijk zijn, zoals Zimbabwe, hebben vaak niet de middelen om zulke moleculaire technieken te ontwikkelen. Daarom was het voornaamste doel van mijn thesisonderzoek om een op DNA gebaseerde methode te ontwikkelen om zo goedkoop en efficient mogelijk trematode-infecties in slakken te diagnosticeren. De techniek vermenigvuldigd specifieke DNA fragmenten van de parasiet in wat men noemt de polymerase kettingreactie. Eens het DNA voldoende vermenigvuldigd is kan het zichtbaar gemaakt worden als een bandenpatroon. We hebben deze techniek toegepast om te onderzoeken welke slakkensoorten er geïnfecteerd zijn en of dit parasieten waren die de mens, al dan niet kunnen infecteren. Een tweede doelstelling was het identificeren van alle slakken- en trematodensoorten in het Karibameer op basis van DNA sequentie analyse. De zoetwaterslakjes zijn soms maar enkele millimeters groot terwijl de parasietenlarven amper waar te nemen zijn met het blote oog. Vergeleken met klassieke identificatie op basis van uiterlijke kenmerken, is identificatie op basis van DNA veel nauwkeuriger.
Indringers en zombieslakken
Onze zoektocht leverde verrassende resultaten op. Zo vonden we ondermeer slakken die geïnfecteerd waren met visparasieten. Dit kan erg belangrijk zijn voor de lokale bevolking daar de visserij en viskweek één van de meest belangrijke economische activiteiten zijn van het Karibameer en Zimbabwe.
Boten varen in de ochtend uit voor het vissen van Kapenta, één van de voornaamste activiteiten in het Karibameer.
We ontdekten ook dat het meer enkele nieuwe en invasieve slakken en trematodensoorten herbergt. Invasieve soorten zijn plant- of diersoorten die buiten hun leefgebied voorkomen en daar de inheemse of lokale soorten verdringen. Ze doen met andere woorden aan ‘invasie’ van andere leefgebieden. Eén van de meest opmerkelijke ontdekkingen was de invasieve slak Pseudosuccinea columella uit Noord-Amerika, die frequent voorkomt in het Karibameer en die een trematodensoort van het Fasciola geslacht kan verspreiden. Fasciola staat bekend als de leverbotworm en kan de lever van mensen, vee en zelfs olifanten infecterenNooit eerder werd deze slakkensoort en parasiet waargenomen in het meer, nog in Zimbabwe. Verder vonden we een invasieve slakkensoort uit Zuidoost-Azië en verschillende trematodensoorten die nog nooit in Zimbabwe of zelfs Afrika zijn waargenomen. Een voorbeeld hiervan is de parasiet Leucochloridium die bekend staat om het brein van zijn slakgastheer over te nemen. De ‘zombieslak’ zoekt dan hoger gelegen, zonnige plekken op terwijl de parasiet in de antennen van de slak kruipt, waar hij pulserende kleurrijke banden vormt. Zo wordt de slak opgemerkt door de eindgastheer van de parasiet: een vogel met een appetijt voor slakken.
Een ‘zombieslak’ geinfecteerd met Leucochloridium. © Gilles San Martin
Een sierplant als game changer
Tenslotte onderzochten we de ecologische voorkeuren van de verschillende slakkensoorten. Ecologie is de studie van de biotische (of levende) en abiotische (of niet-levende) factoren in het leefmilieu van een organisme. We onderzochten hoe het aantal slakken wordt beïnvloed door de waterkwaliteit, waterplantvegetatie en tal van andere factoren op verschillende plaatsen rond het meer. Zo konden we aantonen dat sommige slakkensoorten eerder voorkomen in vervuild water en andere soorten in wateren waar de invasieve waterhyacint (Eicchornia crassipes) aanwezig is. We bewezen zo dat kunstmatige waterreservoirs zoals het Karibameer, kansen creëren voor invasieve soorten zoals de waterhyacinth en de slak Pseudosuccinea columella. Zowel slak als plant zijn van Amerikaanse oorsprong en waarschijnlijk geïntroduceerd via handel in waterplanten zoals de waterhacinth, die wereldwijd als sierplant voor vijvers wordt verkocht. Zij kunnen het risico op de verspreiding van lokale èn invasieve parasieten in het gebied aanzienlijk vergroten met catastrofale gevolgen. De waterhyacinth zorgt ook voor tal van andere problemen, zo bedekt hij uitgestrekte delen van het meer die hierdoor niet meer toegankelijk zijn voor boten, dieren en mensen. Het bestrijden van zulke invasieve plantensoorten zou daarom meerdere voordelen kunnen hebben.
Behalve de opmerkelijke diversiteit aan slakken en parasieten, is het Karibameer een verborgen schat aan wetenschappelijke informatie.
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