Gastropod-borne trematode communities of man-made reservoirs in Zimbabwe, with a special focus on Fasciola and Schistosoma helminth parasites

Ruben Schols
Artificiële reservoirs creëren een verstoorde omgeving die invasieve soorten helpt verspreiden en die op hun beurt kunnen leiden tot een invasional cascade. Daarom bestudeerden we slakken en trematoden in vijf artificiële reservoirs in Zimbabwe met behulp van shedding experimenten en moleculaire analyses. In het kader van de One Health benadering, identificeerden we de volledige levenscyclus van drie verschillende parasieten van het nijlpaard. Deze studie toont het belang van invasieve slakken in ziekte overdracht in artificiële reservoirs en zal hopelijk bijdragen tot een verdere bescherming van het nijlpaard in Zimbabwe.

De verborgen passagiers in het nijlpaard

Voor ik het goed en wel besefte zat ik met mijn medestudente Kudzai in een tot de nok gevulde versleten wagen samen met negen soldaten. Het was een vreemd beeld dat me altijd zal bijblijven: ik, als bioloog, met een dissectie setje op de schoot, naast een gewapende soldaat met een hand vol kogels. Hij vond de munitie bovendien uitermate geschikt als wattenstaafje en pulkte er regelmatig mee in zijn oor. Na een hobbelige rit over smalle zandweggetjes kwamen we aan op de locatie. Hier lag het dan, het dier waar ik al dagen op zat te wachten.

Hoe ben ik hier terecht gekomen?

Het verhaal begint eind 2017 toen onderzoekers van het Afrikamuseum op een ongewone zoetwaterslak stuitten tijdens een exploratie in het Karibameer in Zimbabwe. Deze slak is afkomstig uit Noord Amerika, en bleek een onbekende leverbot parasiet met zich mee te dragen. De naam verraadt het al, de leverbot leeft in de lever van vele dieren, gaande van mensen en koeien tot giraffen en nijlpaarden. De onderzoekers vonden dat een uitzonderlijk hoog aantal van deze zoetwaterslakken besmet waren met deze worm, wat betekent dat de infectiedruk zeer hoog ligt. Maar wie de uiteindelijke eindgastheer was, bleef een raadsel. Het doel van mijn thesis was dit mysterie op te lossen. We besloten ons te focussen op het nijlpaard om twee voorname redenen. Ten eerste zijn ze enorm talrijk in het Karibameer en ten tweede bestaat er een leverbot specifiek voor het nijlpaard waar bijna niets over geweten is.

Figure 1

De leverbot heeft een complexe levenscyclus. Via stoelgang van de gastheer komen eitjes in water terecht waar ze zoetwaterslakken infecteren. Binnen de slak ontwikkelen ze zich tot larven. Wanneer ze volwassen zijn, verlaten ze de slak en hechten ze zich geduldig vast aan waterplanten. Wanneer mensen of dieren deze besmette planten opeten, ontwikkelen de larven zich tot volwassen leverbotwormen in de lever van hun finale gastheer waar ze eitjes gaan leggen. Zo kan de cyclus weer opnieuw beginnen.

Bron: sciencejournalforkids.org/articles/why-do-invasive-species-like-artificial-lakes/

 

image 381Het Karibameer vormt deels de grens tussen Zambia en Zimbabwe en is op basis van volume het grootste kunstmatige meer ter wereld. Op locaties 3 en 16 verzamelden we slakken en hun parasieten.

Bron: sciencejournalforkids.org/articles/why-do-invasive-species-like-artificial-lakes/

 

Het verlossende telefoontje

Maandag kwamen Kudzai en ik aan in Kariba en zaterdag ochtend om 5 uur zouden we alweer moeten vertrekken naar Harare, de hoofdstad van Zimbabwe. De park wachters die het natuurreservaat van Kariba beschermen tegen stropers, krijgen elk jaar een quotum aan wilde dieren om te schieten. Ze schieten de dieren om de populaties in evenwicht te houden maar ook als een vorm van verloning. Ze zouden ons contacteren wanneer ze een nijlpaard gingen doden. We moesten daarom zorgen dat we de komende dagen steeds bereikbaar waren. De spanning was daardoor te snijden en veel slaap zat er voor ons niet in. De dagen vlogen voorbij en tegen donderdag had ik voldoende slakken verzameld. Jammer genoeg was er nog geen nieuws van de wachters. Vol spanning plakte ik vrijdag, als het ware, aan mijn gsm. Tegen de avond was er nog steeds geen nieuws en de moed zakte me in de schoenen. Na het avondeten kwam dan toch het verlossende telefoontje. Een kwartier later zat ik in die versleten wagen naast de soldaat met zijn hand vol kogels op weg naar het nijlpaard.

De verborgen passagiers

De wachters verwerkten het nijlpaard in ijltempo en ondertussen onderzochten Kudzai en ik zoveel mogelijk organen op platwormen. Bij de eerste incisie in de lever vonden we ongelooflijk genoeg meteen twee leverbot wormen. Bovendien kregen we de kans om ook een deel van de maag te onderzoeken. Op de maagwand troffen we een duizendtal platwormen aan, behorende tot twee verschillende soorten.

 

image 382

De drie verschillende platwormen gevonden in het nijlpaard. Foto A is de gevonden leverbot. In ware grootte is deze parasiet meer dan 10 cm lang. Bovendien werden er nog twee andere soorten platwormen gevonden in de maag (B en C). Hiervan bleek C het talrijkste met bijna duizend individuen op een stukje maagwand ter grootte van een A4 papier.

 

Het detective werk

Eens terug in België, onderzochten we het DNA van deze wormen en vergeleken dit met parasitair-DNA gevonden in de vorige studie. Op deze manier konden we de levenscyclus reconstrueren - zoals echte detectives - en bleken de exotische slakken twee van de drie gevonden platwormsoorten (soorten A en B in de figuur) over te dragen op het nijlpaard. Bovendien was het inderdaad de verwachtte nijlpaard-specifieke leverbot waar weinig over geweten is. Daarom namen we foto’s van de volwassen wormen en gaan we het DNA online beschikbaar maken. Hierdoor kunnen toekomstige studies eenvoudiger DNA van deze soort uit stoelgang, slakken of volwassen platwormen identificeren.

figuur 4

Nijlpaarden voelen zich veilig in het water. Zolang we hun kortste weg naar het water niet blokkeren zijn we veilig, al moeten we dan nog steeds opletten voor de talrijke krokodillen die ons rauw lusten.

 

De gevolgen voor het nijlpaard

Voorlopig is er nog niet veel geweten over de schade die deze wormen aanrichten in het nijlpaard. Als we kijken naar de ernstige problemen die nauw verwante soorten veroorzaken in bijvoorbeeld koeien en mensen, voorspelt dit niet veel goeds. Zo veroorzaken leverbot infecties inwendige bloedingen en verlagen ze de vruchtbaarheid. In Zimbabwe staan nijlpaardpopulaties, net zoals in de meeste delen van Afrika, onder druk door illegale jacht en habitatverlies. Daarom staat de soort sinds 2006 op de rode lijst van de IUCN als ‘kwetsbaar’ geclassificeerd. Populaties die verzwakt zijn door ziektes zijn extra kwetsbaar voor andere bedreigingen. Daarom is er meer onderzoek nodig naar de verspreiding van deze parasiet. Mede dankzij onze resultaten kan dit nu gebeuren door DNA te isoleren uit slakken en stoelgang zonder een nijlpaard te moeten doden.

Heb je misschien de kriebels gekregen van al die wormen? Geen nood, want voor mensen in België is de kans op infectie tegenwoordig minimaal. Als je dan toch een tropische reis plant, kan het echter geen kwaad om je goed te informeren.

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Universiteit of Hogeschool
Master in de Biologie
Publicatiejaar
2019
Promotor(en)
Filip Volckaert en Tine Huyse
Kernwoorden
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