How Fasciola hepatica gained a double life
Go to the Fasciola home page to see state of the art pictures of the life cycle stages
There is no better example, in my view, of animal evolution.
Look at the life cycle diagram and you have to question how a parasite developed a life cycle with two hosts.
How did it happen?
Re-word the question 'Which came first the chicken or the egg?' as ...'What did Fasciola parasitise first? a snail or a vertebrate?
The First Host:
Fasciola 's ancestors were free living flatworms that had been around for a long time (some scientists reckon 50 Million years)
before the evolution and arrival of mollusc (snails) on the planet 500 Million years ago. i.e the first flatworms arrived 550 MYa
ago. These turbellarians (Fasciola 's ancestors) had feeding apparatus and enzymes (pre-adaptations) that were going to allow
exploitation of a new environment i.e. a parasitic life style by penetrating a host (snail) 's surface and feeding on their internal
and highly nutritious tissues.
Looking at parasitism as originally an opportunism will allow you to appreciate how they are now benefitting from a
stunningly intricate set of internal and external features wrought through evolution which were a successful fit to
each challenge of its survival.
Fasciola is a flatworm but it is not a simple thing! It is said that 10% of the body volume of the free living miracidial stage, that
finds and penetrates the snail, is the nervous system! This is a creature with a program for survival if ever there was one.
The one host life cycle of Fasciola 's ancestors would have had two free living stages to ensure that it dispersed in the
environment (and ultimately found new hosts (snails) and new generations of snails.
1. Something like the cercaria that swam away from the snail host and turned into a free living adult.
2. Eggs produced by the free living adult that developed into a ciliated stage that was able to swim and locate new molluscan
hosts to parasitise.
i.e. The parasite had to get in and out of the snail:
Getting in: Penetration by the miracidial stage. You can see this taking place in the image on the Fasciola home page
(taken from our movie).
Getting out: Cercariae emerge from snails but how ? Finding a weak point? Enzymes? Using the tail to drill through?
Bursting out of a weak point in the snail's anatomy? We don't know it all!
How did Fasciola gain a second host?
Accepting that snails were around for a long time before vertebrates arrived through evolution the flatworm parasites of
snails now loaded with the evolved features of parasitism were ready and waiting to accept the challenge of being accidentally
ingested by a vertebrate. Probably the vertebrates initially ate the parasitised molluscs which simply then ate their way into a
second host and wandered around in the body. Exploiting the body of a vertebrate once again used all of the apparatus of
movement and digestion, penetration through the gut and migration through the body. With the liver as the most nutrituous
organ evolution found the route to more egg laying and release through that organ to it's bile ducts.
Getting out :
Dispersal occurs in both space (three dimensions) and time (the fourth dimensions).
Passive dispersal: parasites move around with their hosts before and during release (of offspring / eggs).
Active dispersal: in Fasciola occurs through free living stages swimming
Dispersal in space means physical movement either actively or passively:
Active dispersal: miracidia swim (after hatching from their eggs) and survive ~24 hours in which they move to find a snail host.
Cercariae swim, after active emergence from a host snail, by beating their tails and also can be carried away passively
by rainfall and flood water.
Dispersal in time involves movement in time when in a host and parasite longevity during the egg laying period (patency).
Dispersal in time includes development time of eggs released from the host (vertebrate) via the bile
duct gut and in faeces. Miracidial embryonation takes place outside the host and takes more time, e.g. 17 days at 22C, to mature
into a miracidium that can hatch and swim.
The metacercaria (encysted juvenile) is a Master of time travel. It becomes quiescent (nobody knows exactly how) and
remains immobile in it's protective and dessication resistant cyst with a much reduced metabolic rate. It slowly stashes away it's
excretory products in excretory concretions in its excretory ducts and bladder.
(But what are the biochemistry and genetics that allow quiescence?)
Go back to the Fasciola home page to see the life cycle and inside and outside of Fasciola hepatica.
© Dr. Clive Bennett 2013