"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

April 2, 2012

Diplostomum pseudospathaceum

Most animals are infected by multiple species of parasites or multiple strains of the same parasite species that are not close kin. Rarely does an individual parasite (and its close kin) gets to monopolise the resources of an entire host. So do parasites like to share? The answer to that depends on the host (and parasite) in question. Because different species often exploit a single host in different ways, a parasite can share a host with many other species without ever coming into conflict with them. But, when a parasite finds itself sharing a host with members of its own species, competition is more likely to occur as they are all going to be after the same (limited) resource (whatever that may be) from the host.

For some parasites, competition arising from coinfection can favour the most virulent strains, leading to greater overall harm to the host. But more recent studies have shown that the most competitive strains are not always the most virulent. In some cases, competition between co-occurring parasites can actually be beneficial for the host as the parasites end up mutually suppressing each other. How this plays out depends on how the parasite uses its host, and in parasites that have complex life-cycles, this can change from host to host.

The parasite we are looking at today is Diplostomum pseudospathaceum - more commonly known as the eyefluke. As with most parasitic flukes, D. pseudospathaceum uses a snail for the clonal stage of its life-cycle to make thousands of larval stages (called cercariae), which are then released into the water to infect any nearby fish. In the fish, the parasite migrates to its eye and uses it as a temporary vehicle to reach its next host - a fish-eating bird. Diplostomum pseudospathaceum essentially turns its snail host into a parasite factory, using the snail's bodily reserves as raw material to produce its army of clones. There is only so much to go around inside a snail, and when a particular D. pseudospathaceum strain has to share a snail with other strains, it ends up producing many fewer cercariae than if it had the whole snail to itself. But while it is detrimental for D. pseudospathaceum to have close company in the snail, it is a different story in the fish.

While D. pseudospathaceum undergoes a resource-hungry spree of rampant asexual multiplication inside the snail, it is relatively dormant inside its fish host. All it needs from the fish is a space to tuck into within its eye - and there is plenty of room in the fish's eye for these flukes. In fact the more the merrier given that at high numbers the eyeflukes can induce the formation of cataracts - and a half-blind fish is more likely to be eaten by a bird. But there is also another reason for D. pseudospathaceum to be more welcoming to strangers in the fish host.

The immune system of the fish is remarkably adept at responding to intruding parasites, and the immune system can be quickly "primed" towards recognising and destroying particular parasite strains. To reach the fish's eye, D. pseudospathaceum has to complete a treacherous journey through the fish's body, all while under close scrutiny and assault by the fish's immune system. In fish that have previously been exposed (and thus "primed") to D. pseudospathaceum cercariae, the chances of the parasite successfully reaching its destination are greatly diminished. But while a fish might be "primed" towards cercariae of a particular strain, a double- or even triple-prong attack is likely to overwhelm its finely-tuned targeting system. When exposed to simultaneous attacks from multiple, genetically-diverse strains of D. pseudospathaceum cercariae, the fish's immune system becomes overwhelmed, which in turn allows more eyeflukes to slip by.

In the life-cycle of D. pseudospathaceum, there is a time and place for everything - while there is a time to be on the look out for number one, there is also a time to love thy neighbour.

image by Tina Loy, modified from here

Karvonen A, Rellstab C, Louhi KR, Jokela J. (2012) Synchronous attack is advantageous: mixed genotype infections lead to higher infection success in trematode parasites. Proceeding of the Royal Society B 279: 171–176


  1. Another fascinating post- thanks. How does the described dynamic play out in the final host?

  2. No one really knows at the moment Storm, most parasitic flukes are usually pretty benign in their final host, of course the ones that cause pathology gets the most attention, but the vast majority are tiny little worms living in the intestine of their final host, and don't don't cause any pathology unless there is a huge number of them, so I don't know if there is much competition.

    In intestinal parasites which obtain nutrients from their host by absorbing the digested content of of the small intestine such as tapeworms and acanthocephalans, there are number of studies which show there is competition between different species, with the more competitive species displacing other species from the most ideal spot to attach (basically upstream - the intestinal content lose its nutrient level further downstream as they transform into poop).

    However, most trematodes are tissue-feeders which "graze" on the tissue and mucus of the intestinal wall. So far, because of a number of technical challenges, there hasn't been any study which looked at how intraspecific interactions play out in intestinal flukes, though I look forward to the day when we can have the means to do so.

    I'd imagine doing it with the methods available to us now would require experimental infection of bird hosts with metacercarial stages of know genotypes, then sacrificing the host a few weeks later and quantify the establishment success of the different genotypes - but this will not provide an accurate picture of the dynamics involved. When we have accessible microscope-level magnification on endoscopes and means of taking biopsies of the parasites from the intestinal wall of the final host, then perhaps we can work towards providing an answer to your question!

  3. thats all fine and dandy....i want to know how to get rid of them....my fish deffinately shows the signs of having eye flukes....can my other fish get them from him? is there a cure? do i have to destroy all my fish?

  4. my fish killed the 2 snails we had, now his eye is all cloudy and bulging...what do we need to do? can our other fish get this from him? is there a cure? should we destroy him or our other fish?

  5. This parasite has a complex life-cycle and infected fish cannot pass it between each other.

    There might be antiparasite chemical available for the treatment of of these parasites, but you should seek advice from your local aquaculturist/veterinarian for a proper diagnosis and which chemicals to use for treatment. Please take note of the disclaimer on the side:

    "We do not give any medical or veterinary advice. If you think you or your pets have a parasite, please seek the appropriate care you need from your own doctor or veterinarian."

  6. I’m curious if the parasite was to say enter a humane host could it i cause a Cataract? Or will it simply be just a case of swimmers itch

    1. No, they will simply perish inside of a mammalian, not that cercariae will even seek out a mammal in the first place since their target are fish.