"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

March 14, 2012

Ligula intestinalis

The star of today's post is a fish tapeworm call Ligula intestinalis, and today's post is about a recently published paper that resulted from a 20 year-long study that monitored the presence of this parasite in the fish community of a reservoir in north-eastern France. The reservoir was originally created in 1986 for buffering the thermal discharge of a nuclear power plant, and data about parasitism of fish in the reservoir have been recorded since 1991. The main parasite infecting fish living in the reservoir is the tapeworm Ligula intestinalis. This parasite infects many different species of freshwater fish, but it mainly parasitises cyprinids (the carp family) such as carp, roach, and dace. A fish becomes infected through eating infected copepods and once inside the fish, the tapeworm develops into a larval stage call a plerocercoid in the fish's body cavity, which goes on to infect fish-eating birds such as herons and cormorants. As you can tell from the photo, the plerocercoid can reach alarming size and mass.

Over the 20 years since records were kept about parasitism of fish in that reservoir, L. intestinalis had progressively shifted its preferred host from roach (Rutilus rutilus) to silver bream (Blicca bjoerkna). The data from this study provide a picture of how this host transfer occurred over the two decades. Prior to 1998, the tapeworm was commonly found in roach and only occasionally found in other fish such as bream. But the roach population suffered from a series of sharp declines during the two decades - once in 1993, and then a more severe collapse in 1997. It was after this second decline that everything changed - in 1998, L. intestinalis began showing up frequently in bream.

It is more surprising that the switch hadn't occurred sooner - the bream made an ideal host for L. intestinalis in the reservoir. Not only was it abundant when the roach population collapsed, it was also more resilient to environmental stressors - such as thermal effluents from a nuclear power plant. Although the reservoir was restocked with additional roaches for anglers in 2002 and 2004, by that time, the parasite had already made the switch to having bream as its preferred host, and it was only found sporadically in roaches - the original host. So even though it seems as if the sliver bream made the ideal host for L. intestinalis in that reservoir, it took a dramatic event - the collapse of the roach population in 1997 - to bring them together. Ligula intestinalis adapted to changes in its circumstances by making an occasional host (bream) into their main host of choice.

Another interest finding of this study was the way L. intestinalis exploits the bream host. The tapeworm adopts a different strategy depending on the host's sex. When L. intestinalis infects a female fish, it diverts resources from her reproductive tissue, but if it infects a male fish, the tapeworm obtain nourishment from the fat reserves. This corroborates earlier studies that found L. intestinalis infection inhibits the reproductive capacity of its host.

But, the most surprising finding was that despite the grotesquely large size of the tapeworm compared to its host, the overall health of infected fish was not noticeably different from uninfected fish - which is remarkable when you consider how many resources the worm has to drain from the fish in order to grow so large. The fact that L. intestinalis was able to divert energy from the fish without compromising its health suggests that it is capable of manipulating the host's physiology with great finesse - fine tuning the physiology of its host in a way that diverts as much energy as possible for its own growth, but at the same time keeping the host alive long enough for it to be eaten by the parasite's next host.

Image from: Trubiroha et al. (2009) International Journal for Parasitology 39: 1465–1473

Reference:
Vanacker, M., Masson, G. and Beisel, J-N. (2012) Host switch and infestation by Ligula intestinalis L. in a silver bream (Blicca bjoerkna L.) population. Parasitology 139: 406–417.

6 comments:

  1. How does it "choose" a host? If any fish eats an infected copepod, it would seem to me that tapeworm is not getting a second chance. Either it can infect the fish and possibly be evolutionarily successful, or its chances of reproduction are nil.

    Unless the ability to infect bream impairs the chances of successfully infect roaches? But you said it could infect both.

    ReplyDelete
  2. Well, apart from call the bream "the main host of choice" - I never used the word "choose". But the fact that while this tapeworm can infect and develop in a range of fish, yet consistently end up in bream indicates that some type of mechanism is ensuring that infected copepod end up in bream instead of roach.

    While "generalist" parasites can successfully infect a range of host, not all host can be consider equal from the perspective of the parasite - and some species serve as better host than others even if they are all equally susceptible. Generalist can also evolve to become specialists, and as you suggested, perhaps the process of adapting to the bream host has attenuated this tapeworms ability to exploit the roach.

    ReplyDelete
  3. "in bream indicates that some type of mechanism is ensuring that infected copepod end up in bream instead of roach"

    Not necessarily, it could be that the parasite is just more successful in Bream.

    ReplyDelete
  4. Go back and read the post again - keep in mind that Ligula intestinalis was doing perfectly fine in roach prior to the population crash:

    "Prior to 1998, the tapeworm was commonly found in roach and only occasionally found in other fish such as bream."

    Additionally, you point was also addressed here:

    "So even though it seems as if the sliver bream made the ideal host for L. intestinalis in that reservoir, it took a dramatic event - the collapse of the roach population in 1997 - to bring them together."

    Hence my comment.

    ReplyDelete
  5. I have recently found this parasite in fresh water minnows ; will it infect the lake trout which in turn eat the minnows ? Is this a problem ?

    ReplyDelete
    Replies
    1. Well the final host for this parasite are birds, so if the infected minnow is eaten by a trout, it probably spells the end for the tapeworm.

      Delete