16 June 2017

Host-Hopping Eye Fluke

Welcome Back. I’ve got a socko idea for a horror flick: A creature (I’ll call it a parasite) invades a body (host). When the parasite is mature and ready to mate, it manipulates the host so it can move on to the parasite’s next host.

What do you think? It’s been done before? Really? OK, so, I guess I’ll just review the source for my idea, a recent study by researchers from Russia’s A.N. Severtsov Institute of Ecology and Evolution and Finland’s University of Jyväskylä. 

Osprey catching fish
(multiple websites)

That study examined different ways a parasite, the eye fluke, manipulates the behavior of its host, a fish, to make the fish more susceptible to the parasite’s next host, a bird.

Eye Fluke Lifecycle
The eye fluke occurs widely in fresh and brackish waters and in most fish species. The different species of eye fluke are placed taxonomically in the class Trematoda and genus Diplostomum.

Their host-hopping lifecycle has been known for quite some time. Writing in 1941, for example, researchers from the Rockefeller Institute for Medical Research and the New Jersey State Fish Hatchery reported that the parasite found in eyes of fish usually requires three hosts to complete its lifecycle.

Permit me to summarize that lifecycle. The eye fluke’s early development is in freshwater snails (host 1) from which free-swimming larva (“cercaria”) move on to fish (host 2), where they migrate to the fish’s eye lens for development of their last larval stage (“metacercaria”). The fish must then be eaten by birds (host 3), because it’s in the bird’s digestive tract that the flukes attain sexual maturity and produce eggs. Those eggs are distributed with bird droppings; they hatch and infect snails.

Eye fluke lifecycle (modified from fishpathogens.net/pathogen/diplostomum-spathaceum)
The recent study built on earlier work to expand the understanding of how the eye fluke predisposes the fish to be eaten by birds, the final host.

The researchers used holding tanks and lots of young rainbow trout to investigate how eye fluke, in their last larval stage (metacercaria, both mature and immature), influences three behaviors the fish uses to evade bird predators: swim deeper, stop activity (“freeze”) and delay before resuming activity.

Fish tested for their swimming depth response as well as for their activity response were either infected with mature metacercariae or used as control (not infected). Fish tested for their delay in resuming activity following a simulated attack were divided into four treatments: infected with mature metacercariae, infected with immature metacercariae, infected with a mixture of mature and immature metacercariae and control.

To simulate an attack, a plate attached to a stick was passed several inches above the tanks, producing a shadow and the fish response.
Depth, Activity and Delay Affected
Analyses of test data on depth and activity after simulated attack showed infection status had a significant effect. Infected fish preferred shallower water and were more active than the control fish. Notably these two behavioral traits were moderately correlated with each other suggesting that eye fluke larvae could alter both with the same underlying mechanism.

Analyses of data on the delay in resuming activity after a simulated attack found fish infected with mature metacercariae or with a mix of mature and immature metacercariae became active significantly sooner than did control fish or fish infected with immature metacercariae.

Overall, fish infected with mature eye fluke larvae were significantly more active, preferred upper water layers and recovered earlier following simulated predator attacks. 

Fish eye infected with eye fluke (from

Wrap Up
Interestingly, the influence of the parasite was not intensity dependent. There was no correlation between the amount of metacercariae used to infect the fish and the effect on behavior. Still, the study revealed that the eye fluke’s effects were significant both when infection intensities were relatively low and when they were comparable to intensities found in natural populations.

Rather than think about infected fish, I should probably go see one of those parasite-hopping movies. Anyway, thanks for stopping by.

Eye fluke study in Behavioral Ecology and Sociobiology journal: link.springer.com/article/10.1007/s00265-017-2300-x
Article on study on New Scientist website: www.newscientist.com/article/2129880-parasite-living-inside-fish-eyeball-controls-its-behaviour/
Memo on eye fluke life history in 1941 The Progressive Fish-Culturalist journal: afs.tandfonline.com/doi/abs/10.1577/1548-8640%281941%298%5B1%3ATLHACO%5D2.0.CO%3B2?needAccess=true&journalCode=uzpf20

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