When Parasites Invade: A Need for Hosts

When the host of a parasite invades new territory, the parasite species might invade with it.  In a recent Journal of Parasitology paper, Novak and Goater (2013) explored how the lung fluke Haematoloechus longiplexus invaded Vancouver Island when it’s definitive host bullfrog (Lithobates catesbeianus) was introduced in the 1930’s and subsequently established in the wild.

H. longiplexus has a complex life cycle that requires three hosts.  Therefore, in order for H. longiplexus to establish in new territory, the two intermediate hosts must be present along with the invading bullfrog.

H. medioplexus life cycle

This life cycle of Haematoloechus medioplexus is very similar to that of Haematoloechus longiplexus. Photo Credit: Graphic Images of Parasites.

Physa snails are geographically widespread, and occurred in Vancouver Island before the bullfrogs invaded.  Furthermore, Novak and Goater (2013) found H. longiplexus metacercariae in six damselfly species in sites along the East coast of Vancouver Island.  Therefore, H. longiplexus went to Vancouver Island with its bullfrog hosts, and was lucky enough to find functional intermediate hosts already present.  Invasion success!

Interesting note:  No dragonflies had H. longiplexus metacercariae, so H. longiplexus is a damselfly specialist in Vancouver Island.  One explanation for this host specificity might be that when dragonfly larvae undergo metamorphosis, they lose their metacercariae, but metacercariae in damselfly larvae aren’t lost.  So, if a bullfrog snacks on an adult dragonfly, it won’t get infected, but if snacks on an adult damselfly, it might!


Its a snack, its a hat…
Photo Credit: Today.com

Can you think of any other reasons for host specificity in H. longiplexus?


Novak, C.W. and T.M. Goater.  2013. Introduced Bullfrogs and Their Parasites: Haematoloechus longiplexus (Trematoda) Exploits Diverse Damselfly Intermediate Hosts on Vancouver Island. Journal of Parasitology, 99(1): 59-63.

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