Kudoa islandica

Today's post features a newly described species of parasite, which is found in the muscles of some fish that are not exactly prized for their appearance. Regardless of how they look, these fish are commercially prized. But today's featured parasite has a queasy trick that ruins their host's value on the market - its tendency to liquefy fish fillet.

SEM photos of K. islandica spore (from the paper)

Kudoa islandica is a species of myxozoan parasite which infects a number of different marine fishes from the coasts of Iceland. The first of these are two species of wolffish - the Atlantic wolffish and the Spotted wolffish. Both have short bulldog-like faces and a formidable set of teeth to match. Wolffish is harvested for its flesh and it is commonly eaten in Iceland, but on top that, its skin can also be turned into a type of designer leather. The other host of K. islandica is the lumpfish, which is harvest for its flesh which are usually dried or smoked. Lumpfish eggs are also used as a caviar substitute.

Because of the many commercial uses for the wolffish, it was considered as a candidate for aquaculture and experimental farming of wolfish was initiated in the early 2000s. Samples of these farmed fish were also sent regularly to the Fish Disease Laboratory at the University of Iceland to examine them for any pathogens. It was during these routine examinations that K. islandica was discovered. While the parasite was not described at the time, its presence has been known informally for decades. Icelandic fishermen called soft-fleshed wolffish “hárasteinbítur”, which means “hairy wolffish” (the "hair" are the parasite's plasmodia stage).

Since it was initially found in farmed fishes, the scientists at the Fish Disease Laboratory decided to see if this parasite was also found in wild marine fish of Icelandic waters. They caught some wild wolffish and lumpfish from Bay Faxaflói off the west coast of Iceland and found that the wolffish had relatively light to moderate level of infected by K. islandica. In contrast, some of the lumpfish were more heavily infected. In fact, some of them so were so loaded with the parasite that large proportion of their flesh had been replaced by K. islandica plasmodia. This parasite proliferates in the fish's flesh, taking over much of the muscle fibres they invade. However, it does not seem to cause the fish much ill effect, and the lumpfish seems surprisingly fine with their muscle tissues being replaced by parasites, with no signs of inflammation or fibrosis.

Photo of infected lumpfish fillet (from the paper)

It is after the host has died that this parasite begins to unleash its mayhem. Heavily infected fish exhibit "soft flesh syndrome" which seems to be caused an enzyme that is activate by changes in pH which accompanies fish death. This cause the flesh to literally liquefy. In the wild, this process would liberate the infective stages of the parasite into the environment where they can be ingested by the next host in the life cycle, which are small invertebrates such as marine worms. This process cannot be halted by freezing and the melting fish fillets becomes unmarketable.

One of K. islandica's host - the lumpfish - is currently being trialled as a potential cleaner fish that can be used to combat sea lice in salmon farms. Considering that parasites from the Kudoa genus are generally are not picky about what fish it hops into, there is potential for K. islandica to jump host from lumpfish to salmon (which is already infected with its own Kudoa parasite - K. thyrsites), making it key priority to work out the ecology and life-cycle of this flesh-melting parasite.

Reference:
Kristmundsson, Á., & Freeman, M. A. (2014). Negative effects of Kudoa islandica n. sp.(Myxosporea: Kudoidae) on aquaculture and wild fisheries in Iceland. International Journal for Parasitology: Parasites and Wildlife 3: 135-146.