Cardicola forsteri

Today's parasite is a blood fluke that has been turning up in tuna ranches in South Australia. The blood fluke lives the tuna's circulatory system, and lays eggs that can become lodged in the fish's gills or other organs such the heart, and cause significant lesions in those tissues. This is obviously of great concern to the tuna ranchers, so they set out to find a way of alleviating their fish from infection.

Being a trematode, Cardicola fosteri must have an invertebrate host that is the source infection for the tuna. In their search for the first host of C. forsteri, researchers undertook a a truly heroic effort - sampling over 9000 (!) invertebrates, including all kinds of bivalves, snails, and polychaete worms from the pontoons on the tuna ranch and nearby areas, then meticulously dissected and examined every single one of them for parasitic infections. Those who have been following this blog would know that trematodes usually have a mollusc host in which they undergo asexual multiplication - usually a snail, but C. forsteri is very unusual - it turns out that it uses a polychaete worm, specifically tube-dwelling terebellids - also known as spaghetti worms - for asexual multiplication. Infected worms were packed with hundreds of sac-like sporocysts which continuously churn out the free-living cercarial stages that go on to infect the tuna.

The researchers then used specific sections of the DNA obtained from the parasites to match up the sac-like sporocyst stage in the worms with the adult stage in the tuna, and they were able to confirm that the blood flukes in the tuna were indeed originating from those infected tube-dwelling worms. As those sedentary worms usually live on the seafloor, researchers recommended that simply by moving them to deeper waters, the tuna would be infected by far fewer blood flukes. This study shows how understanding the ecology and life-cycle of a parasite can help us take straightforward measures that can mitigate their impact.

Photo by Robert Adlard

Reference:
Cribb TH, Adlard RD, Hayward CJ, Bott NJ, Ellis D, Evans D, Nowak BF. (2011) The life cycle of Cardicola forsteri (Trematoda: Aporocotylidae), a pathogen of ranched southern bluefin tuna, Thunnus maccoyii. International Journal for Parasitology 41:861-70.