Throwing out amoebae with the bathwater
Gill detached Neoparamoeba perurans trophozoites in seawater, one day after a 4 hour freshwater exposure. Photo courtesy of Chris Stratford.
Amoebic gill disease (AGD) has significant impact on global production of Atlantic salmon in marine net pens. The disease is caused by Neoparamoeba perurans which attach to the gill, causing gill pathology, respiratory compromise, reduced productivity and leading to fish losses. AGD is traditionally treated by transferring the salmon for 2 – 3 hours into a freshwater filled tarpaulin which is then removed to return the fish back into their pen while the bathwater is released in situ. More recently, treatment is undertaken in large enclosed wellboats which are capable of retaining the used bathwater for some time after fish have been returned to their pen.
A previous CSIRO in vitro study demonstrated that some amoebae can survive freshwater exposure by transforming to a rounded ‘pseudocyst’ form, but it was unknown whether these surviving amoebae are capable of causing AGD. Therefore, we freshwater bathed AGD affected salmon for 2 hours and mixed the used bathwater into seawater (at 30 ppt salinity) to expose AGD-naïve salmon. Furthermore, to explore the impact of holding used bathwater in a wellboat prior to release, we held the used bathwater for a further 2, 4 or 6 hours before exposing naïve fish. At 20 days post inoculation (dpi), advanced AGD had developed in the fish that had been exposed to the 2 h bathwater, confirming that infective amoebae had survived freshwater exposure and were able to cause disease. When bathwater was held prior to release into seawater the infectivity was greatly reduced. By 40 dpi, AGD was seen in the fish exposed to bathwater that had been held for a further two hours. AGD signs were variable and reduced following longer bathwater holding times.
This research demonstrates that infective amoebae can be released into the marine environment following freshwater bathing, risking infection to nearby fish. However, wellboats provide the opportunity to limit the release of infective amoebae by retaining used bathwater for an extended period. There is potential to explore wastewater treatment methods that could further improve wellboat biosecurity.
More details of this work can be found at https://doi.org/10.3390/microorganisms9050967
CSIRO Aquaculture contact
Dr Richard Taylor– Senior Experimental Scientist