University of Tasmania
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Ectoparasites and associated pathogens affecting farmed salmon during marine grow out in Chile and Australia

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posted on 2023-05-27, 12:24 authored by Gonz‚àö¬8lez Poblete, L
This research characterized some outbreaks of the ectoparasite Caligus rogercresseyi in Chile and compared them to similar infestations in Australia by Caligus longirostris and Ceratothoa banksii, in relation to co-infections by piscirickettsiosis and viral infection salmon anaemia (ISA virus (ISAV)) in Chile and/or amoebic gill disease (AGD ‚- both in Australia and Chile), and according to husbandry management practices. Understanding how and why co-infections occur allows the development of preventive integrated strategies. The co-infection of these pathogens could occur because 1) salmon, normally resistant to a pathogen become, more susceptible to it, due to pre-infection with an ectoparasite or vice-versa, and 2) salmon farms have single-pathogen oriented control strategies that are ineffective. To identify the reservoirs for Neoparamoeba perurans (AGD etiological agent) on ectoparasites from farmed salmon in Tasmania, the presence of this amoeba was investigated on the gill isopod Ceratothoa banksii collected from farmed salmon. Using quantitative real-time PCR analysis it was demonstrated that N. perurans was either present in low concentration or absent on the surface, and absent inside the gill isopods from salmon that had not been freshwater bathed for AGD during 200 or more days. Thus it is unlikely that these isopods acted as reservoirs or vectors for the amoeba. In contrast, with the potential advent of alternative (non-freshwater) AGD treatments it is possible that Ceratothoid parasitism will become a potential problem on Tasmanian salmon farms. An investigation of the environmental reservoirs of N. perurans during tank-based experimental infections demonstrated that the concentration of N. perurans was significantly higher in the running water (13 ¬± 7 cells/L after 16 days) than on the interphase surface air-water-tank (0.01 ¬±0.1 cells/L). Although the number of N. perurans on fish gills was not determined in this tank-based study, on farmed Atlantic salmon from the Huon estuary chronically affected with AGD, amoebae detected from gill swabs ranged from 0-1042 cells/swab, which could indicate that fish themselves are reservoirs of this pathogen. To characterize the outbreaks of the sea lice with the other main pathogens (N. perurans, Piscirickettsia salmonis and ISAV), moribund salmonids were sampled in Chile in 2011 from two neighboring farms. On one farm, rainbow trout were bath-treated with deltamethrin for sea lice fortnightly in October and November and Atlantic salmon from the other farm treated only after the monitoring on 9 November. Co-infection with P. salmonis was present in 28% of rainbow trout and 10-12% of Atlantic salmon, with asymptomatic ISAV (pathogenic strain HPR 7b) in 17% and 80% of the trout and salmon respectively. Fish had not been treated for sea lice despite having a parasite load exceeding 6 lice per fish which is the load where treatment is mandatory. They also showed microscopically and gross gill damage or excess mucus production. The etiological agent of this gill pathology is still unclear, as fish and sea water from farms were free from N. perurans. Water samples did not contain any significant levels of contaminants, diatoms or dinoflagellates and gill damage was not caused by the chemical baths. Apparently healthy Atlantic salmon sampled in 2013 in a farm in Chile that had been subjected to monthly cypermethrin lice treatments, showed no P. salmonis but 33% of the fish in April and 22% in June, were asymptomatic carriers of the pathogenic strain ISAV (HPR 7b). The routine monitoring mortalities in this farm determined that 2% and 12% died by piscirickttsiosis in April and June respectively. Sea lice abundance was lower in most netpens in April with higher water temperatures and AGD presence compared to June with lower temperatures and without AGD. A diverse level of chronic gill lesions were observed on fish samples such as the undetermined gill pathology on fish in 2011, seasonal AGD in March-April 2013 and multiple mucous cells in June 2013. This gill pathology could be the source of fish mortality during sea louse treatments. It could also cause stress on farmed fish that can promote the fast multiplication of the remaining sea lice on the immunosuppressed farmed fish. This could effectively maintain a chronic infestation. Systematic chemical baths that are compulsory for sea lice treatment in Chile can increase gill damage, they can cause stress and predispose fish to outbreaks of ISA or piscirickettsiosis. A reduction of frequency of chemical baths is advised, particularly considering that in Chile, there are no wild populations of fish that need protection from C. rogercresseyi released from farms. This will also help with the fulfilment of the good practice measures to control sea lice and ISA by the reduction of salmon handling and limited use of chemical therapies. In addition, on farms with piscirickettsiosis outbreaks, the daily removal not only of mortality but particularly of moribund fish is required. In relation to environmental friendly treatments, although freshwater baths for AGD could be an alternative control for sea lice, the location far from estuarine areas of most salmon farms in Chile and the lack of plentiful freshwater indicate difficulties for the implementation of this method.


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