Effectiveness of hydrogen peroxide in the control of Caligus rogercresseyi in Chile and implications for sea louse management
Introduction
Caligus rogercresseyi Boxshall and Bravo (2000) has been the most important parasite of farmed salmon in the south of Chile since it was first recorded infecting Atlantic salmon (Salmo salar) in spring 1997 (Bravo, 2003). The most susceptible hosts are Atlantic salmon and rainbow trout (Oncorhynchus mykiss). A wide variety of medicines (Table 1) has been used to control this parasite since Caligus teres was first recorded in Chile in 1981 (Reyes and Bravo, 1983). However, emamectin benzoate (EMB) (SLICE®, Schering Plough) is the medicine that has been used for the longest period as it was introduced to the Chilean market at the end of 1999. EMB was recognized to have high effectiveness in controlling all development stages of sea lice and application through the feed was easily facilitated (Stone et al., 1999).
Evidence of the loss of sensitivity of C. rogercresseyi to EMB in Chile was observed by fish farmers in early 2005. This situation was corroborated through sensitivity studies carried out in the period from 2006 to 2007 (Bravo et al., 2008b). The EC50 values recorded from C. rogercresseyi collected from 18 sites indicated a strong loss of sensitivity of C. rogercresseyi to EMB at all farm sites compared with an EC50 control. The industry therefore investigated alternative medicines to control sea lice, and hydrogen peroxide was the only available alternative in Chile during that period as it did not require a veterinary license (www.sag.cl).
The use of hydrogen peroxide to treat sea lice was first developed in Norway (Thomassen, 1993) and was later adopted in Scotland in the early 1990s (Rae, 2002). Hydrogen peroxide became the primary treatment for sea lice in Scotland (Lepeophtheirus salmonis) from 1993 to 1998 due to the loss of sensitivity to Aquagard (dichlorvos) until Excis (cypermethrin) and oral treatments were authorized by the Veterinary Medicines Directorate in 1998 and 2000 respectively (Treasurer et al., 2000a). Hydrogen peroxide was marketed in Chile from February 2007 as an alternative to emamectin benzoate, without restriction, as it was perceived as an environmentally friendly product. An advantage with hydrogen peroxide is that the application of oxygen is not required during the treatment due to the liberation of O2 by H2O2. The first trials with hydrogen peroxide were carried out in Chile as long ago as 1994 (pers. obs.). The concentrations used were 0.5 and 0.8 g l−1 utilizing an enclosed tarpaulin. Although there was good control in the adult Caligus there was a poor effect over the chalimus stages and farmers therefore chose to treat the parasitised fish with ivermectin which was effective over all developmental stages of Caligus (Johnson and Margolis, 1993). During this period C. teres was the sea louse species parasitising trout and coho salmon (Bravo, 2003).
Hydrogen peroxide induces mechanical paralysis caused by the formation of bubbles in the haemolymph which detaches the lice and they float to the water surface (Thomassen, 1993, Bruno and Raynard, 1994, Treasurer et al., 2000b). Manufacturers have consistently recommended a treatment concentration and duration of 1.5 g l−1 for 20 min based on the work of trials as reported by Thomassen (1993). This early work identified poorer efficacy at lower temperatures and toxicity at higher temperatures and especially > 14 °C. A later study by Treasurer et al. (2000a) showed that there was trade off between concentration and temperature that had to be considered when conducting treatments. In that study poor efficacy of hydrogen peroxide treatment was reported, with a reduction of only 7.5% in the lice number when fish were treated with 2000 ppm for 20 min, and this led to the conclusion that L. salmonis had developed resistance to hydrogen peroxide after multiple treatments (Treasurer et al. 2000a). Thomassen (1993) indicated that no reinfection of salmon with L. salmonis had been reported following treatment with hydrogen peroxide. Another study showed that, although L. salmonis recovered from treatment, there was little evidence that L. salmonis reinfested their hosts (Treasurer and Grant, 1997). Although Caligus elongatus is also found on salmon in Europe and North America most researchers have focussed on L. salmonis and there have been no published reports of the efficacy of hydrogen peroxide in controlling Caligus.
The aim of the present study is to assess the efficacy of treatments with hydrogen peroxide in the control of Caligus in field applications in Chile. In this respect it should be noted that hydrogen peroxide is being used again in Scotland and Norway after an absence of 10 years (Ian Armstrong, pers. comm.). This renewed interest and the likelihood that this medicine will also be used in Chile in the future has given the development of optimal treatment regimens for hydrogen peroxide a high profile, especially against the back drop of likely global loss of sensitivity to current oral treatments as has been demonstrated in Chile (Bravo, 2008b).
Section snippets
Materials and methods
The effectiveness of hydrogen peroxide in the control of C. rogercresseyi in Atlantic salmon was evaluated on two farms, operated by the same company, located in Puerto Montt, Region X (Lat 42ºS) (Fig. 1) in the period June to August 2007. Ten fish were randomly sampled from each of a range of cages on each farm 1 day before the treatment and also 10 fish were sampled 24 h post treatment, following the instructions recommended by the National Fisheries Service, Sernapesca. A tarpaulin skirt of 120
The effectiveness of hydrogen peroxide by skirt treatment
The optimum results obtained 24 h after the application of the treatment with hydrogen peroxide in Farm A were obtained in cage A-5 (Table 2), where there was a reduction of 84.5% of all lice stages. This was followed by cage A-9 (83.4%) and the lowest values were recorded in cage A-2 (51.8%). In adult lice 59.3% and 87.1% were removed compared with values of between 45.1% and 82.3% for chalimus.
The highest abundance of lice before treatment was recorded in cage A-6, where the average number was
Discussion
This study demonstrated that efficacy of farm treatments of salmon using hydrogen peroxide at a concentration of 1.5 g l−1 at a temperature range of 9.7–10.1 °C, varied from 59.3 to 87.1% for adult lice and 57.4 to 85.4% using the skirt method in on-growing fish and also in broodstock (Tables 2 and 3). In contrast, there was a wide range in effectiveness using an enclosed tarpaulin method in post smolts. In some cages a higher number of lice were recorded after the treatment application and this
Acknowledgements
The authors give special thanks to Francisca Erranz, Pier Bassaletti and Veronica Pozo for their assistance with field sampling.
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2020, Marine Environmental Research