Archive Building immunity
Jim Treasurer, Simon Wadsworth and Andrew Grant of Marine Harvest (Scotland) report the findings of an experiment with hydrogen peroxide
POSSIBLE development of resistance by sea lice to medicines in therapeutant treatments and how it may be managed is an important issue in sea lice control. This is particularly critical where the range of medicines available to treat salmon is limited. Until recently, dichlorvos (Aquagard, Novartis) and hydrogen peroxide (Salartec, Brenntag; Paramove, Solvay Interox) were the only medicines licensed in the UK for the treatment of Atlantic salmon with a sea lice burden. While dichlorvos is now used rarely, azamethiphos (Salmosan, Novartis) and cypermethrin (Excis, Novartis) have been licensed and are alternative bath treatments.
Resistance of insects to pesticides, particularly organophosphates and pyrethroids, is well established, and is thought to develop by genetic selection for individuals producing detoxifying enzymes and alterations in target sites and in the balance of acetylcholinesterasae. Resistance of sea lice to dichlorvos has been demonstrated and its development described (Roth, 1999). Roth also quoted the possibility of resistance to pyrethroids in Norway.
Recent anecdotal evidence from fish farms in Scotland suggested that treatments with hydrogen peroxide on some farms have been less effective than when first used. Hydrogen peroxide has been used in Scotland since 1992 and has been the main medicine used on many farms due to lice resistance to dichlorvos.
The manufacturers’ recommended treatment is 1500 ppm for 20 minutes, but efficacy depends on water temperature and this recommendation would give ineffective treatments at temperatures below 10 deg C. Treatments are rarely fully effective and 85 to 100 per cent of the mobile stages may be removed. Resistance has been difficult to demonstrate conclusively in farm treatments as the concentration of peroxide varies, with highly variable volumes of water enclosed within the tarpaulin. Temperature and duration of treatment also affect the success of treatments.
Resistance of the salmon louse Lepeophtheirus salmonis to hydrogen peroxide was investigated on a farm in Scotland following regular use of hydrogen peroxide. Resistance of lice on fish from this farm was also compared experimentally in bins containing a known concentration of hydrogen peroxide with lice from a farm where peroxide had not been used.
Efficacy of routine treatments
The effectiveness of hydrogen peroxide was assessed in January 1999 (water temperature 6 deg C, salinity 28 ppt) on farm A (total stock 78,000 fish in cages 16m square and 8m deep) where the fish had been treated exclusively with hydrogen peroxide over six years.
Ten salmon were removed from each of four cages by handnet 24 hours prior to treatment, anaesthetised in 15 ppm benzocaine and total mobile lice (all were L. salmonis) counted. All cages were treated at a peroxide concentration of 2,500 ppm for 23 minutes by the bath method as there was poor efficacy in previous treatments with lower concentrations.
The nets were first raised from 8 to 3m depth, oxygen was supplied, the cage enclosed with a ‘wedge’ tarpaulin and hydrogen peroxide, 50 per cent w/v diluted 1:1 seawater, pumped into the cage to give a concentration checked by titration. The treatment was terminated and the tarpaulin removed after 23 minutes. Lice were counted 24 hours after treatment and the percentage reduction in numbers calculated.
Despite the high concentration and long exposure time, an average of only 63 per cent of motile lice was removed. These poor results confirmed verbal reports of reduced efficacy from farmers and were further tested in experimental trials in large bins, carried out in January 1999.
Development of resistance
From these experiments it can be concluded that Lepeophtheirus were resistant to hydrogen peroxide in experimental bin treatments on a farm treated regularly compared with a farm where hydrogen peroxide had not been used. These results support the findings of less effective farm treatments. In the fullscale treatments, only 63 per cent of lice were removed, even when high concentrations of hydrogen peroxide were used, 2500 ppm for up to 23 minutes.
Resistance of insects to pesticides develops through genetic selection of individuals and, in lice, this may be selection for individuals with cuticle that provides a barrier to penetration by hydrogen peroxide or the presence of detoxifying enzymes, for example, catalase and glutathione reductase.
Lice may have been pre-exposed to ineffective doses of hydrogen peroxide, and concentrations used to treat salmon do not kill most lice. A treatment that immobilises the parasite without a killing action at the treatment dose is likely to develop resistance. However, resistance may also be encouraged by poor application methods.
Hydrogen peroxide cannot be used at a standard concentration and duration in different water temperatures, and treatment is a balance between achieving efficacy and minimising toxicity to the fish.
In the first farm treatments with hydrogen peroxide in Scotland in 1992, only 83 per cent of lice on average were removed. A proportion of the resident lice population therefore remained after treatment, giving scope for selection of characteristics likely to promote resistance.
Implications of resistance for sea lice control
With increasing resistance to hydrogen peroxide, more treatments are required at higher concentrations to achieve a useful reduction in sea lice numbers and this may increase the risk of fish mortalities. An effective sea lice control strategy requires intervention before the ovigerous females shed eggs, and resistance to hydrogen peroxide and the reduced susceptibility of ovigerous lice to treatment compared with other mobile stages, even on a farm that had not used peroxide, may have reduced the effectiveness of the national sea lice control strategy in Scotland in 1998 and 1999. More effective treatments, particularly in-feed compounds, are required to improve the success of the strategy.
A range of medicines that can be used alternately could reduce the development of resistance. The industry in Scotland and all major suppliers of sea lice medicines are therefore discussing the best strategy for using medicines in an attempt to achieve this.