DISEASES OF AQUATIC ORGANISMS Vol. 17: 101-105, 1993 Published November 18 Dis. aquat. Org. Efficacy of ivermectin for control of the salmon louse Lepeophtheirus salmonis on Atlantic salmon 'Department of Fisheries and Oceans, Biological Sciences Branch. Pacific Biological Station, Nanaimo, British Columbia, Canada V9R 5K6 '~epartmentof Zoology, University of British Columbia, Vancouver, British Columbia, Canada V6T 2A9 ABSTRACT The eff~cacyof orally administered lveimectin against the common salmon louse Lepeophthelrus salmonls on Atlant~csalmon Salmo salar was invest~gatedunder laboratory condl- tions Both 3 and 6 doses of ivermectln at a targeted dose of 0 05 mg kg-' f~shadmin~stered in the feed every third day airested the development and reduced the Intensity of lnfect~onby L salmon~sThis IS the first report of dn eff~cacioustreatment agd~nstthe chalimus stages of sea lice Ser~oushead dnd doi- sal body lesions, typical of L salmon~sfeed~ng act~vity, which developed on the control f~shwere ab- sent from the ~vermect~n-tredtedf~sh lvermectin fed at these dosage reglmes resulted in a darken~ng of the fish, but appealed not to reduce their feeding activity KEY WORDS Ivermectin Lepeophthe~russalmonis Parasite control Paras~tetreatment - Salmon louse . Salmo salar Sea lice INTRODUCTION dichlorvos into the marine environment, have made the development of alternative treatment methods fol The marine ectoparasitic copepod Lepeophtheirus sea lice a priority salmonis is one of several species of sea lice that Orally administered ivermectln (22,23-Dihydro- commonly infect, and can cause serious disease in, avermectin B,) has been reported to be effective for sea-farmed salmonids (Brandal & Egidius 1979, the control of sea lice and other parasitic copepods on Kabata 1979, 1988, Pike 1989, Wootten et al. 1982). Atlantic salmon (Palmei et a1 1987, Smith & Cla~ke This species has a direct life cycle consisting of 1988, O'Halloran et a1 1992, Smith et a1 1992, 1993) 5 phases and 10 stages. These include 2 free- Currently, ivermectin administered orally at a dosage swimming naupliar stages, 1 free-swimming infectious level of 0 05 mg kg fish twice weekly is used copepodid stage, 4 attached chalimus stages, 2 pre- as a treatment for sea lice in Ireland and posslbly adult stages, and an adult stage (Johnson & Albright Scotland (Jackson & Costello 1992, Roth et a1 1993) 1991). At ths dose, ivermectin has been reported to be Currently, epizootics of Lepeophtheirus saln~onison non-toxic and effective In controlling sea lice farmed Atlantic salmon Salmo salar are treated in (Jackson & Costello 1992, Smith et a1 1993) Johnson Europe by bath treatments with dichlorvos, the active et a1 (1993) reported on the toxiclty and ingredient in 'Nuvan 500EC' or 'Aquaguard' (Grave et hstopathological effects of lvermectin on salmon al. 1991a, b, Jackson & Costello 1992, Roth et al. 1993). They reported that ivermectin fed to Atlantic salmon These treatments are labour-intensive and highly at a targeted dose rate of 0 05 mg kg-' eveiy second stressful to the fish, and populations of L. salmonis are day for 50 d resulted in 10 % cumulative mortality reported to be developing resistance to these chemi- over the 65 d of the study After 6 doses of ivermectin, cals (Jones et al. 1992).These problems, as well as con- the treated f~shshowed a darkened colouration, cerns of environmental groups about the release of reduced feeding activity, and some loss of equilibrium 0 Inter-Research 1993 102 Dis. aquat. Org. 17: 101-105, 1993 No histopathological effects of ivermectin on the major fish and 10 control fish) days post-treatment. Thirteen organs were observed in their study fish in the 6-dose group were sampled at 21 d post- The objective of the present study was to investigate treatment. Twenty fish died during the trial and the re- the efficacy of orally administered ivermectin for the maining 6 fish were not sampled. control of the salmon louse Lepeophtheirus salmonis In Trial 2, 106 Atlantic salmon that had been infected on Atlantic salmon. in the laboratory were divided into 2 equal groups and assigned to either a 6-dose or a control group. Prior to treatment. 5 fish from each group were sampled and MATERIALS AND METHODS the intensity of Lepeophtheirus salmonis determined. Ten fish from each group were sampled at 21 and 28 d Ovigerous Lepeophtheirus salmonis were collected post-treatment. Twenty-one fish died during the trial from wild chinook salmon Oncorhynchus tshawyts- and the remaining fish were not sampled. cha and sea-farmed Atlantic salmon Salmo salar from Fish were killed by a blow to the head, and their fork the Strait of Georgia, British Columbia, Canada. Eggs lengths and wet weights determined. The body sur- and developing larvae were cultured in a 45 1 tank faces, including the gills, were examined for copepods supplied with flowing sand-filtered seawater (ca 4 1 and the distnbution of the copepods on the fish was re- min-') at 10.8 to 12.1 OC and ambient salinity (29 to corded. The number of copepods present was cor- 31 L.Circulat~on within the tank was maintained by rected to a standard wet body weight to compensate gentle aeration. Nitex screens (100 Km mesh) were at- for differences in size among hosts. tached to the drain to prevent copepod loss. Intensity data were log (x+l)transformed to normal- Naive Atlantic salmon, 16.1 to 21.7 cm fork length ize their distnbution, and differences in copepod inten- and 47.0 to 149.3 g wet weight, were transferred to a sity (number of parasites per infected host) investi- 500 1 tank and infected with newly moulted copepodid gated by analysis of variance (ANOVA) procedures or larvae under conditions of darkness, low water flow, t-tests. Comparisons of copepod intensity for the con- and aeration. Nitex screens (180 pm mesh) were at- trol and treatment groups over time as well as between tached to the tank drains to prevent copepodid loss. different groups at each sampling period were made Fish were maintained in flowing sand-filtered sea- using either t-tests or Scheffe's tests. Stage distribution water at 11.0 to 13.3 "C and ambient salinity (29 to data were compared between groups using a chi- 31%). square statistic (Zar 1984). Ivermectin used in our experiment was in the form of a veterinary preparation of 1 % w/v oral drench (Eqvalan; Merck Frosst Canada). Commercial salmon RESULTS pellets (White Crest) were allowed to dry for 24 h at room temperature and sprayed with a solution of Prior to treatment there was no significant difference Eqvalan diluted with distilled deionized water. Diets (l-way ANOVA and t-test; p < 0.05) in the intensity of were stored frozen in sealed containers until used. The Lepeophtheirus salmonis between treatment and con- level of ivermectin in the treated feed was confirmed trol groups in either trial (Fig. 1). by the Health of Animals Laboratory, Agriculture In Trial 1, the intensity of infection for both the con- Canada, Saskatoon, following the methods given in trol and 3-dose group was significantly different over Doherty et al. (1990). The diets of Trials 1 and 2 con- time (l-way ANOVA; p < 0.001). The results of multiple tained 62.1 and 61.0 %, respectively, of their targeted range tests of copepod intensity showed significant dif- concentrations. ferences among all 3 samples of the control fish A 1.0 %body welght food ration containing ivermec- (Scheffe's test; p<0.05). In the 3-dose group there tin at a targeted dose of 0.05 mg ivermectin kg-' fish were significantly more copepods present pnor to was fed every third day. Fish received either 3 or 6 treatment than at either 14 or 21 d, and there was no doses of ivermectin. Untreated diet was fed on the significant difference in intensity between 14 d and other days at a level of 1.0 %of body weight per day. 21 d (Scheffe's test; ~~0.05).There were significantly In Trial 1, 105 Atlantic salmon that had been infected fewer parasites present at Day 21 than prior to treat- in the laboratory were divided into 3 equal groups. ment in the 6-dose treatment (t-test; p < 0.01). There Each group was assigned to either a 3-dose, a 6-dose, was no significant difference in the intensity of cope- or a control group. Five fish from each treatment and pods between the control and 3-dose group at 14 d control group were sampled prior to treatment and the (t-test; p>0.05), or between any of the 3 groups at intensity of Lepeophtheirus salmonis determined. Fish 21 d (l-way ANOVA; p>0.05). in the 3-dose and control groups were sampled at 14 In Trial 2, the intensity of infection for both the con- (10 treated fish and 10 control fish) and 21 (11 treated trol and 6-dose groups was significa.ntly different over Johnson & Margolis: Efficacy of ivermectln In controlling the salmon louse 103 m Control 3-Doses a6-Doses C, - -4 Pre-Treat. Day 14 Day 21 *G l00 d) m control W Controls 80 -4 70 m 6-Doses 60 cl cd 50 d) 40 30 2 20 10 0 Pre-Treat. Day 21 Day 28 Time Fig. 1. Mean (+SDI intensity of Lepeophtheirus salmonis on W Controls control and ivermectin-treated Atlantic salmon in Trial 1 (upper)and Tr~al2 (lower).Fish were fed either 3 or 6 doses of ivernlectin at a targeted level of 0.05 mg kg-' every third day. Fish were maintained at 11.0 to 13.3 "C and ambient salinity (29 to 31 W.An ' above error bars indicates significant differ- ences in intensity between groups (t-test; p < 0.05) time (l-way ANOVA; p < 0.001).