High level efficacy of lufenuron against sea lice (Lepeophtheirus salmonis) linked to rapid impact on moulting processes

Abstract

Drug resistance in the salmon louse Lepeophtheirus salmonis is a global issue for Atlantic salmon aquaculture. Multiple resistance has been described across most available compound classes with the exception of the benzoylureas. To target this gap in effective management of L. salmonis and other species of sea lice (e.g. Caligus spp.), Elanco Animal Health is developing an in-feed treatment containing lufenuron (a benzoylurea) to be administered prior to seawater transfer of salmon smolts and to provide long-term protection of salmon against sea lice infestations. Benzoylureas disrupt chitin synthesis, formation, and deposition during all moulting events. However, the mechanism(s) of action are not yet fully understood and most research completed to date has focused on insects. We exposed the first parasitic stage of L. salmonis to 700 ppb lufenuron for three hours and observed over 90% reduction in survival to the chalimus II life stage on the host, as compared to vehicle controls. This agrees with a follow up in vivo administration study on the host, which showed >95% reduction by the chalimus I stage. Transcriptomic responses of salmon lice exposed to lufenuron included genes related to moulting, epithelial differentiation, solute transport, and general developmental processes. Global metabolite profiles also suggest that membrane stability and fluidity is impacted in treated lice. These molecular signals are likely the underpinnings of an abnormal moulting process and cuticle formation observed ultrastructurally using transmission electron microscopy. Treated nauplii-staged lice exhibited multiple abnormalities in the integument, suggesting that the coordinated assembly of the epi- and procuticle is impaired. In all cases, treatment with lufenuron had rapid impacts on L. salmonis development. We describe multiple experiments to characterize the efficacy of lufenuron on eggs, larvae, and parasitic stages of L. salmonis, and provide the most comprehensive assessment of the physiological responses of a marine arthropod to a benzoylurea chemical.

Effects of chitin synthesis inhibitor treatment on Lepeophtheirus salmonis (Copepoda, Caligidae) larvae

“…No significant difference was found in the transcriptional levels in LF-treated nauplius II, while for DFB, HX, and TFB only slightly up- or down-regulation were found in one investigated gene. These results are in accordance with other studies where the transcriptional level of chitin metabolism genes was not affected by CSI treatment [17,54]. In this study, LsCHS1 was the only gene that showed a different transcriptional level in more than one CSIs tested.…”

“…A recent study in L . salmonis showed that lufenuron-treated nauplius had a less electron-dense layer, poorly organized epicuticle, and a diffuse procuticle [17], which could be a result of less chitin production and could explain the flexible cuticle structure of the affected copepodids observed in the present study.…”

Increasing temperatures accentuate negative fitness consequences of a marine parasite

“…We infested the fish by exposing them to copepodid (i.e., larval) L. salmonis as described in Poley et al 81 and Whyte et al 82 . Briefly, adult female lice with egg strings were collected from sea-cage sites in Bay of Fundy, New Brunswick (NB), in August 2019.…”

Evaluating the potential for sea lice to evolve freshwater tolerance as a consequence of freshwater treatments in salmon aquaculture

“…Freshwater treatments contributed 5.7 and 5.0% of the total registered NMM treatments in 2017 and 2018, respectively (Hjeltnes et al 2019). In addition to freshwater and warm water treatments and prophylactic skirts, other methods in development include novel chemotherapeutants (Poley et al 2018) and selective breeding for sea louse resistant Atlantic salmon (Gharbi et al 2015).…”