Evaluation of the performance of pyrethroids on different life stages of Caligus rogercresseyi in southern Chile

Highlights

• We evaluated the performance of pyrethroids on C. rogercresseyi in the field.

• One cypermethrin and two deltamethrin-based products were assessed.

• All products were associated with significant reductions in all sea lice stages.

• Pyrethroids appeared to be less effective on juvenile compared to adult lice.

• Cypermethrin showed a greater effect on gravid female lice than deltamethrin.

Abstract

Control of sea lice in Chile is largely based on antiparasitic treatments, synthetic pyrethroids being the most used drugs. In recent years, farmers in Chile have reported decreased performance of pyrethroid-based treatments. The aim of this study was to assess the performance of two deltamethrin-based (Alphamax® and a generic product) and one cypermethrin-based (Betamax®) product on the different life stages of Caligus rogercresseyi, while controlling potential confounders. We found that both deltamethrin products and the cypermethrin product had a significant effect on the reduction of juvenile, mobile adult, and gravid female lice, compared with untreated pens; however, the effect on juvenile lice was less than on mobile stages. There was no evidence that pyrethroids performed better on certain mobile life stages, such as gravid females. When the three products were compared, no significant differences were observed in the numbers of juvenile, adult male, and non-gravid female lice after we controlled for potential confounders; however, cypermethrin exhibited a small, yet significantly greater effect on the gravid female group when compared with one of the deltamethrin-based products. We also confirmed that other factors besides the product choice, such as the pre-treatment sea lice abundance, water temperature and salinity, and time elapsed to the post-treatment sample, affect the post-treatment sea lice level as well, and therefore, they should be taken into consideration when assessing the effect of immersion treatments.

Introduction

Sea lice is considered one of the most economically significant parasites to salmon industries around the world (Costello, 2006, Costello, 2009). In Chile, the most important species of sea lice is Caligus rogercresseyi, affecting mainly Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss), while Coho salmon (Oncorhynchus kisutch) appear more resistant (Bravo, 2003, González et al., 2000). Heavy infections can result in reduced growth and feed-conversion efficiency, as well as reduced marketability of fish due to skin lesions (Costello, 2009). For this reason most salmon industries, including the Chilean industry, try to maintain relatively low sea lice levels.

Sea lice control in Chile largely relies on pharmacologic treatments that target either the adult or the juvenile life stages of the parasite on the fish. Decision to treat infected fish is made at the individual farm level; however, farms within a particular farm management area (i.e. neighborhood) are encouraged to co-ordinate their bath treatments within a few weeks of each other (Sernapesca, 2012). At present, five drugs are approved for use against C. rogercresseyi in Chile: emamectin benzoate (EMB), deltamethrin, cypermethrin, diflubenzuron, and azamethiphos (SAG, 2013). Currently, the synthetic pyrethroids deltamethrin and cypermethrin are the most commonly used bath products for controlling C. rogercresseyi. These products are administered topically through immersion treatments, commonly called ‘baths’. Deltamethrin is approved for mobile (adult males, non-gravid, and gravid females) stages, while cypermethrin is labelled for controlling both juvenile and mobile stages of C. rogercresseyi (SAG, 2013). There are two deltamethrin-based products available: Alphamax® (Pharmaq) and a generic deltamethrin (FAV), and one cypermethrin-based product, Betamax® (Novartis) (SAG, 2013). Synthetic pyrethroids are frequently combined with EMB and, to a lesser extent, with diflubenzuron (R. Ibarra, pers. comm.), which targets juvenile stages. Pyrethroids interfere with nerve impulse transmission by stimulating the sodium channels in neural cells that produce spastic paralysis and subsequently death of the parasite (Burka et al., 2012, Corner et al., 2008, Torrissen et al., 2013).

As with other parasite organisms, sea lice copepods, specifically Lepeophtheirus salmonis, have expressed resistance patterns against synthetic pyrethroids in Norway, Scotland and Ireland, where cypermethrin and deltamethrin have been used since mid 1990s (Sevatdal and Horsberg, 2003, Sevatdal et al., 2005).

In Chile, there is no information published regarding synthetic pyrethroid efficacy or resistance of C. rogercresseyi to these chemotherapeutants; however, there are anecdotal reports of reduced efficacy after some pyrethroid-based treatments (R. Ibarra, pers. comm.). Greater insight about the susceptibility of particular lice development stages to specific drugs is needed, because it would ensure better selection of products, allowing the application of targeted therapies, which may delay the development of resistance to chemotherapeutants. The aim of this study was to assess the performance of pyrethroid-based immersion treatments in Chile over the last two years and compare the three available products on the different development stages of sea lice, while controlling other important explanatory variables of the post-treatment sea lice levels.