SARF069: Evaluation of sensitivity to chemotherapeutants in successive generations of Lepeophtheirus salmonis from a resistant population

Start Date: 04/01/2010
End Date: 31/12/2010
Main Contractor(s): University of Stirling
Other Sponsor(s): Intervet UK Limited

Objectives

To maintain populations of naïve and resistant lice in culture through multiple generations.

To monitor sensitivity levels to a given treatment over a period of 12 months and up to 6 subsequent generations in a population of resistant lice in the absence of any treatments.

To measure the fecundity and successful hatching and development of lice from each generation compared to a naïve control population of lice.

To determine the sensitivity to a given treatment of hybrids of resistant and naïve lice strains

Summary

Optimising sea lice treatment regimes to minimise the development of resistance.

This study will examine and evaluate the effects on lice sensitivity to specific treatments of withholding treatments from a population of lice already shown to have reduced sensitivity to the active ingredient of the treatment.  There are currently reports of reduced sensitivity to certain lice treatments in different parts of Scotland and world wide.  As research is still ongoing into the mechanisms of resistance to different treatments, it is unclear if the inheritable resistance mechanism is 1) reversible or disappears in subsequent generations or 2) infers a cost to the lice which puts them at a disadvantage in population terms when new naïve lice enter the system or 3) infers a greater reproductive capability on resistant lice which would have consequences on the spread of resistant strains between farm sites.

MERL is in a unique position to be able to conduct this study as we have two verified strains of L.salmonis maintained onsite.  Our naïve strain was originally taken from a farm site where the only treatment that had been used was hydrogen peroxide.  Lice from this site were collected approximately seven years ago and have been cultured through many generations without exposure to lice treatments.  Our resistant strain were collected from a farm site in 2008 where a perceived reduction in treatment efficacy suggested a reduction in sensitivity to the treatment compounds used.  These lice have been shown to be five times less sensitive to an existing licensed treatment than our naïve strain using bioassays and in vivo treatments (unpublished data).  We also suspect they are also resistant to other licensed treatments that have been employed by the farm site and this will be tested this year.  Both populations are maintained onsite under a Home Office project license.

By accelerating the reproduction of both lice strains through multiple generations and by the hybridisation of lice in controlled conditions, this study will determine if resistance to lice treatments can be altered at the population level by strategic use of existing lice treatments in conjunction with current knowledge on the epidemiology of lice from wild fish.

This study will also aim to refine the bioassay procedure used to monitor resitence in lice populations and validate this against in-vivo challenge models.