Sea Lice Research Update

Sea lice (Lepeophtheirus salmonis and Caligus elongatus) research in Stirling originated at the time of the first outbreaks in the early days of salmon mariculture development. However, the first funded project was jointly awarded to Chris Sommerville and Rod Wootten whilst Rod still worked at DAFS (now Marine Scotland). Little was known at that time of the biology of the sea lice. The early research at Stirling described, amongst other things, a detailed life cycle of L. salmon is and generation times at different seasonal temperatures, information which was passed on directly to the fish farmers and which aided in the development of treatment schedules. A laminated illustration of the parasite and its life stages was made and soon found its way onto the wall of the site office at most farms. Further work established a suitable practical sampling regime for fish farmers and they were advised how to monitor the parasite development in cage systems.

Farmers were trained through site visits and short courses at Stirling on how to recognise the different species of lice, L. salmonis and C. elongatus, how to identify each life stage and how to monitor the farm sites using our sampling regime; a technique which is basically still widely used today.

Clarification through experimentation showed that there were susceptible and non-susceptible stages to the then current treatment with dichlorvos and showed that it was important to treat at the most appropriate time to maximise efficacy and to minimise cost in labour and in environmental terms. This data, together with the epidemiological data (see below) determined the optimum time for treatments to deliver a) a more effective treatment, b) a reduced need to treat, and c) better long term disease management. A system was setup which invited farmers to send samples to Stirling for identification and for counting.

A further study on the method of application of treatment chemical, compared full, partial and no- enclosure of cages during treatment and made recommendations for their use. Treating a whole site simultaneously was stressed to be important in delaying the development of resistance. However, detection of reduced sensitivity of lice to dichlorvos was observed as early as 1989 and at about to same time we developed an assay for cholinesterase activity and the effects of dichlorvos treatment on fish acetyl cholinesterase activity was assessed. Experiments which looked at repeat treatments at 3 and 6 day intervals showed that there was a cumulative effect of treatments with short intervals which resulted in incremental reductions in acetyl cholinesterase activity. Farmers were advised on the minimal interval to achieve between treatments as treatments were becoming more frequent with the spread of reduced sensitivity in farm lice populations.

A study carried out on behalf of the Scottish Salmon Growers investigated the sensitivity to dichlorvos in farms across Scotland using a specifically designed bioassay. The published results established for the first time the extent of reduced sensitivity to dichlorvos, the only treatment available at that time. The industry very quickly moved towards a crisis with sea lice becoming very difficult to control.

In anticipation of there being some considerable time before a new product could be brought to license to replace dichlorvos, we set about finding non-chemical ways of minimising infection. Working again very closely with the industry we determined the epidemiological pattern of infection in relation to a number of factors associated with farm husbandry and management. As a result of this study we were able to show that lice built up on farms over the growing cycle and the clearly infected 2+ fish were acting as a major source of infection of newly stocked vulnerable smolts. The first major recommendation was, therefore, for single year class stocking. We also showed that a fallowing period was highly beneficial in reducing the build up of lice epizootics at any one farm site.

Parallel studies on the biology of lice took an in depth look at the copepodid stage, which is the transmission stage which develops from the free living nauplius to become the stage infective to fish. This then moults into the first chalimus stage after forming a filament attachment. We described the mode of attachment of the copepodid and the process of the filament formation in the chalimus I stage and their distribution on the salmon host. A histopathological study showed the feeding behaviour of the copepodid and the chalimus stages and concluded that there were minimal pathological effects on the epidermis of the host and a rapid healing of the lesion. This information was able to lead farmers to concentrate on the control of the mobile stages. A study of the functional morphology of the alimentary canal laid the basis for the subsequent major attempts at vaccine development which sought to find antigens which, taken in orally, would disrupt digestion. We have also described the morphology of the cuticle and its associated glands, also a target for future vaccine attack

A study of the eye structure and studies on the copepodid’s responses to light were taken up in the anticipation of finding treatment or control methods using phototactic responses. Of particular interest was the strong positive response to light of certain wavelengths, which led to the invention of light lures. Unfortunately, a successful light lure has yet to be designed.

In the early 1990s we embarked on a long term programme developing alternative chemotherapies which continues to this day. Our approach has always been to work with pharmaceutical companies for whom we have screened many potential products over many years.

We have subsequently contributed to a number of new products for sea lice control which have reached the market. For example, the preliminary work on pyrethrins was published as long ago as 1993. We successfully tested and completed trials for licence for hydrogen peroxide treatments with Solvay Interox and the organophosphate azimethiphos, marketed as Salmosan, with Ciba Geigy. The most successful product tested was emamectin benzoate (SLICE) for Schering Plough. We then carried out trials sponsored by Schering Plough to obtain a licence for this product. This was an innovative oral treatment and had, for the first time, a long duration of efficacy. SLICE has been the most effective and successful chemotherapeutant for lice to date and is used globally.

The ability to use a variety of treatment products made it possible to slow the development of resistance so, alongside the development of new chemotherapies, we developed and promoted the concept of an IPM (Integrated Pest Management) strategy. An IPM strategy involves many non-chemical approaches such as facets of management, husbandry techniques and biological control and only finally chemotherapies used in rotation. We were the first to promote the idea of an IPM for management and control of sea lice which we presented through conferences and trade press. We have also pursued aspects of biological control to be used in an IPM. The most familiar biocontrol method is the use of cleaner wrasse, first developed in Norway where wild, captured wrasse are used to stock salmon cages. We looked at their pathogens to see if any were likely to act as vectors of disease to salmon and showed that they were susceptible to two important viral diseases - IPN and pancreas disease. Thus we were able to warn farmers that infected wrasse on site could create a situation where restriction orders may be imposed on a salmon site under the Diseases of Fish Act.

Other programmes on-going will define the genes which cause the development of resistance and may even produce a lice resistant salmon population for the industry. Population studies using a variety of molecular techniques helped understand the dispersion of lice around the Scottish coast and contributed information to the wild versus farmed debate by comparing the two populations. We found that there was more variation between individuals than between the wild and farmed populations. Microsatellite analysis indicated a high level of gene flow around the Scottish coast with implications for the spread of resistance.

Current research at IOA and Machrihanish Marine Environmental Laboratory involves the screening of further possible treatment compounds and the development and use of a bioassay method for determining the sensitivity of lice to therapeutants. The latter is now used by industry to screen lice populations on farms. Further research involves studies on the mechanisms of resistance in lice, a very significant issue currently.