ANALYSIS - In part two of Fish Vet Group’s article on the reasons and methodology behind carrying out a sea lice bioassay, Chris Findlay explains the challenges of interpreting the results of a bioassay and considers potential alternatives.
A bioassay should ideally produce a clear and unambiguous result that accurately reflects the sensitivity of sea lice to a medicine.
However, results of a bioassay can be inconsistent with the results of treatment carried out on the farm. There is no guarantee that the dose to which lice are exposed in the bioassay will precisely reflect the dose that lice experience during treatment on the farm. Reasons behind this include estimation of treatment volume on the farm; differential adsorption of sea lice medicines to the
containers holding lice during the bioassay and increased exposure of lice to medicines during a bioassay. Lice within a bioassay are more likely to be exposed across dorsal and ventral surfaces than lice on fish.
Secondly, the bioassay tests only Pre-Adult II stage lice (PAII). The farmer wants to know whether a medicine will be effective against all stages of lice present on the farm (variations in relative sensitivity between stages of lice to the same therapeutant are known to occur).
Rather than directly testing sea lice collected from a farm, ‘second generation’ lice can be tested by hatching eggs collected from gravid female lice. The copepodites developing from these eggs can be used to challenge fish and sufficient lice collected at PAII stage. This approach can only be carried out in specialised and licensed laboratories. A bioassay result will take longer with this method due to the time it takes for the lice eggs to hatch and develop through to PAII stage.
Bioassays using copepodite stage sea lice allow a rapid result to be obtained using a larger and more easily collected number of test subjects. However, difficulties exist when extrapolating such results to the real world, due to the potential for life-stage dependent differences in resistance gene activation and regulation.
A ‘sensitivity test’ can be used to assist in the selection of a medicine. The test provides a basic indication of the expected effect of a medicine used as a bath treatment against all stages of mobile lice. The test compares survival rate of lice treated at a therapeutic dose of a sea lice medicine compared to a control group.
Work is progressing on the identification of genetic markers for resistance to a range of sea lice medicines. In future, this may provide farmers with rapid pen-side means of monitoring sea lice for the emergence of resistance. However, such tests will be specific to particular routes of resistance – for example in the identification of enzymatic resistance but without the development of avoidance reactions, cuticle thickening or other chemical detoxification methods.
Ultimately, while molecular methods will add to the sophistication with which decreased sensitivity to medicines is identified, bioassays are likely to remain an important part of the wider picture of sea lice management.
Go to Part 1 on this story by clicking here.