Bioassays are used in toxicology to determine the relative toxicity of chemicals to living organisms and therefore may be an effective means of diagnosing and measuring resistance to a sea lice medicine.
Most often, bioassays are undertaken where lice counts have shown that treatment has not effectively reduced lice numbers. Apparent lack of efficacy can have a number of causes, and investigation should focus on testing for resistance when treatment errors (for instance incorrect dose rate, short dose duration and poor dispersion of medicine) have been ruled out.
Alongside investigating instances of apparent lack of efficacy, bioassays (or related basic sensitivity tests) are occasionally used to establish whether a given licensed medicine is likely to provide an effective treatment for a population of lice. Increasingly, therapeutic strategies specific to the degree of resistance identified are being developed, making bioassays a useful tool in the identification and subsequent management of cases where lice populations demonstrate decreased sensitivity to an available medicine.
A bioassay exposes lice collected from the farm to a range of concentrations of a sea lice medicine, for the length of time advised in the label instructions for the particular medicine being tested. Lice are assessed after 24 hours in clean seawater as live, moribund or dead. Results are reported as an EC50, which is the concentration of medicine immobilising 50 per cent of lice present, or an LC50, the concentration killing 50 per cent of lice present.
To ensure reliability, bioassays are typically carried out at five different medicine concentrations with a negative control group not exposed to any of the therapeutant. Each of the doses used should show some response to the medicine, with at least one of the doses resulting in a full or 100 per cent response. At least 10 lice are tested at each concentration, including the control. The tests are run in duplicate, meaning a minimum of 120 healthy pre-adult stage II lice is required.
Protocols developed for sea lice bioassays include the use of pre-adult stage II lice (PAII). While pre-adult stage I lice are thought to have the same sensitivity as PAIIs, from a practical point of view they are smaller and consequently more difficult to handle without causing damage to the lice in either the collection procedures or in the set up and reading of the bioassay. Selecting a specific stage of lice standardises the bioassay, allowing relevant comparisons to be made between bioassays over time.
Healthy lice are required for a bioassay. After exposure to medicines in the bioassay, lice are assessed as ‘live,’ ‘moribund’ or ‘dead’ primarily on the basis of their ability to swim normally and attach themselves to the walls of the bioassay vessel. Differentiation between live and moribund lice can be subjective — the difference between live and moribund lice after 24 hours can be clouded by the use of lice that were not in full vigour at the start of the bioassay.