Ivermectin is used in the UK in veterinary products for the control of parasitic infestations of insects, nematodes and mites in livestock. However, its use to control sea lice infestations has recently been investigated

Ivermectin is a xenobiotic compound and would therefore not be found naturally in the environment.

Codling et al (1998) approached the appropriate regulatory authorities (Environment Agency in England and Wales, and the Scottish Environment Protection Agency (SEPA) in Scotland) for data on the concentrations of ivermectin in the environment.

Concentrations of ivermectin in sediments have been measured by the Scottish Salmon Growers Association (SSGA) around fish cages in Loch Nevis and Loch Ainort. In most cases, levels were undetectable but some low concentrations were found in Loch Nevis. Research at the Institute of Aquaculture, Stirling, has shown that ivermectin does not cause liquifaction of fish faeces and there is no leaching of ivermectin from fish faeces. These were suspected as being the reasons why so little ivermectin had been found in the sediments (Codling et al 1998). However, research by SEPA is still on-going.

Ivermectin is one of a structurally similar group of compounds known as avermectins. The principal features of these chemicals in relation to their fate and behaviour are their low solubility

in water (ivermectin 5 mg a.i. l-) and their log Koc values greater than 3.6, suggesting that they are likely to have an affinity for organic matter in soil and sediments and suspended solids in the aquatic environment. Ivermectin is likely to photodegrade rapidly in aqueous solution (half-life <13 hours) and is likely to biodegrade in aerobic soil (ivermectin: half-life = 7 to 240 days). However, a study on the rate of degradation in marine sediments has found degradation to be slow (half-life in excess of 100 days) (Codling et al 1998) .

An exhaustive literature review on the toxicity of ivermectin to marine organisms has not been carried out for the purposes of this profile. The information provided in this section is taken from existing review documents (Codling et al 1998). The most sensitive groups of organisms have been identified.

Few relevant toxicity studies for saltwater organisms are available in the open literature. However, the Scottish Salmon Growers Association (SSGA) has recently commissioned a number of studies on the toxicity and bioaccumulation of ivermectin (via waterbourne exposure) to marine invertebrates in order to provide sufficient information to the Scottish Environment Protection Agency (SEPA) to consider the use of ivermectin to be licensed for the control of sea lice. These data indicate that certain species of crustaceans may be particularly sensitive to ivermectin, with a 96 hour LC50 of 0.07 &micro;g l^-1 cited for the mysid shrimp Neomysis integer (SSGA 1996, cited in Codling et al 1998).

Due to the high adsorptive capacity of ivermectin and with releases to water liable to result in adsorption to sediment, information on the toxicity of ivermectin to sediment-dwelling or sediment associated organisms is of importance.

Studies on sediment-dwelling or sediment associated organisms have been reported by Codling et al (1998) .

A 10 day LC50 of 18 &micro;g ivermectin kg^-1 (wet sediment) for the lugworm Arenicola marina (suggesting high sensitivity). Sub-lethal effects on feeding activity were apparent at all concentrations tested (0.005 - 0.105 mg kg^-1). A re-burial test with surviving worms indicated that a concentration above 0.008 mg kg^-1 (wet sediment) affected the ability to burrow into clean sediment. The data for other organisms exposed in sediment, i.e. 10 day LC50s of 180 and 23,600 &micro;g kg^-1 dry sediment (18,400 &micro;g kg^-1 wet sediment) for the sediment re-working amphipod Corophium volutator and the sediment scavenging echinoderm Asterias rubens respectively, indicate lower sensitivity.

Data on bioaccumulation is limited.

The mussel Mytilus edulis was exposed to an average concentration of 6.9 &micro;g l^-1 ivermectin over 6 days with maximum levels of 5.2 mg kg^-1 detected, resulting in a bioconcentration factor of 752. On transfer to clean water, tissue concentrations dropped to 0.06 mg kg^-1 over 150 days and a slow depuration half-life of 22 days was determined. No adverse effects were reported for the mussels during the 6 day exposure period or the 150 day depuration phase.

However, Codling et al (1998) proposed that despite a high log Kows and high calculated BCF for fish, the large size of the ivermectin molecule may prevent bioaccumulation in fish. However, further experimental data are needed before this can be confirmed.

Potential effects include:

• acute toxicity to invertebrates (particularly crustacea) at concentrations above the EQS of 0.001 mg l^-1 (annual average) and 0.01 mg l^-1 (maximum allowable concentration) in the water column;
• accumulation in the sediments and potential toxicity to sediment dwelling organisms.

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