Clik interferes with the lifecycle of the larvae and its presence on the fleece protects the whole animal. Crovect is a poison which kills larvae and works only at those areas where it has been applied. So you don't actually need to massage Clik into the fleece though Novartis recommend half the spray on the back and half on the bum.
It's not a nice chemical which is why it has a 40 day meat withhold and persistent effect. This is part of what it says on the label
Special precautions to be taken by the person administering the veterinary medicinal product to animals[/color]Operators should wear synthetic rubber gloves, a face mask and PVC trousers when applying the product.[/color]In case of skin contact remove contaminated clothing and thoroughly wash the affected parts of the body with soap and water.[/color]In case of eye contact rinse eyes with clean water for several minutes and contact physician for advice.[/color]In case of ingestion call for medical advice.[/color]Always wash hands with soap and water after handling sheep and before eating and drinking or smoking.[/color]Wash hands and exposed skin after working with recently treated sheep.[/color]Do not eat, drink or smoke whilst using the product.[/color]Handling sheep in the weeks following treatment:[/color]Do not shear sheep in the 3 months after treatment.[/color]Handle sheep as little as possible after treatment as residues remain on the fleece for some weeks.[/color]If you need to handle sheep after treatment, wear coveralls and Wellington boots. If sheep are wet also wear waterproof trousers and coat.[/font]