It has been known for head lice to become resistant to chemical insecticides and survive treatment. But it is now thought that if head lice are found after treatment, it is likely to be for one of the following reasons:

• Misdiagnosis after treatment €“ the person does not have live head lice, just empty eggs
• Non-compliance €“ the treatment was not used correctly
• Re-infection €“ the person may have become re-infected, either from the original source or from another person.

However, if a customer finds that an insecticide hasn't worked, even though they used it properly, you can advise them to try a different one. So, for example, if a permethrin treatment was tried initially, the customer could try malathion. Alternatively, they could try a physical insecticide treatment.

Once the presence of live head lice has been confirmed, it is important to try to contact everyone with whom the infected child may have had direct head-to-head contact in the previous four to six weeks. This is called 'contact tracing'. The aim is to check everyone for head lice so that they can be treated if necessary to prevent any re-infections.