According to Associate Professor Rick Speare of Australia's James Cook University, there are four main causes why headlice treatments fail.

1. Inadequate application of the headlice product

This is usually a result of failing to cover the length of the hair or not applying enough product. One solution is to comb the product through the hair with a normal comb, making sure it covers the hair from root to tip. Also make sure hairs are lying parallel as you comb - this makes a nit comb easier to use.

2. Lice are resistant to the insecticide

Headlice can become resistant to one or more products - and there's no way of knowing in advance which products they are resistant to. But you can test for resistant lice. First, apply the product and leave it on for at least 20 minutes. Then, using a fine-tooth comb, comb from roots to tip and wipe the comb on a paper tissue. Look for movement. If the lice are active and walking around, they are probably strain-resistant. If they're dead or still, they are sensitive to the insecticide.

3. Failure to re-treat headlice

No product kills all the eggs, which take about seven days to hatch. So you have to re-treat one week after the first treatment. If things are going well at this stage, you should expect to find nymphs (young lice) but no adults. Nymphs are smaller than adult headlice. To ensure that the lice have been completely eradicated, make sure you use the same product again. You should also recheck the hair at 14 days by wet combing. But only re-treat at this point if you find young lice. None of the products are preventive - they will only work on actual lice (young and old).

4. Reinfection

This is hard to detect. Check the hair weekly, by using the conditioner technique or a good metal comb. If the hair is clean one week, and lice are present the next, reinfection is highly likely. But don't blame reinfection until you have checked the first three failures. If you suspect reinfection, ask your child who they have had head-to-head contact with and see if these other children can be checked. (Being in the same room as an infected person does not transmit headlice.)

Headlice myths and misconceptions

Herbal and essential oil headlice treatments.

There is no good evidence that tea tree oil or other such substances kill headlice.

Other oils

Like conditioner, oils (including essential oils) make the lice easier to dislodge. Some people use olive oil, hair gel and even mayonnaise.

But although headlice have died after being immersed in olive oil for some hours, the researchers who discovered this said there was no proof it would work on the human head. Lots of oil can be very difficult to remove.

Repellents

You can buy products that will supposedly keep lice away, but there's no evidence they work and they could even build up resistance. Don't use them.

Nit-checks at school

Some schools organise parents to come in and do nit-checks. Harvard lice expert Dr Richard Pollack says there is no evidence this reduces infestations. Parents often misdiagnose lice, and the process uses up precious school time.

Regular brushing

Despite frequent advice, brushing or combing may not reduce the risk of headlice. One study actually showed the opposite!