There are four reasons why treatment of head lice can fail.
Cause 1) the head lice treatment was not applied properly.
In this situation, the chemical does not come into contact with the lice long enough at a high enough concentration. This is usually due to not covering all the hairs adequately with the chemical. Cover the hair from roots to ends with the product. One way to make sure that you have covered all the hair is to comb the product in with a normal comb. Check how you are applying the product and read the instructions on the package. If you have done all of this then application is not a problem.
Cause 2) the lice are resistant to the insecticide used.
Head lice can become resistant to insecticide. Some strains appear to be resistant to at least two insecticides at the same time. You can tell if the head lice are resistant by treating the head adequately as described in Cause 1) and then checking for live lice. Use a fine tooth comb and look for movement. If they are all dead or immobile, the lice are sensitive to the insecticide. If the lice are still active they are resistant to the insecticide in that product.
KEY POINTS: note the “active ingredient”, not the product name. If the lice are resistant, switch to another brand with a different “active ingredient”.
Cause 3) a second treatment was not used to kill the newly hatched eggs.
No product kills 100% of eggs, even if the strain of lice is sensitive to the active ingredient. Eggs take 7 days to hatch. Therefore starting from 1 day after treatment to 7 days, nymphs may be hatching. Retreatment at 7 days is a mandatory part of treatment. At this treatment if things are going well, you should expect to find nymphs, but no adults. At 14 days, you should recheck the hair, and retreat a third time if you find nymphs.
Cause 4) reinfestation occurs.
Reinfestation can complicate treatment of head lice. This happens usually from head to head contact with a person who has lice. Reinfestation from the environment is very rare. Reinfestation is hard to detect. Suspect it if you have a sensitive strain as shown by testing as described in Cause 2) and are treating it correctly (Cause 1 and 3), but you unexpectedly find adult lice in the hair. Check the hair daily by using a fine tooth comb. If the hair is clear one he has had head to head contact, and see if they can be checked. Don’t just concentrate on the classroom; look at head to head contact opportunities at play and in the family. Be systematic. Don’t expect a cure in less than a week. Keep a balanced view. Head lice infestations are a nuisance but do not spread disease.
Do not use measures that put your child's health at risk. With the growing concerns about resistance of head lice to current treatment with pyrethrin and permethrin as the “active indgredient”, many people have turned to alternatives. Some are ineffective or dangerous or may create more emotional harm to the child:
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