Head Lice Resistance

Head lice have become resistant to many of the original brands of head lice treatment and are currently continuing to become resistant to many of the available treatments. This has become a major problem amongst primary schools, with many schools and parents not knowing what to do. In many cases, schools have been giving the wrong advice, advice in which parents also wholly believe in. This is the idea of treating the children every night or few days until the lice are gone.
This is actually incorrect advice, as treating this often gives lice a better chance to build resistance against these products. Reports of resistant lice have become more frequent, especially those who are chronically infected and have been treated many times with the same or similar products. In the US, it has been found that some lice have become totally resistant to permethrin, the ingredient used to kill lice.
Resistance happens over multiple attempts to kill the lice. After a treatment, there may be a few lice that were for one reason or another able to survive. When these then reproduce, the new lice will most likely carry this trait. Over multiple generations and treatments this will continue, until you have strongly resistant lice.
Human head lice co-evolved with us, so it comes as no surprise that they are building a resistance against treatments designed to kill them.