Natural Selection- Lice

Lice, nits, or cooties are (Pediculus humanus) are tiny insects that live in the fur and hair of mammals. They feed on skin, blood and other secretions. They cause itching, sores, and can spread diseases like typhus. #ewwwww

EWWWWWWWW

Lice is commonly spread through close contact and sharing items like hats, scarves, brushes, etc. Cleanliness has nothing to do with it, and just washing your hair doesn't get rid of them.

Up until the 1970's, lice treatments included grossed out parents picking out individual lice from their kids' head. Yes, really. They also used combs to pull out the eggs that the lice would stick onto the hairs. It took a long time and involved a lot of tears. But the alternative was shaving their heads.

Pyrethroids are chemicals produced by flowers that are a natural insecticide. The work by attaching to protein channels in the insect's nervous system and this prevents the nervous system from working. These have been put into "lice shampoos" to help get rid of the lice.

The first pump is normal. It allows sodium to enter the cell and the nerves work normally. The second pump is inactivated. The pyrethroid chemical blocked the pump. Sodium can not enter the cells so the nervous system can not work anymore.

Doctors and scientists noticed something strange. Starting in the 1990's, the pyrethroid-based compounds stopped working.

No, they aren't super heros- they are lice that are resistant to being killed by the pyrethroid chemicals. They have differently-shaped pumps. The pyrethroid doesn't block their pumps, so their nervous systems can work normally.

The purple represent the lice with normal pumps. The orange represents the resistant lice with the differently-shaped pumps. Examine the pictures. On the next slide, you are going to describe what happened to the lice population.

• There are a lot of lice.

• Variations in lice- some have normal pumps, some have different pumps.

• The lice with normal pumps were killed easily with the pyrethroid. Lice with different pumps were unaffected and survived.

• Lice with different pumps mated and passed on the genes to have different pumps to the baby lice. Now we can't kill the lice. Gross.

Examine this graph. It shows the percentage of lice cases that were resistant to pyrethroid. You will use this graph to answer the following few questions.

Suppose you use a pyrethroid product in 1983. What is the percent chance that the lice would be resistant to the pyrethroid product?

This is becoming a big problem in the USA. The orange states show were they found at least some resistant lice in 100% of the samples they collected. Let's use this information. Suppose your head is feeling itchy and it turns out you have lice. What are the chances you will be able to kill the lice easily with an over-the-counter pyrethroid lice shampoo?

In Europe, people do not use pyrethroid shampoos anymore because they are ineffective. They use silicone and oils to "suffocate" the lice. Another common at-home remedy is covering your head in mayonnaise. At least it's better than shaving your head...

Suffocating the lice doesn't affect the nits, or the eggs they leave stuck on our hair. And when they hatch....you have your lice problem all over again!

Many people find themselves using a combination of treatments, including going back to old-school treatments to help their lice problems!