Caution reading this may well make you itch.
Head Lice are wingless insects who live their entire life on the human head living of our blood. Pets and other animals cannot be the host. Their legs are designed for walking up and down our hair and cannot cope with flat surfaces very well. They do not carry any disease apart from very occasional secondary infections from excessive scratching.
Head Louse infection of any part of the body is known as pediculosis capitis and Head lice most probably co-evolved with humans.
They do not always cause symptoms, the itching comes from their movement, reaction to their saliva and faecal matter, when they are feeding on our blood. But this reaction will often pass after a few weeks so you can be infected with out the itch.
In strong artificial or natural light the eggs are easy to see, usually laid 1.5 cm from the scalp and then as they mature are moved further away with the growth of the hair. Nits or Head Lice eggs are usually quite firmly fixed to the hair and do not slide easily up it, so if debris is found and moves freely up they are most probably not Nits but just normal hair detritus. They are traditional egg shaped, about 0.8 mm long. Light to very dark brown in colour when containing an embro but appear white after hatching. The are called nits both before and after hatching and the casings can last up to 6 months still attached to the hair.
The unhatched eggs are much more difficult to see and the white egg casings are not a sign of live infection so you are really looking for live adults as a sign of continued infection.
Egg to egg laying adult 15 – 23 days
Ideal conditions are 27 – 31 C and 45 – 75% relative humidity.
Eggs laid 15-18 hours after mating, attached to the hair using a lipid and peptides cement, which is similar to human hair hence why egg-looseniing agents can some times damage hair. The nits at this stage are quite resistant to Pedicullides, chemicals used to control infections.
The egg stage takes 7 to 12 days.
The Nymph stage takes 8.5 to 11 days. The newly hatched Nymph will need to feed as soon as it leaves the egg. It will then molt three times. After the second moult it will be able to transfer to the next host.
A newly matured female can lay up to 9 eggs daily (averaging 3 – 4) from one insemination for 8 – 9 days. So up to 81 eggs in ideal conditions in just over a week. After that egg production drops during the remaining three week of the Head Lice adult life.
Adults will eat every 4 – 6 hours and will try to find new hosts. They can crawl 6 – 30 cm a minute. But they cannot survive long off a host, suffering dehydration and starvation and will quickly become unable to feed and breed. But can live in ideal conditions up to 48 hours off an host.
The main transmission of Head Lice, through head to head contact, is mainly in children between 4 – 14 years and 2 to 4 times as likely to infect girls as boys.
There are number of chemicals that can be used to treat them but all have some draw backs, including being toxic to humans, flammable, messy and the development of resistance to them.