The Secret Life of a Louse
|Blood filled gut of a head louse|
Parents shudder when the dreaded note comes home in the school bag stating that there has been an outbreak of head lice in the classroom. The note – as always – deals with methods of detecting and defeating the lice but tells nothing of their secret, rather disgusting, little lives….
Given that head lice can only survive on a human scalp – and nowhere else on earth – and given that they have been evolving for over a million years, one has to salute them as the most durable and audacious of insects.
|Is there anyone who can honestly say they can look at this picture without scratching their head or eyebrow?|
The louse has a total life span from egg until death by old age of just 50 days. An adult female louse, about the size of a sesame seed, will lay four eggs every day which totals two hundred and ten eggs in her lifetime. Each egg is meticulously attached to the shaft of hair using a glue secretion from her reproductive organs mixed with her saliva. The glue rapidly hardens into a “nit sheath” which is such a strong adhesive that the most effective way of removal is manually (hence, nitpicking). The positioning of the egg is crucial as it is temperature dependant from the body heat of the host so as to allow optimum incubation for the developing embryo. In cooler climates, the eggs are laid a centimetre from the scalp whereas in warmer climates, eggs are laid about ten centimetres down the shaft of hair. After seven days, the eggs hatch into nymphs and the discarded eggs remain attached to the shaft until they are removed. The nymph will moult three times before reaching adulthood, each time discarding its exoskeleton on the hosts scalp. Once mature, pairing begins within ten hours. Adult lice copulate frequently for bouts of up to an hour. The louse’s legs are short and end with a single claw and opposing “thumb” with which it grasps hair– should the louse fall, it will die within forty-eight hours of losing its host and food source.
|The incision making claw.|
Lice feed exclusively on human blood and do so about five times a day. They make an incision on the scalp with their claw and inject saliva into it to prevent the blood from clotting. Bloodsucking will continue for a long time if the louse is not disturbed, yet it will not become engorged as a tic would. A dark red faeces is excreted onto the scalp as the louse feeds. As alarming as that sounds, head lice are not known to be vectors of any disease although excessive scratching can cause bacterial infection.
Head lice are extremely common throughout Ireland and the UK with two in ten children infected every year. Statistically girls are four times more susceptible than boys and children aged between four and thirteen (and their parents) are the most frequently infected group. There is the old adage that lice only like “clean hair” but the jury is still undecided with a lot of conflicting evidence to suggest the same. Some surveys suggest that lice do prefer clean hair as natural oils in dirtier hair tend to clog the lice’s ability to breath and to adhere to whilst other surveys have shown that lice do not discriminate between clean or dirty, they just love it as a nesting ground regardless of condition. Due to their lengthy evolutionary success, the latter would seem more accurate – especially as the louse cannot jump, nor fly and relies solely on head to head contact for distribution – it would seem that there is little scope to be choosy.
Head lice are part of our social fabric – they stretch back as far as remains of mummified Egyptians showing signs of infestations and Inca Kings employing dextrous fingers to be “nit picker to the king” to contemporary appearances on television in episodes of CSI Miami (where a complicated murder was solved by a louse carrying incriminating blood!) and having an appreciation page dedicated to the feisty little beasts on Facebook. It would seem that the head louse will carry on having a close relationship with the human race for a long time to come.
For more information on head lice, advice on chemicals and for a video on how to correctly comb for nits, visit http://www.headlice.org/