Lice Flashcards
What are the three species of medically important lice
Body louse - Pediculus humanus
Head louse - Pediculus Capitis
Crab/ pubic louse - Pthirus pubis
Why are lice medically important
Biting pests
Body louse is a vector
All of life’s spent on the host and dispersal is through close contact
Morphology of lice
Similar for all three
Wingless ectoparasites
Head and body lice identical
Dorso-ventrally flattened with clawed feet for gripping hair
Blood feeding at all stages
Eggs hatch into adult like nymphs
Eggs laid at the base of the hair
Distance from hair base to egg position is the age of infestation (hair growth is 1cm per month)
Life cycle of lice
All three nymphal stages are blood feeding
3 instar stages, adult, eggs
Eggs laid on hair, clothes and coarse hair and hatch after 7-10 days
Adults live for around a month
Feed 5x per day
Lay 6-8 eggs per day
Optimum temp is 31 C and eggs don’t hatch if below 22
Adults and nymphs die if kept off the host for more than 48 hours
Body louse borne diseases
Louse-born epidemic typhus - Rickettsia prowazeki
Trench fever - Bartonella quintana
Epidemic relapsing fever - Borelia recurrentis
Clinical manifestations of lice infections
Bite marks along clothing seams
Can develop prurigo nodules after chronic rubbing
Considered rare but outbreaks still occur in the disadvantaged
Transmission of head lice
Touching heads is the only important route of transfer
Rarely more than 100 lice per head
Biting nuisance and can cause secondary infections
Transmission of pubic lice
Transfer overwhelmingly through sexual intercourse but can be transferred by sharing a bed and through discarded clothing
A biting nuisance - pruritis
Transmission of body lice
eggs attached to clothing - especially the seams
proliferate if clothing is not removed so particularly effects people like refugees and prisoners
rarely more than 100 lice (record is 20,000)
spread by close contacts (and fomites, clothing etc)
Control of head lice
Washing with soap and water - reduces numbers but does not rid infestation
regular use of lice comb (every 2 days for 3 weeks)
insecticides (usually pyrethroids with PBO) (3x weekly applications)
dimectone lotion or spray
shaving of head hair
Control of pubic lice
Traditionally shaving of all course hair
Insecticides
Emulsions and lotions applied to the entire body (do not wash for 24 hours after application)
Use vaseline or a 10 day course of 0.25% physostigmine opthalmic ointment for eyelashes
Control of body lice (vector)
Frequent washing and changing of clothes and bedding
Drying clothes in direct sunlight
Delousing using insecticides e.g. permethrin dust
