Mites Flashcards
Mites background and morphology
Huge diverse group ranging in size and form
Most microscopic
Includes free living forms, pests of stored food and some parasites
Resemble ticks but hypostome without teeth
Feed on tissue juices and cellular liquids but don’t swell up after feeding like ticks
Medically important mites
Parasites - scabies
vectors - scrub typhus
dermatitis - itch mites, hair follicle mites
allergies - house dust mites
Life cycle of scabies mites - Sarcoptes scabiei
egg, 1st instar nymph (larva), 2nd (protonymph), 3rd (duetonymph), adult
3 days, 2 days, 1.5 days, 2 days
fertile females (<0.5mm; <15 per person) create tunnels, lay eggs in surface skin, lives less than a month and rarely leaves the tunnel, feeding on tissue and cell liquids
larvae and nymphs create pockets in which to moult - moulting pockets - 9-19 days
males (0.5 size of females) wander on skin surface looking for females
not a vector but secondary infections are often a problem
worldwide distribution
Scabies transmission
human-human transmission only
transmission by skin-skin contact mainly - highly contagious -1 female mite can set off infection but 15 minutes contact thought to be necessary
transmission from bedding and clothing is rare - mites live only 2-4 days off the host
> 80% of cases from shared bed occupation
incidence increases with poverty and during wars and disasters (overcrowding
How is scabies prevalence affected by age
younger people generally more likely to be infected
Scabies diagnosis
Rash on body (not mite distribution) - appears in 4-6 weeks in 1st infection and earlier in subsequent occasions, characterised by increasing itchiness
confirmation by finding mites or tunnels by scraping at ulcers or crusted areas, pick from end of tunnels with needle, examine at x50
secondary infections may confuse
treatment of scabies
topical application so hot soapy bath is recommended first
conventional treatment - 20-25% benzyl benzoate painted from the neck down
5% permethrin cream is more effective and has fewer side effects
Permethrin 10-20g/l in liquid parafin
treat all members of the same family
systemic - ivermectin
treatment of fomites - 2% temephos 10 mins at 50 C
thorough laundering and ironing
Immunosuppression in Crusted norwegian scabies
suppression of the itch response which causes a huge increase in the mite population causing profuse crusting and hyperkeratosis
large numbers of mites on the skin means bedding and clothing becomes infective
can be caused by burns, immunosuppressive drugs, HIV related infections
Scabies evidence for immunity
30 year cycle of epidemics
infection of non natives more difficult
primary infections result in higher mite density
immunosuppression results in extreme infestation
What are the vectors of scrub typhus
Leptotrombium sp
Scrub typhus mites
larvae of many species worldwide can cause irritation when they infest the skin (temporary)
Leptotrombium sp mites are the only medically important group and only in Asia
only the larvae take a blood meal and are parasitic on humans
wild rodent hosts for larvae
Leptotrombium life cycle requires arthropod prey for nymphs and adults
locations called mite islands
Scrub typhus life cycle
egg, deutovum, larva on vegetation and soil, feeding on vertebrate hosts, engorged larva, falls to soil, protonymph, deutonymph/nymph, tritonymph , adult
larvae in scrub-jungle, neglected gardens or modified vegetation (fore, clearing, agric abandonment etc),
wait on vegetation,
on skin especially under tight clothing (waist, ankles) for less than 10 days,
saliva injection produces irritation,
larva drops off onto the ground to moult - life cycle 1.5-10 months,
mite reservoir - trans stadial and trans ovarian transmission of O. tsutsugamushi produces the next generation of infective mite larvae,
wild rodents probably not a significant reservoir
Morphology of Leptotrombidium spp
<0.3mm - 6 legs (unlike scabies adults)
feathery hairs on body
dorsal plate (scutum)
pigmented eyes on body
Prevention and control of scrub typhus mites
wear boots and long trousers
clothing impregnated with repellant (DEET, DMP, benzyl benzoate)
permethrin
avoidance of mite islands
clearance or burning of mite islands
acaricide spraying (ULV, fogs or emulsions)
chemoprophylaxis (200mg weekly doxycycline) of high risk groups, e.g. soldiers in field
