History and biology of scabies

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History and biology of scabies

• Ian F Burgess
• Medical Entomology Centre
• Insect Research Development Limited

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History of scabiesJust how wrong can we get it?
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In the mists of time

• Scabies has been known for more than 3000 years
• It is suspected that some biblical leprosy was
  actually scabies
• Laws of Moses
• Naaman general of the King of Syria
• King Uzziah
• Job
• Scabies was known in every culture but curing it
  was another thing

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Its all Galens fault

• Early medicine of Hippocrates (C 4th BC) and
  Galen (C 2nd AD) was based on the four humours
  - blood, phlegm, yellow bile and black bile
• Scabies, like other diseases, was believed to be
  caused by an imbalance in the humours
• Germ theory was a long way off so treatments
  were by purging, bleeding etc.

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Some people got it right - almost

• Aristotle (384-323 BC) mentioned "lice in the
  flesh" that produced vesicles
• Abu Marwan Abdal Malik Ibn Zuhr (Avenzoar) 12th
  century Moorish physician found the mites but did
  not link with disease

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Some people got it right - almost

• St Hildegard of Bingen (1099-1179) discussed
  scabies in her treatise, Physika, naming the itch
  mite "snebelza"
• She was not believed by physicians due to
  prevalence of Galens ideas
• She treated scabies using sulphur ointment

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Bonomo a truly scientific observer

• The first real advance came with Giovanni Cosimo
  Bonomo (1663-1696) a student of Francesco Redi
• Saw peasants in the market place removing mites
  using needles

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Bonomos letter

• He wrote a famous letter to his teacher on July
  18th 1687 ..having frequently observed that
  the poor women, when their children are troubled
  with the itch, do with the point of a pin pull
  out of the scabby skin little bladders of water,
  and crack them like fleas upon their nails. it
  came into my mind to examine what these bladders
  might really be.

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Bonomos letter

• Bonomo looked at one under the microscope
• I found it to be a very minute living creature,
  in shape resembling a tortoise of whitish color,
  a little dark upon the back, with some thin and
  long hairs, of nimble motion, with six feet, a
  sharp head, with two little horns at the end of
  the snout."

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Bonomos letter

• Redi publicised the work in his booklet
  Observations about the 'pellicelli' of the human
  body, made by Gio. Cosimo Bonomo and written by
  him with other observations in a letter to
  Francesco Redi, Florence, 1687"
• There was widespread interest but the proposal
  was largely suppressed by intervention of the
  Popes doctor who said it was unscriptural

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The slow dawning of light

• 1805 Joseph Adams infected himself and
  developed scabies
• 1805 - Jean Louis Alibert extracted mites from
  scabies patients
• 1851 Ferdinand von Hebra linked Norwegian
  scabies with mites
• 1865 Thomas Hiller wrote that nearly all
  doctors believed the mites caused the disease

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Biology of scabies
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What are scabies mites?

• Scabies mites are in the
• Class Arachnida spiders, ticks, scorpions etc.
• Order Acarina mites and ticks
• Sub-order Astigmata mites with no spiracles
• Family Sarcoptidae
• There is one species Sarcoptes scabiei that
  affects humans and animals

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What are scabies mites?

• Scabies mites have
• 4 pairs of legs
• No head
• Various spines and bristles on the body
• These anatomical models were made by Émile
  Deyrolle in the mid 19th century

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Scabies mites

• Adult females and males are different sizes
• Females 400 microns
• Males 250 microns
• They have different appendages for different life
  styles
• Females are found most often live in skin
• Males are rarely seen as they remain on the
  surface

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Scabies life cycle

• Newly matured females form a short burrow and
  wait for a male to find them
• Males and females mate in the burrow
• Males then wander off in search of other
  females
• The female enlarges the burrow and stays there
  for the rest of her life burrows about 2-5mm
  per day

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Adult female

• Adult female mites form burrows in the stratum
  corneum (dry horny layer of skin)
• They cut into the cells using sharp elbow
  joints on the front pair of legs

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Adult female

• The female lives in the stratum corneum at the
  boundary of the living and keratinised tissue
• The adult female mite is the only stage making a
  permanent burrow

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Adult female

• As they burrow the mites feed on living skin
  cells rasped away by their legs
• They lay eggs, defecate, and die in the burrow
• A female mite does not leave the burrow unless
  physically dug out

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Scabies life cycle

• Eggs in burrows hatch to 3 legged larva
• Larva makes its way to skin surface
• Feeds on skin debris in hair follicles 3 days
• Makes a moulting pocket temporary burrow for
  moulting

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Scabies life cycle

• Scabies mites have two nymphal stages that look
  alike
• Protonymph
• Tritonymph
• Males and females have both stages but both
  male nymph stages about the same size
• Each nymph lasts 3 days
• Nymphs make moulting pockets

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Scabies life cycle
3 days
3 days
3 eggs per day
3 days
3 days
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Fertility and fecundity

• The whole cycle egg to adult normally takes
  about 12 days
• Adult females
• May live 60 days
• Can lay 150-180 eggs
• Can travel 10-15 cm under the skin

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Fertility and fecundity

• Dermoscopy has shown fertility index
• 11.5 12.6 eggs per burrow (Sept Dec)
• 6.0 8.9 eggs per burrow (Jan July)
• More mites around in the autumn and winter when
• More humid
• Colder
• Fewer mites when
• Less humid
• Warmer

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Population of mites

• Most people have few adult mites
• This means
• Few active juvenile stages
• Few eggs waiting to hatch
• Low risk of transmission

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Population of mites

• Numbers of adult mites (burrows) build up slowly
• Symptoms only start after several weeks
• The number of active females increases until
• Immune response inhibits increase
• Scratching eliminates many burrows