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Sarcoptes scabiei

ISSN 2398-2977


Synonym(s): Sarcoptes scabiei var. equi S. scabiei

Introduction

Classification

Taxonomy

  • Phylum: Arthropoda.
  • Class: Insecta.
  • Genus:Sarcoptes.
  • Species:scabiei.

Distribution

  • Rare disease - been eradicated from large areas of the world (occurs in UK).

Significance

  • Reportable in USA.
  • Induces marked pruritus   Sarcoptic mange    →   severe itching and scratching   →   exudation, keratinization, connective tissue proliferation, thickening of skin with alopecia.
  • Zoonosis: will transfer to humans in close contact with infected horses (eg bare-back riding) and will induce lesions (usually mild) usually on the body and arms   Sarcoptes scabiei: var canis - human  (lesion distribution is different from human strain ofS. scabiei). The lesions resolve spontaneously about 3-5 weeks after contact with the horse ceases as the mites of the horse strain will feed and lay eggs but will not reproduce on man.
  • Thought to be a number of host-adapted strains in a singleS. scabieispecies.

Active Forms

Active Form 1

Morphology

  • Roughly circular with female 330-500 µm by 250-400 µm   Sarcoptes scabiei: female  .
  • Males smaller as are 6-legged larvae and 8-legged nymphs.
  • Legs short and legs 3 and 4 do not protrude beyond body margin.
  • Long pedicels (stalks)   Sarcoptes scabiei  ending in suckers on legs 1, 2 and 4 on male and 1 and 2 on female.
  • Epimeres (chitinous extensions of the legs on the body) of legs 1 and 2 join in mid-ventral line.
  • Very sclerotosed genital apparatus on male.
  • Small triangular spines on central part of dorsal surface.
  • Strong pegs on dorsal surface.
  • Terminal anus.

Color

  • Cream white with brown legs.

Tolerances

Other
  • In vivosurvival only.

Development

Growth
  • Develops from larvae in egg   →   nymph   →   adult in ~17-21 days.
Reproduction
  • Egg production: possibly 3-5 per day, maximum of 40-50 eggs.
Longevity
  • Up to 4-5 weeks.

Resting Forms

Resting Form 1

Morphology

  • Oval   Sarcoptes scabiei: egg    Sarcoptes scabiei: empty egg case  .

Color

  • Brown, translucent   Sarcoptes scabiei: egg    Sarcoptes scabiei: empty egg case  .

Tolerances

Other
  • In vivosurvival only.

Development

  • Develops and larvae hatches in ~2-3 days.

Clinical Effects

Epidemiology

Habitat

  • Epidermis of the horse and will survive a few days to 2 weeks on man.
  • Female in burrow in epidermis as far as spinous cell layer.
  • Male on surface of skin and enters burrows.
  • Eggs in burrows.
  • Larvae/nymphs in pockets off burrow, or larvae tunnel to surface and enter at new skin fold or hair follicle to form pockets.

Lifecycle

  • Egg.
  • Larvae.
  • Nymphs (3).
  • Adult male and female.

Transmission

  • Incubation: from a few hours to several weeks depending on severity of exposure and prior sensitization.
  • Transmission by direct contact when probably larvae or newly fertilized females are transferred.
  • Very contagious with rapid transmission between in-contact horses.
  • Mites can survive up to 3 weeks in infested stables.
  • Little evidence of transfer via the environment, even those that are heavily infected.

Pathological effects

  • Antigens are assumed to be mite bodies, feces and hatching and moulting fluids.
  • Immune hypersensitivity reactions, both immediate type I and delayed type IV, seem to be involved.
  • Infiltrating cells are primarily mononuclear cells (T lymphocytes, macrophages and B lymphocytes).
  • IgE and mast cells and eosinophils may be found in the vicinity of the mites, burrows and papules.
  • Disease is due to the immunopathologic response of the horse   Sarcoptic mange  .

Pathogenesis

  • Following infection, female mites begin to burrow into the skin, produce eggs and mite numbers build up.
  • Initially there is no immune reaction to the parasites, the period of sensitization, for 3-5 weeks.
  • Mite numbers continue to increase, but now the animal is sensitized and the interaction between the hypersensitivity reaction and antigen produces the clinical signs and pathology.
  • Mite numbers may now decrease, removed physically by scratching, and developing stages are probably killed or inhibited by a protective immune response.
  • Some animals may clear the infection but, in others, large or small numbers of mites will persist producing chronic antigen exposure and chronic disease.
Clinical
  • Sarcoptic mange   Sarcoptic mange  .
  • Lesions begin on the head, neck and ears. Can then spread over entire body.
  • Itchy, non-follicular papules and vesicles may be seen initially.
  • Rubbing and biting lead to excoriation. Focal or generalized hyperkeratosis can develop with crusting of the skin, alopecia, and thickening of the skin (lichenification).
  • Secondary bacterial infections can occur.

Histopathology

  • Changes are variable   Sarcoptic scabiei: mange 01 - histopathology    Sarcoptic scabiei: mange 02 - histopathology    Sarcoptic scabiei: mange 03 - histopathology  .
  • In outer layers, simply the burrow, mites and feces.
  • Cytolysis in living epidermis and dermo-epidermis junction with increasing parakeratosis around the necrotic burrow.
  • Perivascular and dermal infiltration of mononuclear cells, eosinophils, etc.
  • Some vasculitis and spongiotic lesions beneath the burrows.

Other host effects

  • Some skin is eaten.
  • Female mite begins to burrow in skin crease and enters by chewing with its chelicere and moving its mouthparts from side-to-side. A flap forms and is lifted and the burrow is enlarged in this way with the claws on the legs. Female burrows down as far as spinous cell layer; 0.5-5 mm a day.
  • Growth of epidermis lifts the burrows, eggs, etc, and the keratinized epidermis of the earlier part of the burrow is rubbed off.

Control

Control via animal

  • Topical treatment - repeated washings may be needed.

Control via chemotherapies

  • Topical treatment with chlorinated hydrocarbons or organophosphates   Therapeutics: parasiticides  also lime sulfur 1 week for 4-5 weeks.
  • Oral ivermectin   Ivermectin   at normal dose rates, weekly for 4-8 doses.

The injectable form should not be used in horses as it causes tremendous reactions and secondary clostridial infections.

Control via environment

  • Isolate all infected horses and fumigate premises.

Diagnosis

Useful samples

  • Deep skin scrapingof fold of skin until blood oozes   Dermatology: scraping  .
  • Several scrapings from edges of reddened, crusty areas or papules, difficult to find.

May help to coat the scalpel blade with mineral oil, glycerol or saline.

Specimen storage

  • At room temperature or 4°C for a few days.

Transport of samples

Field diagnosis

  • Clinical signs.
  • Presence of concurrent pruritus in the owner may be helpful.

Laboratory diagnosis

Skin scrape

KOH digestion

  • 10 min in 10% KOH in a boiling water bath or longer if the scraping is placed in hot KOH directly on a slide.
  • Mites and eggs will be visible.

Skin scrapings are only about 65% sensitive because mites may be present in very small numbers in a horse that is developing hypersensitivity.

Some people prefer to look at scrape in liquid paraffin to pick up movement of parasite (not too much material on each slide).

Biopsy

  • Mites or their burrows may be visible in the epidermis. Legs and mouthparts or the spines on the dorsal surface are useful for recognition of the mite in cross-section

Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • De Pennington N & Colles K M (2011) Sarcoptes scabiei infestation of a donkey in the UK. Equine Vet Educ 23 (1), 19-23.