Equis ISSN 2398-2977

Trombicula spp

Synonym(s): N. autumnalis, Harvest mite, Trombicula autumnalis, Chiggers, Forage or 'scrub itch' mites, Eutrombicula spp, Red bug

Contributor(s): Maggie Fisher, Sheelagh Lloyd, Rosanna Marsella, David Senter

Introduction

Classification

Taxonomy

  • Phylum:Arthropoda.
  • Order:Acari.
  • Family:Trombiculidae.
  • Genus:Trombicula/Neotrombicula, Euschoengastica/Neoschoengastica.
  • Species: alfreddugesi, autumnalis, sarcina, splendens.
  • Subspecies:Eutrombicula.

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Clinical Effects

Epidemiology

Habitat

  • Damp, but drained, usually chalky soil.
  • The natural hosts are usually small rodents, eg field mice, but larvae will attach themselves to any grazing or other domestic animals or humans in the fields in late summer/early autumn.
  • 'Out of season' infestation can occur due to survival of mites in preserved hay.

Lifecycle

  • Adult mite lays eggs in soil.
  • Larvae hatch and climb to the top of long grass to locate a host.
  • Only the larvae are parasitic.
  • After feeding on a host, the larvae drops to the ground, undergoes three nymphal stages before becoming an adult.
  • Nymphs and adults known as velvet mites because of numerous pilose setae that cover the body and legs.
  • Adult mites have a 'waist'.

Transmission

  • The mites are acquired from the chalky soils that they favor.
  • Larvae will climb up to 8 cm on grass, harvested corn stalks etc to locate and attach to a host.

Pathological effects

  • A hypersensitivity reaction (presumably on response to antigens in saliva and feeding tunnel) that persists for days to weeks after the mite has been removed or fallen off.
  • The proteolytic enzymes and hypersensitivity reaction lead to severe pruritus and irritation.
  • Infested horses become irritable with leg-stamping, nose-rubbing and head shaking.
  • Small papules and wheals develop on distal limbs after 2-3 days.
  • Pruritus is variable suggesting that some horses may develop hypersensitivity lesions: papules, edema, crusting and ulceration.

Other Host Effects

  • Larvae attaches to host by its chelicere for up to 4-5 days and feeds on host's tissue fluids.
  • Attachment and feeding is aided by cytolytic enzymes that dissolve tissues.
  • A 'feeding tunnel' is formed of a hardened hyaline material secreted by the mite and this penetrates through the epidermis.

Control

Control via animal

  • Treatment.
  • Topical application of acaricide.
  • Horses lose infestation when removed from source so treatment not always required.

Control via chemotherapies

  • Infection is self-limiting so treatment is not usually required.
  • 5% lime sulfur is effective to kill mites and is a strong anti-pruritic agent.
  • Topical pyrethrins   Pyrethrin  .
  • Topical application of 0.25% fiponil spray (extralabel) may prevent reinfestation.
  • Palliatives, eg glucocorticoids, maybe required to reduce itching and self-trauma.
    The hypersensitivity reaction to embedded antigens can be intense even after the mites have been dislodged. Topical symptomatic therapy can be useful.

Control via environment

  • Remove animal from source of infection.
  • If at pasture, animal may be returned normally after 8 weeks, but infection will probably recur the following year.
  • If infection occurs in a stabled horse then clean out stable and check bedding and hay, etc for mite source.

Diagnosis

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Further Reading

Publications

Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Mair T S (1994) Headshaking associated with Trombicula autumnalis larval infestation in two horses. Equine Vet J 26 (3), 244-245 PubMed.

Other sources of information

  • Higgins A J & Wright I M (1995) The Equine Manual. W B Saunders, USA. pp 260-261.

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