Introduction

Classification

Taxonomy

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

Active Forms

Resting Forms

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

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.