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Basics ⬇

Outline

BASICS

Definition!!navigator!!

Chorioptes (equi) bovis, Psoroptes (equi) bovis, and lice (Werneckiella equi and Haematopinus asini) are ectoparasites that affect horses. They complete their life cycle on the horse and are transmitted to other horses by contact.

Pathophysiology!!navigator!!

The mites cause disease by direct irritation and inducing a cutaneous hypersensitivity reaction while the sucking lice (H. asini) can induce anemia.

Systems Affected!!navigator!!

Skin

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

  • Incidence is sporadic and secondary to contact with another infested horse or the immediate environment containing scabs or hair shafts with eggs or larvae present
  • More common in barns with horses that are hauled frequently and return with subclinical infestations

Geographic Distribution!!navigator!!

Worldwide

Signalment!!navigator!!

Chorioptic acariosis is more common in breeds with feathered fetlocks, such as draft breeds.

Mean Age and Range

Young and geriatric animals may be more prone to severe lice infestations.

Signs!!navigator!!

General Comments

Mite and lice infections are more severe in colder months. Weight loss can be seen due to the chronic irritation. Infestations may produce typical clinical signs in some animals whereas others may exhibit few clinical signs or may be asymptomatic carriers.

Historical Findings

Often involves travel to an equestrian event where exposure had occurred.

Physical Examination Findings

Lice Infestation

  • Common in sick, old, or otherwise debilitated animals
  • Secondary seborrhea can camouflage the lice from detection. As the louse hangs onto the hair shaft when feeding, do not expect to find lice in alopecic areas
  • Hair coat is of poor quality, represented by multifocal areas of alopecia and scale
  • Pruritus is variable and affects the neck, shoulders, mane, tail, and less often the legs
  • Lice glue their nits to hair shafts.
  • W. equi, the biting louse of equids, prefers sites on the body such as the forehead, neck, and dorsolateral trunk rather than the neck and tail

Chorioptes (leg mange)

  • Chorioptes is an important cause of pastern dermatitis, particularly in draft horses (Figure 1. Mites live on the surface of the skin. These infections are generally less severe and less pruritic than Psoroptes
  • Chorioptic acariosis starts with pruritus, irritation, and restlessness. Pruritus may be mild or absent in some cases. A mildly erythematous papular to crusted dermatitis involving the distal legs is the first sign. Exudation of serum with matting of leg hair and thick adherent crusts may develop over limited or extensive areas. Self-trauma results in secondary bacterial dermatitis

bovis (ear and body mange)

  • P. equi is rare but highly contagious. Psoroptic mites do not burrow, and feeding results in exudation and crusting
  • Lesions are found on regions such as under the forelock, base of the mane and tail, and the axillary region. The mites prefer areas with thick hair such as the ears, mane, tail, and intermandibular areas
  • Intense pruritus is the hallmark resulting in marked head shaking and tail rubbing. Papules, vesicles, crusts, scaling, alopecia, excoriations, and exudation on the skin and ear margins are common. Lichenification of the ears, mane, and tail-head is seen in chronic cases
  • The species Psoroptes hippotis and Psoroptes cuniculi cause otoacariosis, which results in aural discharge, head rubbing, head shaking, and carrying the ears in a downward droopy flat position

Trombiculidiasis (harvest mite, chiggers)

Trombicula autumnalisinfestation leads to papular dermatitis and seasonal pruritus of the sides of the face (peri ocular, perioral, and muzzle), feathers of the fetlocks, mane and tail, and sometimes the ventrum in pasture-grazed horses. Pruritus is typically intense, leading to significant self-trauma.

Causes!!navigator!!

Opportunistic infestations secondary to contact with another infested horse.

Risk Factors!!navigator!!

  • Travel to equestrian events or trail rides
  • Asymptomatic carriers of C. bovis serve to perpetuate the infection from season to season as Chorioptes may survive off the host for months
  • Horses put out to pasture with tall grass in the late spring to late summer are a risk for development of trombiculidiasis

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

Differential diagnoses for crusting and pruritus on the legs besides Chorioptes include:

  • Culicoides hypersensitivity
  • Staphylococcus infection
  • Contact dermatitis
  • Dermatophytosis
  • Vasculitis
  • Food allergy

Differential diagnoses for crusting and pruritus on the body besides Psoroptes include:

  • Culicoides hypersensitivity
  • Staphylococcus infection
  • Atopic dermatitis
  • Food allergy
  • Dermatophytosis
  • Dermatophilosis
  • Pemphigus

Differential diagnoses for tail and mane pruritus besides Psoroptes include:

  • Culicoides hypersensitivity
  • Onchocerca
  • Oxyuris equi infestation (for tail pruritus)
  • Tail pyoderma (for tail pruritus)
  • Food allergy

Differential diagnoses for pruritic otitis besides Psoroptes include:

  • Culicoides hypersensitivity
  • Atopic dermatitis
  • Food allergy

CBC/Biochemistry/Urinalysis!!navigator!!

  • Anemia with heavy H. asini infestations
  • Nonspecific eosinophilia with mange infestations

Other Laboratory Tests!!navigator!!

N/A

Imaging!!navigator!!

  • Microscopy for differentiating the causative arthropod
  • W. equi are 1–2 mm in size, have a relatively broad body, and a square head, while H. asini are 3–3.5 mm in size, have a longer narrower body, with a sharp conical head and piercing mouth parts
  • Chorioptes can be easily seen with a magnifying lens. This mite is 0.3–0.5 mm and has an oval body, a small head with blunt mouthparts, and long legs with suckers directly fixed to the extremity
  • Psoroptes are 0.4–0.8 mm in size with an oval body and elongated mouthparts and long legs with triarticulate sucker-bearing peduncles
  • Trombiculid larvae are orange to red and 0.25–1.0 mm, have 6 long legs and an oval body, and can sometimes be seen by the naked eye

Other Diagnostic Procedures!!navigator!!

  • Clipping of the affected area before skin scraping is helpful. Use a no. 10 blade coated with mineral oil to help collect the mites when performing superficial skin scrapes
  • Multiple superficial skin scrapings are needed to recover mites such as Chorioptes and Psoroptes. Skin scrapings can be negative in asymptomatic carriers or in chronic disease. Trombicula are found in crusts of dried serum. Soften crusts in 10% KOH before examination under the microscope. The orange-colored larval stages are seen under low power
  • Other collection methods include acetate tape preparations or a firm toothbrush used to brush material downward onto a glass slide or Petri dish
  • An otoscope provides both a magnifying lens and good light in 1 tool and is useful for ectoparasite identification among the hairs

Pathologic Findings!!navigator!!

Skin biopsies are nonspecific unless fragments or entire parasites are present. Findings include varying degrees of superficial perivascular dermatitis with numerous eosinophils and possible deep lymphoid nodules, a description compatible with both ectoparasites and hypersensitivities.

Treatment ⬆ ⬇

TREATMENT

  • Prevent reinfestations with use of long-acting insecticides or acaricides. Treat all horses on the premises to prevent a single reservoir supplying the mites or lice for reinfestation
  • Outpatient medical management is appropriate for most cases
  • As additional dispersal is by host mobility and transportation of infested hosts, limit the horse's contact with others in pastures, barns, and transportation vehicles
  • Poor nutrition predisposes to many illnesses beyond lice and mites
  • Prevention is achieved by the use of pyrethroid sprays or wipes during periods off premises and aids in avoiding infestations when the horse returns to the barn

Surgical Considerations

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Shampooing the mane and tail with pyrethrin, pyrethroids, lime sulfur, or selenium sulfide products is an initial step in removal of adult lice, or washing the fetlocks in the case of a chorioptic mange infestation. Clipping of the feathers is important in cases of Chorioptes. This in itself is insufficient as eggs will hatch after bathing. Removal of the nits from the mane and tail is beneficial. Long-acting pyrethroid, cypermethrin, or resmethrin sprays or wipes must then be applied to kill hatching larval stages of the arthropod. Fly wipes or sprays with pyrethroid compounds may require weekly application to achieve success
  • Fipronil is not approved for use on horses. Fipronil is highly effective against related arthropods. Application of the spray at 3 week intervals for 3 treatments should be completely effective
  • Ivermectin 200 μg/kg PO given twice with a 14 day interval is effective against H. asini but not W. equi
  • For chorioptic mange, clipping is advised before applying the antiparasitic agents. Ivermectin reduces mite numbers but is less effective. Treat the entire body as the mites can migrate to the neck, trunk, and face
  • For psoroptic acariosis, ivermectin at 200 μg/kg PO given twice with a 14 day interval is effective
  • For control of mange, a combination of a systemic macrocyclic lactone with the application of a topical acaricide is the best treatment
  • Trombiculidiasis is self-limiting if the horse is removed from the pasture; however, using topical acaracides effectively kills the larvae and corticosteroid treatment may be necessary to provide immediate relief from the pruritus

Contraindications!!navigator!!

Amitraz use is contraindicated in horses.

Precautions!!navigator!!

  • Feline susceptibility to pyrethroid compounds is dose dependent and the formulations for use on horses (0.05–0.10%) are rarely of a level to achieve this toxicity, but some topical formulations for dogs and horses reach >50% permethrin, which is toxic to a cat. After applying a pyrethroid product directly to horses, a 2 h drying time generally limits any toxic transfer from the horse to a cat
  • Injectable ivermectin can cause serious side effects in some horses

Possible Interactions!!navigator!!

Not known.

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Observe for clinical signs of reinfestation.

Prevention/Avoidance!!navigator!!

Treat all horses on the premises to prevent reinfestations. Parasiticidal treatments must be combined with isolation of all contaminated horses, avoidance of infested areas, and disinfection of barns and material.

Possible Complications!!navigator!!

Reinfection can occur when using short-acting insecticides or acaricides.

Expected Course and Prognosis!!navigator!!

Excellent prognosis if all horses on a farm are treated.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Age-Related Factors!!navigator!!

Exposure is required, thus more common in juveniles or adults rather than foals or weanlings, but infestation is possible if present on the breeding farm.

Zoonotic Potential!!navigator!!

Lice are species specific. Transient infestations of mites on humans are possible for a matter of hours, but no establishment is possible.

Suggested Reading ⬆ ⬇

Paterson S, Coumbe K. An open study to evaluate topical treatment of equine chorioptic mange with shampooing and lime sulphur solution. Vet Dermatol 2009;20(5-6):623–629.

Author(s) ⬆ ⬇

Author: Rosanna Marsella

Consulting Editor: Gwendolen Lorch

Additional Further Reading ⬆

Click here for Additional Further Reading