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Common mites and skin disease: an overview

It is becoming easier to treat common ectoparasitic conditions but some risks can be overlooked

20 April 2020, at 9:00am

The common ectoparasitic conditions encountered in general practice include demodicosis, sarcoptic mange, cheyletiellosis, otoacariasis and harvest mite infestations (Table 1). In the last few years it has become much easier to treat these conditions successfully without risk to the pet, owner and environment. However, there are situations when they are overlooked, especially when dealing with more complex dermatological cases. Some of these mites are contagious to in-contact animals and some cause zoonotic infestations.

Content table
TABLE (1) Difference in clinical presentations associated with common mites affecting the skin
Microscopic view of Demodex injai
FIGURE (1) Demodex injai are long-bodied Demodex mites

Demodicosis

Demodex mites are normal residents of canine skin; however, they can cause skin disease when they multiply excessively. In dogs, three species – Demodex canis, D. injai (Figure 1) and D. cornei – have been associated with clinical disease.

Inherited genetic factors and other predisposing factors – including age, poor nutrition, hormonal changes, endoparasite infestation and neoplasia – are associated with the multiplication of the resident mites and subsequent disease. More recently, allergic dogs, patients at a veterinary hospital, were found to have a higher incidence of demodicosis than normal (Bowden et al., 2018). It is not known whether this was due to immunosuppression by the drugs prescribed for the concurrent allergic skin disease, or due to the immune dysregulation associated with the allergy itself. Secondary pyoderma often complicates the condition, resulting in further immunosuppression of the host.

Clinical signs

The clinical forms of demodectic mange are:

  • Localised: fewer than five small circumscribed patches of scaling and alopecia with or without erythema. This form is usually seen in young dogs and the lesions tend to appear around the eyes, muzzle, limbs and/or trunk. Generally, the lesions are non-pruritic, unless there is secondary bacterial infection. In most individuals (90 percent) the lesions will resolve spontaneously (within one to three months); however, in a small number they either persist, or go on to become generalised
  • Generalised: more than five sites are seen on the individual. The clinical signs include multifocal patches of alopecia, scaling with or without alopecia, ulceration, crusting, comedones, hyperpigmentation, follicular casts, papules, pustules, draining sinuses and sometimes nodules in some breeds, such as Bull Terriers and Bulldogs
  • Pododemodicosis occurs in many dogs with generalised demodicosis; however, there are a small number of cases where the feet are the only affected site and occasionally only a single foot is affected. In many cases there is also a concurrent deep pyoderma
  • Adult-onset demodicosis occurs when the condition develops in an adult individual over two years of age which did not have the disease previously. This form can be localised, generalised or restricted to the feet. Adult-onset demodicosis is associated with systemic diseases, such as hyperadrenocorticism (iatrogenic or spontaneous) and neoplasia, and with the use of immunosuppressive drugs
  • Seborrhoea oleosa on the trunk, often accompanied by erythema, hyperpigmentation and sometimes comedones, is seen in West Highland White Terriers (Figure 2), Border Terriers and Fox Terriers. This syndrome is often associated with D. injai
  • Facial pruritus and dermatitis associated with D. injai is seen in some breeds such as Shih Tzus and West Highland White Terriers. Often the dogs have concurrent Malassezia dermatitis and pyoderma
  • The diagnosis is confirmed by microscopy on deep skin scrapings, hair plucks and tape strips. In cases with a purulent exudate, large numbers of mites may be present in the exudate. In dogs with adult-onset demodicosis it is important to look for the trigger, or an associated immunosuppressive condition
Seborrhoeic dermatitis - dog's dorsum
FIGURE (2) Greasy seborrhoeic dermatitis on the dorsum, here seen on a West Highland White Terrier, is associated with D. injai

Treatment

The treatment for demodicosis has been revolutionised in recent years with the availability of licensed products that are easy to administer, and they have excellent efficacy compared to some of the older products.

The isoxazolines, a new class of ectoparasiticide, are potent inhibitors of arthropod nervous systems. They antagonise GABA and glutamate receptors. Currently afoxolaner, fluralaner and sarolaner are the licensed isoxazolines for demodicosis. Amitraz is another licensed product but is rarely used these days given the efficacy of the other products. Moxidectin combined with imidacloprid is licensed but is not always successful in complete resolution. Lotilaner has been reported as efficacious against Demodex mites but is currently unlicensed.

Microscopic image of Sarcoptes scabiei mites
FIGURE (3) Sarcoptic mange is an intensely pruritic condition caused by Sarcoptes scabiei mites

Sarcoptic mange

Sarcoptic mange is an intensely pruritic condition caused by Sarcoptes scabiei var. canis (Figure 3), a mite primarily infesting dogs, but also occasionally cats, foxes and humans. The life cycle of the mite is three to four weeks and involves larvae and nymphs. The adults live for three to four weeks and can survive off the host for a short period if the environmental conditions are right. It is highly contagious, mostly by direct contact, but also through fomites.

Clinical signs

It is a highly pruritic condition and in the more advanced cases, high doses of glucocorticoids, or oclacitinib or lokivetmab, fail to control the pruritus. Typical lesions are erythematous papules and yellow crusts. Lesions often develop on the ventral thorax, elbows, hocks and margins of the pinnae. An itch–scratch reflex on the pinnal margins is present in most dogs with lesions at this site. The disease can become generalised if not treated, exhibiting extensive alopecia, erythema, excoriations, crusts and lymphadenopathy. It is zoonotic.

The diagnosis is based on history, physical examination and skin scrapings. Finding the mite, its eggs or faecal pellets on skin scrapings is diagnostic. The enzyme-linked immunosorbent assay (ELISA) serological test measuring anti-Sarcoptes IgG is shown to have sensitivity ranging between 83 and 92 percent and specificity ranging between 89.5 and 92 percent (Bornstein et al., 1996; Curtis 2001; Lower et al., 2001). False positive reactions are associated with cross-reactivity with house-dust mite antigens and false negative reactions may result from performing the test too soon, or due to the action of glucocorticoids if used.

The parasite can be difficult to find on skin scrapes and one of the best tests, if there is a high index of suspicion, is trial therapy. Trial therapy should be given to any dog suspected of having scabies, even if scrapings are negative as sarcoptic mange is a curable disease.

Treatment

Afoxolaner, fluralaner and sarolaner are the licensed isoxazolines for sarcoptic mange. Weekly amitraz applications, selamectin and moxidectin applied every two weeks (off-label) have been used in the past.

Cheyletiellosis

Dogs are often infested by Cheyletiella yasguri, cats by C. blakei and rabbits by C. parasitivorax, but cross-infestation can occur. The life cycle occurs entirely on the host and lasts three to five weeks. Adult mites live for about 14 days in the keratin layer of the skin and can survive off the host for a few days.

Clinical signs

Excessive scaling on the dorsum is the principal sign, with pruritus being variable. The condition is zoonotic. Diagnosis is based on microscopic examinations of skin scrapings, adhesive tape technique and/or coat brushings.

Treatment

Whilst there are no licensed treatments for cheyletiellosis, the products mentioned for harvest mite treatment below are likely to be effective, in addition to selamectin, moxidectin, ivermectin and amitraz.

Otoacariasis

Otodectes cynotis mites live and multiply mainly in the ear canals where they bite into the epidermis, resulting in irritation and inflammation, and feed on exudate and cerumen. They can survive in the external environment for four to five days. Infestation occurs either by direct contact or from a contaminated environment. These mites are visible to the naked eye (pin-head sized white dots) on otoscopic examination and are found mainly in the ear canal, but may migrate to the neck, gluteal region and tail. Type I and III hypersensitivity responses to mite antigens have been reported.

The life cycle lasts about three weeks. Eggs hatch into six-legged larvae and then into eight-legged protonymphs and deutonymphs.

Clinical signs

Signs are mainly those associated with otitis externa. They include erythema and a brown-black, dry, coffee granule-like exudate in the ear canals. In some cases, pruritus and self-induced alopecia may be seen on the skin around the face and neck. In rare cases, middle ear infestation, through a ruptured tympanic membrane, can occur.

The diagnosis is based on visualisation of mites by otoscopic examination, or by microscopic examination of ear exudate (mounted in liquid paraffin). Mites may be seen on skin scrapings if areas other than the ear canals are involved.

Treatment

Sarolaner, selamectin and moxidectin are licensed for Otodectes cynotis treatment.

Harvest mite infestation

The harvest mite Neotrombicula autumnalis is common in parts of the UK, particularly in areas with chalky soils. The life cycle takes between 50 and 70 days. Adults live in organic decaying material. Eggs hatch into six-legged larvae which are parasitic. These larvae attach to dogs, cats and many other species including humans. The natural hosts are thought to be small rodents such as field mice.

Harvest mites
FIGURE (4) Harvest mites (orange specks) can appear either alone or in clumps on the interdigital skin

Clinical signs

Clinical signs vary in intensity from mild to severe. Pruritus often affects the feet, ear margins and ventral thorax and abdomen. Close inspection may reveal the orange parasite either alone, or in clumps (Figure 4). In the UK, the parasite is generally encountered from July to October, but the length of the season depends on the prevailing environmental factors, such as a mild/severe winter.

Treatment

Harvest mites are difficult to treat and often glucocorticoids are needed to provide relief. Fipronil applied weekly to the predilection sites during the summer and autumn months may prevent attachment to the skin.

Key points

Common reasons for failure to diagnose mites:

  • Assumption that the owner uses routine flea and parasite control on a regular basis, but does not
  • Assumption that the clinical signs are allergic in nature
  • Over-reliance on serum allergy test results
  • Inappropriate site selection for skin scrapes

Common reasons for treatment failures:

  • Failure to treat for long enough, ie two weeks beyond two consecutive negative skin scrapes
  • Resistance to a class of active ingredient
  • Failure to identify and treat concurrent microbial infections
  • Underlying immunosuppression, particularly neoplastic disease
  • Concurrent use of glucocorticoids and immunomodulators such as oclacitinib
  • Failure to treat in-contact pets
  • Failure to treat the environment
  • Improper dilutions or storage of products
  • Poor compliance, when using amitraz

Sarcoptes, Cheyletiella and Otodectes are contagious and so in-contact pets must be treated. These mites can survive in the environment for a few days and so environmental treatment with a permethrin spray is essential.

References
Author Year Title
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Anita Patel, BVM, DVD, FRCVS, is a diplomate and a recognised RCVS Specialist in veterinary dermatology. She has worked exclusively as a dermatologist for the last 15 years and lectures on all aspects of small animal dermatology in the UK, Europe, Africa and Asia.

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