Chapter 13 13.1 Diagnostic approach to skin diseases 13.2 Skin diseases characterized by pruritus (itch) Ectoparasitic skin diseases characterized by pruritus Endoparasitic skin diseases characterized by pruritus 13.3 Skin diseases characterized by alopecia, scaling, and/or crusting Infectious skin diseases characterized by alopecia, scaling, and/or crusting Immune-mediated skin diseases characterized by alopecia, scaling, and/or crusting Miscellaneous skin diseases characterized by alopecia, scaling, and/or crusting 13.4 Skin diseases characterized by ulceration and crusting 13.6 Skin diseases characterized by depigmentation 13.7 Congenital and developmental skin diseases 13.8 Miscellaneous skin diseases History: A client questionnaire aids collection of a complete history, especially for complex or chronic cases. Important background information should include age, sex, breed, husbandry (bedding, rugging), feeding (including supplements), local environment, activity, animal contacts, familial details, and general health. Specific disease information should include seasonality, duration, initial lesion appearance/location, degree of pruritus, and therapies and response. Physical examination: The physical examination should be thorough and include the full body, and careful and complete skin examination, assessing: • Mucous membrane colour and abnormalities (e.g. petechiation, ecchymoses or ulcers). • Specific skin lesions, noting distribution (generalized, regional or localized), size, and morphology. Accurate terminology to describe lesions, and characterization as primary or secondary, is important. Primary lesions are caused by the primary disease process, and are the most useful diagnostic clues and sampling sites: • Alopecia: hairloss; may be localized, regional, extensive, patchy, diffuse, well-demarcated (often primary, due to follicular diseases e.g. infectious: dermatophilosis, dermatophytosis, superficial bacterial pyoderma; or non-infectious: alopecia areata, telogen effluvium), or poorly demarcated (often secondary, most commonly associated with self-trauma). • Scale: loose fragments of stratum corneum; may be greasy, white, yellow, adherent, non-adherent; indicates increased epidermal cell turn-over, mostly a non-specific inflammatory response. • Erythema: redness; may be localized, regional, or diffuse; often from increased vascular engorgement, a non-specific inflammatory response. • Crust: dried exudate, serum, pus, blood, cells on surface of skin; form over erosions and ulcerations. Skin scrapings: Skin scrapings allow examination for external parasites, primarily mites. Scrapings should be superficial for surface mites (majority of species of mites affecting horses), collecting ample scale and surface debris over wide areas (e.g. 5 cm2) and/or multiple sites to maximize yield of potentially sparse mites. Paraffin oil / mineral oil is placed on the skin, a scalpel blade (sharp or blunted; No. 10 or 20) is angled to gently scrape the skin surface, and oil and surface scale are transferred onto a glass microscope slide. Superficial scrapings do not need to produce surface bleeding. Areas with papules may give a higher yield of mites. A cover slip is placed on top, and samples examined microscopically with 4× or 10× lens (40× or 100× magnification). Deep skin scrapings are required for deep dwelling mites, especially demodex (very rare in horses). The skin should be squeezed and scraped repeatedly to produce capillary oozing, to ensure mites are extruded from the hair follicles where they reside. Samples should be examined as for superficial scrapings. Skin scrapings can be used to search for dermatophytes; although trichograms or tape impressions, which contain more hair shafts, are generally higher yield tests for animal dermatophytes. Skin cytology: Dry skin lesions are best sampled with acetate tape (e.g. Scotch® tape, or plain, non-patterned clear tape), pressed sticky-side down onto lesional skin (tape impressions). Initial gentle cleansing with water, saline or 0.05% chlorhexidine scrub is important to clear surface debris from dusty coats; or multiple samples should be collected and initial samples discarded. Tape impressions are useful for evaluating superficial bacterial pyoderma, dry forms of dermatophilosis, and dermatophytosis. Direct impression smears are effective for evaluating moist lesions. After initial cleaning of surface debris with saline, a glass slide is pressed firmly onto the affected area. Special techniques may be required when some organisms are suspected (e.g. maceration of crusts in saline for Dermatophilus spp.). Nodular lesions can be sampled with fine needle aspirates; using a 10 ml syringe and 25–21 g hypodermic needle (smaller gauge in more vascular nodules), with gentle suction and needle redirection within the lesion. The aspirated sample (often in the hub of the needle) is gently sprayed onto a glass slide. Slides from impression smears and aspirates should be air-dried and stained (e.g. Diff-quik®; Gram, Giemsa, or Methylene blue sometimes indicated). Tape impressions can be Diff-quik stained, although the author prefers a few drops of the blue Diff-quik stain alone under the tape, fixed at one end on a glass slide, with the stain gently smeared out using tissue paper to create a flat even surface for examination. Oil can be placed directly onto the tape surface. Aspirates, impression smears, and tape impressions should be examined for the presence of inflammatory cells, bacteria, fungi or neoplastic cells. Tape impressions can also be examined for broken hair shafts containing fungal hyphae and/or spores. Scanning with the 4x or 10x lens is important initially to identify areas of inflammatory cells. Oil immersion (1000×) is required for bacterial examination, and 40x will reveal larger fungi and cell morphology. Trichogram: Hairs are plucked from the periphery of alopecic lesions and laid carefully onto 2 or 3 drops of paraffin oil on a glass slide. The proportions of hairs in telogen (rest) and anagen (active growth) phases can be estimated, important information in some follicular growth diseases (e.g. telogen defluxion). Hairs can be examined for shaft abnormalities (e.g. alopecia areata), parasites or fungal elements. Samples do not require clearing with potassium hydroxide (KOH), as the vast majority of veterinary dermatophytes are ectothrix fungi (live on the outside of hair shafts) unlike the common human dermatophytes (endothrix; living within hair shafts). Microbial culture: Microbial isolation and antimicrobial sensitivity testing is often important for accurate treatment of deep microbial skin infections, particularly when more unusual or resistant bacteria (e.g. Nocardia) or fungi may be involved. Skin biopsies collected with sterile technique are ideal for culture. A detailed history and differential diagnoses should be provided to the laboratory, as special techniques and culture media are required for some organisms. While waiting for histopathology results where the aetiology remains unknown, fresh sterile skin biopsies can be stored by laboratories, and the most appropriate culture techniques employed when results are available. Microbial culture is more problematic with superficial skin infections, as normal skin flora or transient contaminants may interfere with culture and/or be pathogenic, and is less commonly indicated. Interpretation of microbial culture results can be misleading, as negative cultures do not exclude infectious disease, and positive cultures may not always indicate organisms responsible for disease. Culture results must be combined with clinical and historical data, cytology findings and often histopathology for correct interpretation. Skin biopsies: Skin biopsies for histopathology are often important for diagnosis of skin disease, but often misused. They should only be considered after formulation of a differential diagnoses list based on historical and clinical data, and after surface cytology tests are performed. Biopsies are not indicated simply faced with chronic disease of unknown aetiology, where secondary infections, self-trauma lesions, and drug effects may mask primary changes; nor with many superficial skin diseases, including hypersensitivities, where a non-specific perivascular mononuclear and/or eosinophilic dermatitis is common. Biopsies are most useful with deep dermal and nodular diseases, and unusual or extensive unexplained lesions. Secondary infections should ideally be treated before biopsy collection, and samples should be collected carefully; no surface disinfection, and delicate handling to avoid artifactual damage. Biopsies may be collected using local (2% lignocaine, placed subcutaneously beneath lesions) or regional anaesthesia, or occasionally general anaesthesia when fractious animals. Multiple biopsies should be collected; primary lesions whenever present (e.g. pustules, papules), and differing secondary lesions (e.g. crusts, alopecia, hyperpigmentation, scale). A minimum of 4 samples, and ideally 6–8, should be collected. Punch biopsies are useful for uniform lesions, papules, small pustules, and crusts. Elliptical excisions are preferable for transitional regions (e.g. border of normal and abnormal), larger pustules/bullae/vesicles, and deep lesions. Samples should be blotted dry, thin samples pressed gently onto a piece of cardboard to prevent curling, and immersed in 10% buffered formalin. The biopsy defects can be closed with non-absorbable suture material. To attain maximum information from the histopathology report, it is vital to provide the pathologist with relevant history, a description of the lesion(s), and your differential diagnosis list. A histopathologist with a special interest or qualifications in dermatohistopathology will often provide more useful information. Specialized tests: Specialized tests are required for evaluating some diseases, in particular hypersensitivities. Intradermal testing is useful for managing atopic dermatitis; although it does not confirm this diagnosis as false positive and negative reactions can occur. Patch testing is required to firmly diagnose contact allergies. Treatment trials, including insect control, elimination diets, and occasionally symptomatic treatment trials are often essential for diagnosis of skin disease. • This group of diseases is very common, and can be frustrating and time-consuming to diagnose, as lengthy treatment trials may be required (e.g. hypersensitivities). However, long-term control is more likely successful after a diagnosis has been confirmed, because treatment can be targeted. • The most common pruritic skin diseases are allergic or parasitic. The prognosis is: • Classically non-pruritic skin diseases may rarely present with pruritus as a major feature (e.g. bacterial pyoderma, dermatophytosis, dermatophilosis). • Primary lesions are often few or absent, and may be masked by secondary lesions. • Secondary lesions commonly include alopecia, scaling, excoriations, and crusting from self-trauma, and lichenification and hyperpigmentation when the condition is chronic. • Skin biopsies are frequently unrewarding; many pruritic skin conditions will result in very similar histopathology changes. • Pruritus: often a key feature may be absent despite heavy burdens; typically see mane and tail rubbing, flank biting. • Lesion distribution: head, neck and thighs. • Primary lesions: typically none. • Secondary lesions: alopecia (patchy, irregular, poorly demarcated, dull dishevelled coats); superficial erosions and/or crusting may be caused by rubbing. • Anaemia can occur in extremely heavy infestations of sucking lice. Diagnosis: Adult lice (3–6 mm long) and/or eggs (‘nits’; 1–2 mm long, attached to hairs) are typically visible; although may be sparse. Biting species, light brown in colour, tend to distribute dorsally (back, mane, tail, neck). Sucking lice, blue-black in colour when filled with blood, tend to distribute ventrally. All in-contact horses should be assumed infected, even if lice are not seen and skin appears normal. Treatment: Adult lice are easily killed with topical synthetic pyrethroids (e.g. cypermethrin or permethrin), although eggs are more resistant, and treatment beyond the typical 3-week life cycle is important. Permethrin full-body sprays for 4–6 weeks are effective treatment. Sucking lice are also killed by ivermectin (200 µg/kg orally). It is vital to treat all in-contact horses. The environment will not need extensive treatment if all horses are treated beyond 4 weeks, however cleaning of grooming equipment is advised especially with heavy infestations. • Pruritus: ranges from intense, with foot-stamping and/or rubbing of lower limbs, to absent. • Lesion distribution: fetlock and pastern areas. • Primary lesions: papules (rare). • Secondary lesions: scale and crusting frequent (mild, focal to extensive); alopecia usually minimal; marked lichenification and swelling can occur with chronicity. Diagnosis: Typical history and clinical signs are suggestive; although confirmation requires superficial skin scrapings. Mites are relatively large (0.3–0.5 mm long), and superficial dwellers, living in surface scale and debris. Scrapings can be difficult to obtain on heavily haired legs; hair should be clipped or parted, and ample surface crusting and scale collected. Mites are usually plentiful, but may be sparse. Treatment and control: Treatment with topical synthetic pyrethroids (e.g. permethrin), or topical fipronil (off-label) is effective. Treatment must be applied thoroughly to all lower limbs of all in-contact horses, and ideally continued once weekly for 4–6 weeks. Ivermectin or moxydectin are ineffective for treatment. Doramectin may be effective in some cases. Topical pour-ons (e.g. epiromectin) are newer options effective in ungulates. Chronic infestation is common in groups of Draught and Shire horses, flaring most typically in winter and settling again over summer. Sustained treatment of all individuals beyond the 3-week life-cycle, followed by quarantine measures and/or repeated prophylactic treatment prior to exposure to untreated horses should be curative. Trombiculidiasis: Trombicula spp, (harvest or chigger mites) are common in many parts of the world, including Europe and Australia. Clinical signs typically occur in late summer and autumn. The adult and nymph stages parasitize plants and invertebrate hosts; and larval stages cause disease. The complete life cycle takes from 50–70 days. Specific clinical signs: Tiny orange-red mites, just visible to naked eye, may be present in the centre of papules. Diagnosis: Demonstration of larval Trombiculid spp. (0.2–0.4 mm long) on skin scrapings is diagnostic, although they are often absent as they feed only briefly. A presumptive diagnosis is often made based on exposure to known Trombiculid areas at classical times of year, and typical physical findings. Infested environments are frequently well-known locally, although populations may vary considerably each season, depending on climatic conditions. Forage mites: A variety of environmental mites from the Acaridae and Pediculoididae families live in hay and grain, and huge population outbreaks can occur when environmental conditions are suitable. These mites feed on animal and human skin, producing irritation and dermatitis. Specific clinical signs: Lesion distribution: muzzle most commonly and ventral areas; may also affect dorsum of horses stabled beneath food storage areas (e.g. hay lofts). Diagnosis: Demonstration of mites (0.3–0.6 mm long) from skin scrapings, combings, or tape impressions; or from forage samples (where they are usually plentiful), is diagnostic. Poultry mites: Dermanyssus gallinae, the red or poultry mite, will occasionally feed on horses and people. Direct or indirect poultry or bird nest contact is required. Adult mites feed on blood, and are visible as red mites when engorged. Diagnosis: Demonstration of mites (large: 0.6–1 mm long) is diagnostic; mites are often easier to find in infected environments than on horses. Sarcoptes: Sarcoptes scabiei mites of horses, once common worldwide, are now extremely rare, although previously common. They have been eradicated from many countries, including USA and UK, and remain absent from others, including Australia. They cause an extremely pruritic dermatosis, similar to sarcoptes infestation in other species. Sarcoptes is a reportable disease in the horse in most countries. Diagnosis: Demonstration of typical sarcoptid mites in superficial skin scrapings is diagnostic, however mites may be sparse. Miticidal treatment trials are indicated despite negative skin scrapings if history and clinical signs are consistent. Psoroptes: Psoroptes spp. rarely produce skin disease in horses in any part of the world. They are surface dwelling mites, feeding on scale and tissue fluids on skin and within ear canals. Transmission is via direct or indirect contact. Diagnosis: Demonstration of mites on skin scrapings or from ear canal samples is diagnostic; (scrapings may require sedation). Mites are large (0.4–0.8 mm long) and readily seen. Diagnosis: Diagnosis can be difficult; and treatment trials often indicated. Microfilaria (0.2 mm long) can be demonstrated in skin scrapings, direct impression smears, or in macerated tissue biopsies after incubation in saline (37°C [98.6°F] for 10–15 minutes). Skin biopsies for histopathology, revealing superficial dermal eosinophilic inflammation associated with microfilaria, are required for confirmation. Multiple skin biopsies may be required to demonstrate microfilaria, which tend to distribute in pockets. Treatment: Oral ivermectin (200 µg/kg) and moxidectin (400 µg/kg) are very effective at killing microfilaria; typically only one treatment is required. An ocular examination is important prior to treatment, and concurrent administration of glucocorticoids is often recommended initially due to frequent exacerbation of cutaneous and ocular lesions associated with microfilarial death. • Tabanid, Chyrssops and Haematopota spp. (horse flies which are up to 25 mm in size are most active on hot humid days; breed on vegetation near water sources). • Stomoxys calcitrans (stable flies which are 5–8 mm in size feed during day; breed in wet straw, bedding, manure). • Haematobia irritans (horn flies which are 3–4 mm in size feed on the ventrum in warm sunny weather, on the dorsum in cool wet weather; breed in manure). • Simulium spp. (black flies; 1–5 mm in size feed dawn and dusk; breed in running water). • Culicoides spp. (biting midges, gnats, sandflies; 1–4 mm in size feed dawn and dusk; breed in damp decaying vegetation or manure) which are an important and common cause of hypersensitivity in horses. • Musca and Hydrotaea spp. (house flies) do not cause direct irritation as they lack biting mouth parts. Diagnosis: Insect hypersensitivity is often presumed based on typical seasonal patterns of pruritus and typical sites of lesions; however, atopic dermatitis, and possibly food hypersensitivities or adverse reactions, may cause identical clinical signs. Histopathology of skin biopsies is non-specific, with a superficial and deep perivascular to interstitial eosinophilic dermatitis that could be consistent with other hypersensitivities or external parasites. Intradermal testing can provide supportive positive results although false positive results may occur. Testing with extracts from the relevant Culicoides species for each region seems important to help minimise false negative reactions. However, firm diagnosis of insect hypersensitivity requires complete response to a thorough insect control trial. The author’s preferred option is permethrin aqueous spray or oil-based wipe-on applied twice daily (dawn and later afternoon) to all affected areas for 4 weeks. Rugs and hoods can be helpful to limit skin exposure; however, spraying is still required adjacent to, and on exposed areas. Complete response, and ideally relapse on re-exposure, confirms the diagnosis. The trial is very time-consuming for owners, but is essential to confirm the diagnosis and ensure that insect control measures are warranted. Treatment: Measures to reduce insect populations and/or avoid insect exposure are essential. Treatment of un-rugged grazing horses is extremely difficult. Insect repellents (e.g. permethrin) should be timed prior to major insect feeding times; frequency of application will depend on other measures in place and current insect populations. Rugs and hoods; stabling at dusk and dawn, ideally with fans, fine-meshing, and/or sustained release insecticides; and environmental control which involves removal of stagnant water, decaying vegetation, and manure will all help. Anti-inflammatory doses of oral prednisolone (0.5–1 mg/kg sid) or injectable dexamethasone (0.05–0.1 mg/kg SC) can give short-term relief when pruritus is severe, although won’t always be effective and side-effects must be considered, especially with long-term use. Insect immunotherapy is currently not widely available, and response is questionable. • Lesion distribution: face, pinnae, ventral abdomen, ventral thorax, legs most commonly; also lateral neck, mane, tail base. • Primary lesions: wheals (uritcaria, pruritic or non-pruritic). • Secondary lesions: poorly-demarcated alopecia, scaling, lichenification, hyperpigmentation; broken hairs on mane and tail.
Dermatology
13.1 Diagnostic approach to skin diseases
Diagnostic tests
13.2 Skin diseases characterized by pruritus (itch)
Ectoparasitic skin diseases characterized by pruritus
Chorioptic mange
Environmental mites
Other rare mites
Endoparasitic skin diseases characterized by pruritus
Onchocerciasis
Oxyuriasis
Insect irritation
Hypersensitivities
Insect hypersensitivity
Atopic dermatitis
