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Asking the experts about skin disease

Challenging dermatology cases and questions are posed to two leading equine skin experts

01 October 2019, at 9:00am

Dealing with skin diseases is an almost daily task for first opinion equine practitioners. Here, two specialised dermatologists give their views on a variety of equine skin issues.

A recurring corticosteroid-responsive case of urticaria in a horse fails to go into remission when the horse is completely removed from the stable environment and fed grass only for two months. How would you approach this case, and what could be the likely causes?

Tim Nuttall, BSc, BVSc, CertVD, PhD, CBiol, MSB, MRCVS, is Head of Dermatology at the Royal Dick School of Veterinary Studies, University of Edinburgh
Tim Nuttall, BSc, BVSc, CertVD, PhD, CBiol, MSB, MRCVS, is Head of Dermatology at the Royal Dick School of Veterinary Studies, University of Edinburgh

TN There is a very wide range of potential triggers, which aren’t mutually exclusive, and horses can have more than one trigger. There are some cases that appear to be idiopathic. Potential triggers include hypersensitivities (indoor and outdoor environmental allergens, foods and insect bite reactions), physical (pressure, water, heat and/or cold), irritant plants (eg nettles) and chemicals (eg oils, soaps, cleaning fluids, creosote, etc), reactions to drugs and vaccines and post-viral infections.

The key is a thorough history and clinical examination to narrow the differentials. It’s important to determine when and where the lesions occur, looking at activity and routine, stable environment, fly control, etc. Allergy testing may be indicated but equine serology tests have not been fully validated and the results must be interpreted with caution.

In particular, they must make sense in terms of likely exposure and the onset of lesions. Treatment options include avoiding the triggers, allergen-specific immunotherapy, glucocorticoids and antihistamines.

Janet Littlewood, MA, PhD, BVSc (Hons), DVR, DVD, MRCVS, is an RCVS Specialist in Veterinary Dermatology at Landbeach, Cambridge
Janet Littlewood, MA, PhD, BVSc (Hons), DVR, DVD, MRCVS, is an RCVS Specialist in Veterinary Dermatology at Landbeach, Cambridge

JL In cases that are persistent or recurrent for a period of six weeks or more, further investigations are indicated. The list of possible triggers to urticaria is lengthy. In a case such as the horse described here, a type 1 hyper-sensitivity reaction to outdoor environmental allergens is likely. A grass-only diet would rule out a food trigger – and food-induced urticaria, although often suspected, is actually rarely confirmed in the horse. A clinical diagnosis of atopic dermatitis is thus justified, and allergen-specific IgE testing to identify causal allergens would be indicated. This might allow effective allergen-avoidance measures and would provide information for selection of allergens for an allergen-specific immunotherapy (ASIT) vaccine – either injections or oral allergy drops. Tree pollens, grass pollens, weed pollens and insect bites have all been implicated in cases of equine atopic dermatitis, although in case series there is very frequently sensitivity to dust and forage mites in addition, and also moulds.

It is usually held by dermatologists that intradermal testing (IDT) is the most sensitive and specific method for identification of allergen-specific IgE, and although there are multiple laboratories offering serum IgE tests for horses, both intra- and inter-assay correlation and correlation with IDT results are poor. Horses with atopic dermatitis with ongoing symptoms of urticaria and/or pruritus will need symptomatic therapy, even if ASIT is undertaken. A recent publication reported 64 percent of atopic horses derived benefit from ASIT, but there is a lag period before benefit is seen. Symptomatic therapy can include antihistamines, glucocorticoids and, in non-responsive pruritic cases, tricyclic antidepressant drugs can be helpful.

FIGURE (1)
FIGURE (1)

The lesion in Figure 1 is found on the lip of this horse. What are the possible causes and what would your treatment be?

TN This is an ulcerative nodule (or nodules). It’s most likely to be inflammatory, with habronemiasis a consideration. A sarcoid or neoplasia is less likely. I’d recommend cytology to determine the nature of the inflammation with careful examination for infectious agents. Biopsy with special stains for bacteria, mycobacteria and fungi would be required if the cytology isn’t diagnostic.

JL The lesion appears to be dermal and breaking through the epidermis, so dermal pathology rather than epidermal or follicular pathology is likely. The aetiology could be either inflammatory or neoplastic, but at this location and depending on the time of year of the lesions, presence or absence of pruritus, then cutaneous larval migration due to deposition of L3 larvae of the nematode parasite Habronema by flies would be quite likely. Other differentials would include:

  • Deep fungal infection, particularly saprophytic
  • Deep bacterial infection, eg atypical mycobacteria
  • Fibrosarcoma
  • Cutaneous schwannoma (neurofibroma)
  • Sarcoid – although absence of an epithelial component makes this unlikely

Microscopical examination of a surface impression smear might be very helpful, with an abundance of eosinophils being a feature of habronemiasis and, occasionally, larvae might be seen. In fungal granulomata it might be possible to detect fungal elements on surface cytology. Histopathological examination of biopsy material with use of special stains plus or minus culture and/or PCR assays would confirm a diagnosis. Treatment would be dependent on that definitive diagnosis, but if habronemiasis were confirmed then treatment would consist of ivermectin or an avermectin, with glucocorticoids to control hypersensitivity to the dying larvae.

You are presented with a horse with a verrucose sarcoid on the outer surface of the pinna of one ear. How would you deal with this case?

TN One option is monitoring; if it’s not progressive, not bothering the horse, has an intact surface without infection and isn’t attracting flies etc, it could be left. If treatment is indicated, I’d start with topical medication. Imiquimod (Aldara) and the AW5 cream from Equine Medical Solutions are both effective and well tolerated.

If topical treatment fails, laser ablation under standing sedation and a regional nerve block may be possible depending on the size and position of the lesion. Brachytherapy may also be possible, but the lesion would need to be reviewed carefully to see whether it is suitable.

JL If the lesion were near the base of the ear then laser surgery would be my treatment of choice. This modality would probably not be a good option for the less fleshy part of the pinna, as there would be risk of damaging the underlying cartilage and poor wound healing. Radiotherapeutic options would require referral to a suitable centre of excellence. Intra-lesion BCG injections are a good option for lesions around the face, but this vaccine can be difficult to source.

Topical cytotoxic products such as blood-root alkaloids and the Equine Medical Solutions AW5 cocktail are contraindicated for use on the head as they may cause quite severe inflammation and necrosis. 5-fluoruracil ointment is less aggressive and can be used on the head. My personal choice would be to use imiquimod (Aldara) cream, which up-regulates local immune function and cytokine production. It is licensed for treatment of human papillomavirus lesions, and also has anti-neoplastic effects. However, its use had been described for aural plaques/viral papillomata on the medial aspect of the pinnae and the paper reported significant inflammation and discomfort resulting in some horses becoming head-shy.

What are the common mistakes made in taking skin biopsies in horses?

TN The commonest mistake is not sampling representative lesions, which can lead to a misdiagnosis and inappropriate treatment. It’s important to obtain primary lesions – if in doubt, take several biopsies from different lesions (most histopathology labs will process at least three biopsies for the same cost). Deeper lesions may need a full thickness incisional biopsy rather than a superficial punch biopsy. Roughly speaking, for inflammatory disease select early lesions and in atrophic disease go for developed lesions.

Where possible, always perform cytology first, as this can help narrow the differential diagnosis and guide the next steps. Cytology consistent with an inflammatory reaction, for example, can prompt requests for special stains for bacterial, mycobacterial and fungal organisms and/or collection of more tissue for culture.

Other important things include supplying a full history, full lesion description (and it’s easy to send digital photos) and your differential diagnosis. Each biopsy should be submitted in a separate pot with enough formalin to completely submerge it. Each pot should be clearly labelled with the biopsy site.

JL There are lots of common mistakes in taking skin biopsies:

  • Prepping the skin prior to taking samples – this removes the surface crusts and stratum corneum, which may contain vital features of the disease and its progression
  • Use of local anaesthetic with adrenaline, which creates artefacts due to altered blood supply
  • Crushing artefacts due to careless handling of samples with forceps
  • Sampling atypical or old lesions – the lesions that look the worst are often not typical of the disease processρFailure to indicate orientation to enable appropriate cutting of sections – a particular problem if biopsies are taken across the junction of normal to abnormal tissue; a line should be drawn on the sample surface to indicate direction of cutting, ie vertically from normal to abnormal
  • Failure to label sample pots adequately – if multiple samples are taken, they are best put in separate, labelled pots of formalinρFailure to save a sample of unfixed tissue in case it is needed for microbiological, immunological or PCR investigation
  • Most importantly, failure to give a full clinical history on the submission form!
FIGURE (2)
FIGURE (2)

A two-year-old Thoroughbred gelding is suffering from biopsy-confirmed pemphigus foliaceus (Figure 2). How would you manage the case?

TN The prognosis for horses with pemphigus foliaceus is usually good. Most cases are idiopathic, but it’s worth reviewing the history for potential triggers such as drugs. Glucocorticoids are the first-line treatment. Prednisolone is licensed for horses and is normally used at 1 to 2mg/kg daily to remission. The dose is then tapered to the lowest every other day dose that maintains remission.

Some horses respond better to dexamethasone (using 2mg tablets or a 2mg/ml injectable solution orally) than to prednisolone even at equipotent doses. Dexamethasone can be given at 0.1 to 0.2mg/kg daily to remission and then tapered to maintenance. It is longer acting so should be given twice weekly for maintenance. Azathioprine can be used if glucocorticoids don’t achieve remission or to help reduce their dose to avoid adverse effects. However, treated horses should be monitored carefully for side effects, owners must be instructed in safe handling of cytotoxic drugs and human exposure through contact with bodily fluids should be considered.

JL Oral glucocorticoids are the mainstay of treatment of pemphigus foliaceus in the horse. I prefer to use oral prednisolone, but dexamethasone can be used. The initial dose for immunosuppression that I use is 1.5 to 2mg/kg bodyweight once daily until no new lesions are apparent and there is significant clinical improvement, for a period of up to three weeks. The dose can then be gradually reduced, by halving the dose on alternate days for a similar period required to achieve remission/improvement, with successive halving of the alternate day dose leading ultimately to every other day dosing.

The prognosis is dependent on the age of the horse, with young animals sometimes achieving complete remission and able to come off therapy. Adult horses or ponies usually require ongoing maintenance therapy, often at a dose rate of 1.0 to 1.5mg/kg every other day.

If glucocorticoids alone fail to secure good improvement then adjunctive therapy with azathioprine or gold salts can be given. I have had good experience with use of sodium aurothiamalate given by deep intramuscular injection on a weekly basis. Other dermatologists report benefit from azathioprine, but my personal experience with this has been disappointing. Whilst most of the cases I have been involved with have done well on therapy, the prognosis is still guarded, as some cases fail to respond to treatment or develop complications and require euthanasia.