Pruritus is the most common manifestation of skin disease in the horse. Pruritus is exhibited in a number of ways including the obvious scratching, rubbing, chewing and biting, but also in more subtle fashion such as head shaking, foot stamping or "irritability".
Pruritus is the most common manifestation of skin disease in the horse. Pruritus is exhibited in a number of ways including the obvious scratching, rubbing, chewing and biting, but also in more subtle fashion such as head shaking, foot stamping or "irritability". When a clinician begins the work up for a pruritic horse, the history should include questions such as length of time of the skin disease; does the condition appear to be contagious? Seasonal? Recurring? Are multiple horses affected? The most common causes of pruritus in horses include hypersensitivity reactions, bacterial and fungal infections, infestations with mites or lice, and irritation or hypersensitivity reactions caused by biting insects. This lecture will cover only some of the more common causes of pruritus in horses seen by this author.
Cutaneous infections (folliculitis)
Two common infections of the skin include bacterial folliculitis and dermatophytosis which may also involve hair follicles. Many different bacteria are known to infect or colonize the skin, and most infections will stimulate some degree of pruritus, however Staphylococcus species accounts for much of the true skin infections seen in horses. Clinically lesions can be localized or widespread and start as a papular to pustular dermatosis which can progress into serous our hemorrhagic crusting lesions. Alopecia is also common and may produce a "moth eaten" appearance to the coat. A Staphylococcal infection may be secondary to other underlying dermatopathies, particularly hypersensitivity reactions. Cytology of lesions (crust or pustules) revealing intracellular coccoid bacteria will confirm the diagnosis. Culture and sensitivity testing is normally only performed if the patient fails to respond to appropriate systemic antibiotics. Unfortunately methicillin resistant Staphylococcal infections are becoming increasingly common around the world in both veterinary and human medicine. At the Veterinary Microbiological Diagnostic Center, the Netherlands, the percentage of methicillin-resistant Staphylococcus aureus (MRSA) isolates found in equine clinical samples increased from 0% in 2002 to 37% in 2008. Their study found that nosocomial transmission occurs in equine clinics and that personnel played a role in the transmission. An increased awareness of this epidemic should motivate all veterinary personnel to utilize more complete sanitation practices between handling patients, especially hand sanitation with frequent washing and antiseptic rinses or gels. If a patient with folliculitis is failing to respond to appropriate, empirically chosen antibiotics, then a resistant strain should be suspected and cultures of an intact pustule, or fresh exudate underneath a crust should be obtained. Topical therapy is also useful when treating superficial skin infections. Chlorhexidene shampoos, mupirocin ointment and 0.4% stannous fluoride gel all have efficacy, especially for localized infections, or as adjunctive therapy with systemic antibiotics. The author commonly examines horses with bacterial folliculitis which have been treated with antibiotics for seven to ten days, instead of the necessary 21 days.
Microsporum and Trichophyton are the two most common genus of ringworm in the horse. Clinically lesions are similar to bacterial induces lesions, although the sites affected are most commonly at points of friction or under saddle or tackle. Cytology (trichogram) of infected hairs is difficult to perform, and most patients are diagnosed with Dermatophyte cultures. Sabouraud's agar is required to recover some of dermatophyte species which infect horses, such as Trichophyton equinum. Sab-Duet™ (Hardy diagnostics) are ideal culture plates since they contain both Sabouraud's on one side, and a DTM on the other. Others advocate placing 1-2 drops of an injectable multi-B vitamin on the media. Due to the large number of saprophytic mold spores on the coat of horses, cleaning the hair coat prior to sampling is recommended. Wiping the coat clean with alcohol, or even washing the site with a gentle shampoo or detergent is recommended to minimize the numbers of contaminants on the culture. Because many cases spontaneously resolve, therapy may be conservative and limited to topical antifungal products, many of which are available in shampoo, spay, ointment and rinse formulations.
One of the most common sources of pruritus in the horse is irritation or allergic reactions to parasites. Common mite infestations include Chorioptes which is most commonly found in the distal limbs of horses, especially those with heavily haired fetlocks. Psoroptes mites in horses will frequently affect the head/ears and or tail head and cause moderate or severe pruritus at these sites. Chigger mites (Trombicula) will affect lower limbs, and the head area of horses which graze grass. In all cases mites are recovered by skin scrapings. Lice (both biting and sucking) are another parasitic cause of pruritus, and can be diagnosed by visualization of eggs (nits) or adults on the skin or hair. The incidence of all these parasites has been reduced with the widespread use of avermectin drugs.
Culicoides can cause dermatitis in any horse simply due to frequent biting (especially ventral midline) resulting in an irritationreaction. When a horse develops a hypersensitivity reaction to Culicoides, a more severe dermatosis will develop. Culicoides feeds mostly at dawn or dusk, and different species prefer to feed on the dorsal vs. ventral midline region of the horse. The flanks and legs are usually spared, which can be a diagnostic clue. Papules and excoriations develop initially which might progress to widespread alopecia or pustules and crust, especially when complicated by a secondary pyoderma. The diagnosis is based on history (time of year associated with Culicoides), physical examination, response to insect control, and ruling out other causes of pruritus, although atopy can potentially mimic insect hypersensitivity. Intradermal testing for Culicoides is sensitive and specific for diagnosing Culicoides hypersensitivity. Ideal therapy includes insect control or insect repellants as well as control of the inflammatory response, usually with glucocorticoids. Treatment of a secondary pyoderma may also be necessary.
The most common cause of pruritus in horses at Dermatology for Animals in Arizona is atopic dermatitis. This prevalence may partly be due to a lower parasite burden in the Southwestern United States. Two presentations are recognized, the pruritic horse, and the horse with urticaria (hives) which may or may not be pruritic. After ruling out other causes of pruritus, a tentative diagnosis of atopy should be considered. Other causes of hives include adverse drug reactions, vasculitis, and even folliculitis or pemphigus foliaceous which may mimic urticaria due to the hairs sticking straight out as opposed to lying flat on the skin.
The International Task Force on Atopic Dermatitis developed guidelines in 2010 for the treatment of atopic dermatitis which involve a multifaceted approach including
• Treatment of acute flares
• Attempt to ID and avoid all triggers of flare
• Improve skin & coat hygiene
• Treat ongoing pruritus with drug therapy
• Allergen specific immunotherapy should be offered when feasible
Even though this is directed towards atopic canines, the principles certainly apply to the atopic horse. The diagnosis of atopic dermatitis is not based on any laboratory or skin test but is based on a combination of signalment, history, clinical signs and the ruling out other causes of inflammatory skin. When attempting to effectively help a patient with atopic dermatitis it is necessary to understand the pathogenesis of the disease, and teach the client these basic concepts. Atopic dermatitis is known to be an inherited type 1 hypersensitivity reaction to percutaneously absorbed antigens. Research is also showing conclusively that epidermal barrier defects contribute to the pathogenesis. Bacterial and yeast infections provide additional antigens or mediators which exacerbate pruritus.
Treatment of acute flares usually requires systemic and possibly topical corticosteroids. Drugs such as antihistamines are often ineffective when dealing with an intense flare. When attempting to indentify and avoid the triggers of the flare, remember that multiple triggers may be present. The atopic horse is likely to be more sensitive or predisposed to developing hypersensitivity reactions to insects, therefore insect control is particularly important. Intradermal skin testing can identify reactions to both airborne allergens as well as insects such as Culicoides.
Improvement of skin and coat hygiene is multifaceted yet simple. Frequent baths or simply hosing down the coat on a frequent basis (weather permitting) is useful for removing allergens from the coat and skin. Several products have been developed for dogs which claim to restore intercellular lipids within the epidermis, thus enhancing the barrier function. They include ceramides with fatty acids (Virbac), Phytosphingosine (Solgenol) and EFAs (Dermoscent). Clinical trials are ongoing, but these products make sense if they are in fact able to restore the epidermal barrier, reduce transepidermal water loss, and reduce percutaneous absorption of allergens. For the horse prone to secondary bacterial folliculitis, the use of shampoos containing chlorhexidine can be useful in prevention o f these infections. Many clients (and veterinarians!) have the misconception that frequent baths will cause adverse reactions to the skin such as excessive drying, when in practice this is quite rare. Overall frequent baths are part of the solution, not part of the problem.
The use of allergy specific immunotherapy (ASIT) or desensitization is underutilized in the treatment of equine atopic dermatitis. In our practice, compared to dogs and cats, horses are easily the "best" responders to ASIT. The skin test tend to be easier to interpret since positive reactions can be more dramatic compared to other species. Improvement in clinical signs also tends to be much faster in an atopic horse compared to the other species, with many patients showing improvement within the first few months. Immunotherapy seems equally helpful for both the pruritic horse as is does for the atopic horse with hives. At our practice, the presence of hives is not a contraindication to administration of an allergen injection. For ASIT to be its most effective, three things need to be determined. The proper "recipe" of the allergen vial, the volume injected, and the frequency of the injections.
We rely on intradermal skin testing exclusively when we work up an atopic horse. The contents of the vial are based partially on the skin test results, but other factors must be considered. It is wishful thinking to expect formulating a vial based exclusively on test results will give maximum results. The correlation between results and the seasonality of the patient should "match". For example a horse which demonstrates the most clinical signs every fall, then weed pollens should be given a higher priority in the formulation of the extract. Another factor to consider is the likelihood of exposure. This requires the clinician, or whoever is formulating the recipe, to have knowledge of the local flora, their pollinating seasons and the bouncy of the pollens and spores. Consideration should be given regarding the quantity of pollen produced. If the clinician is relying on an outside source (such as a serology company) to formulate vials without vital historical and exposure data, the success of the immunotherapy program may suffer. For these reasons referral to a clinician who is trained and experienced in the diagnosis and management of allergic patients should be offered when available. If this specialized service is not available, then it is likely that serology (blood) testing is the only alternative. In such cases a complete history regarding seasonality of clinical signs, where and when the horse is symptomatic (and when not) should all be available to whoever is formulating the recipe.
The other critical components of ASIT which need to be determined is the injection schedule. Allergy specific immunotherapy is most definitely not a "one size fits all" but the optimal schedule needs to be determined for each individual patient. Normally we utilize a "rush" schedule for our immunotherapy where patients are administered 0.25 ml of concentrated (20,000 pnu/ml) extract weekly, with the volume increasing by 0.25 ml each week until a total volume of 1.0-1.5 ml is administered every 2-4 weeks. Through this process the owners are instructed to watch closely for patterns such as more, or less, pruritus. A horse which flares immediately after 1.0 ml of allergen should have the volume reduced. A patient flaring by the third week after an injection should have the frequency of the injections increased. Balancing this volume and frequency requires good communication and a hands-on approach, but will lead to improved success in the immunotherapy program.
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