Vet Times The website for the veterinary profession https://www.vettimes.co.uk Investigating and treating sweet itch approach and case studies Author : RICHARD MORRIS Categories : Vets Date : April 21, 2014 RICHARD MORRIS Describes the causes of this skin disease in the horse and methods of diagnosis, while various treatment options are illustrated with three case studies Summary Sweet itch is an equine hypersensitivity reaction to the biting insects Simulium and Culicoides. It is the most common allergic skin disease of horses and there is a strong hereditary component with certain breeds being over-represented (Welsh, shire and Icelandic breeds). A seasonal pruritus of the predilection sites (mane and tail base) is the main clinical sign leading to self-inflicted trauma from excoriation, affected individuals usually developing the disease from four to five years of age and getting progressively worse as they get older. The constant skin irritation causes alopecia, ulceration, erosion, excoriation and lichenification of affected areas of skin and the restlessness can interfere with grazing and result in weight loss. Investigation using serology and intradermal skin testing has been well-documented and different treatment approaches including allergenspecific immunotherapy have been tried. Key words equine, skin, allergy, Culicoides SWEET itch is an insect bite hypersensitivity reaction in the horse. The National Sweet Itch Centre says about five per cent of the UK horse population is affected and, in the heavy 1 / 15
horse population, a prevalence of 11.6 per cent has been shown (Littlewood, 1998). It is a serious problem affecting the performance of the horse and should be noted on a prepurchase examination. The insects responsible in the UK are the black fly (Simulium) and the midge (Culicoides; Figure 1), which are active from March to November. The midges swarm at dawn and dusk in wooded areas near slow-moving streams (Figure 2). The females need a blood meal to form and lay their eggs in the soft mud and so settle on any nearby animals to feed. History and clinical presentation The bite is very pruritic and hypersensitive individuals react in an extreme way. Chronic attack by the insects and constant rubbing by the horse on fences, gate posts and so on, leads to excoriation, alopecia, lichenification and hyperpigmentation of the mane (Figure 3), base of the tail ( Figure 4 ), and occasionally the belly. When taking the history it is essential to investigate the husbandry to help rule in or out other differential diagnoses: Is the horse kept in a stable or out at grass? If indoors, what is the type of stall, bedding or hygiene? If outside, what is the condition of the paddock (muddy with abrasive vegetation predisposes to mud fever; lots of trees, slow-moving streams predisposes to sweet itch)? What grooming kit is used on the horse/ what leather treatments are used on the tack? What is the horse used for (hunting, eventing, dressage)? What other horses are in contact and what lesions do they have? What previous treatments has the owner used (for example, Camrosa ointment, tea tree, aloe vera preparations)? Differential diagnosis The list of differentials for pruritus can be broken down into contagious and non-contagious causes (Tables 1 and 2; Pascoe and Knottenbelt, 1999). Because allergies are a diagnosis of exclusion, it is important to carry out a thorough clinical and specific dermatological examination first to rule out other diseases that may have a similar presentation. Laboratory samples for microscopy should be taken, including hair plucks, to look for louse eggs or ringworm damage, skin scrapes to check for mites, stained impression smears to investigate for Dermatophilus and sticky tape strips taken over 2 / 15
the rump to look for worm eggs, such as Oxyuris equi. If there is any suspicion of ringworm, samples for fungal culture should be taken. Treatment Treatment approaches to manage sweet itch can be categorised into medical and management approaches (Tables 3 and 4) and different treatments will be needed to suit individual circumstances. The aim of management approaches is to avoid contact with the allergen (biting flies) and the more effective these are the less medical treatment will be necessary. Case studies Case 1. Hyposensitisation was of no help and control was achieved with allergen avoidance and topical corticosteroids. An eight-year-old Welsh pony gelding was treated with an allergy vaccine. Three intradermal injections were given one week apart followed by a course of floral capsules given once a week. Unfortunately, no improvement was seen and, eventually, control was achieved using a topical steroid spray, fly protective sheet (Figure 5) and keeping the pony in an indoor school throughout the summer months. Case 2. Control achieved with insect repellents and allergen avoidance. A 12-year-old Welsh crossbred pony rubbed its rump constantly, causing alopecia and excoriation (Figure 6). No evidence of ectoparasites or dermatophytes were found. Sweet Itch was diagnosed and initially treated by administrating a short-acting glucocorticoid injection, which reduced the pruritus, but precipitated laminitis. This, fortunately, resolved with supportive care (NSAIDs and frog support for a few days), but any further use of corticosteroid preparations was contraindicated in this individual. Long-term resolution of the symptoms was achieved with insect repellents (benzyl benzoate), a fly protective sheet, moving to an area free from trees and slow-running water, and keeping the pony indoors at dawn and dusk. Case 3. Control achieved by parenteral and topical corticosteroids, oral nicotinamide and allergen avoidance. A six-year old Welsh pony with pruritus and excoriation of the mane and neck (Figure 8) was initially given a short-acting glucocorticoid injection with no ill effects, and then managed with a topical glucocorticoid spray. Oral nicotinamide was introduced into the diet and a fly protective sheet put on resulting in resolution of the symptoms. Conclusion 3 / 15
Insect bite reactions are primarily a Type I hypersensitivity (Hanna et al, 1982) with a strong component of Type IV reaction (Pascoe and Knottenbelt, 1999), hypersensitive animals suffering from even a single or few bites show an extreme response in which pruritus is very significant. Studies using intradermal skin testing (IDT) and serological tests have been carried out (Ferroglio et al, 2006) in an effort to confirm the diagnosis. Some studies have found IDT useful (Lebis, Bourdeau and Marzin-Keller, 2002; Sloet van Oldruitenborgh- Oosterbaan et al, 2009) whereas other studies have found a wide divergence between a positive IDT response and manifestation of clinical signs (Kolm-Stark and Wagner, 2002; Kolm and Wagner, 2004). Serological tests for equine Culicoides salivary antigen have to be specific for immunoglobulin E (IgE) because high levels of anti Culicoides salivary gland immunoglobulin G (IgG) are found in normal and clinically affected animals. Only high levels of IgE are found in sweet itch sufferers and only during the time when clinical signs are apparent while IDT results were positive all year round (Wilson et al, 2001). The results of treatment with allergen-specific immunotherapy (ASIT) have been variable for example, in case one of this article, desensitisation did not prove to be very effective. However, an open study of 10 horses in British Columbia showed significant clinical improvement with ASIT (Anderson et al, 1996), but in two double-blind, placebo-controlled studies (Barbet, Bevier and Greiner, 1990; Ginel et al, 2014) no significant difference between the control group and those given the desensitisation course with the Culicoides extract was seen. However, the allergen preparation, dose, site of injection and duration of treatment may all play a part in the efficacy of immunotherapy. Coppack (2013) suggested weekly intradermal injections with a Culicoides whole body extract was effective in 60 per cent of cases and further research in this area will be useful. Nicotinamide does help some individuals if started early enough in the year before the symptoms develop. It is thought to improve skin barrier function and reduce the production of histamine. However, dietary supplementation with essential fatty acids has been shown to have no benefit (Friberg and Logas, 1999). Affected individuals suffer greatly from the crippling affects of sweet itch. A meticulous approach to history taking and a thorough clinical and dermatological examination is essential to rule out other differential diagnoses (Patterson and Ball, 2013). By explaining the different treatment approaches, and getting commitment from owners, good management can be achieved and effective treatment carried out. Acknowledgement Thank you to Neil McEwan for his advice and assistance in this article. Note some drugs mentioned are not licensed for equine use. References 4 / 15
Anderson G S, Belton P, Jahren E, Lange H and Kleider N (1996). Immunotherapy trial for horses in British Columbia with Culicoides (Diptera, Ceratopogonidae) hypersensitivity, Journal of Medical Entomology 33: 458-466. Barbet J, Bevier D and Greiner C (1990). Specific immunotherapy in the treatment of Culicoides hypersensitive horses: A doubleblind study, Equine Veterinary Journal 22 (4): 232-235. Coppack R (2013). Immunotherapy use for sweet itch, Veterinary Times 43 (46): 16. Ferroglio E, Pregel P, Accossato A, Tarrico I, Bollo E, Rossi L and Trisciuoglio A (2006). Equine Culicoides hypersensitivity: evaluation of a skin test and of humoral response, Journal of Veterinary Medicine, Series A 53 (1): 30-33. Friberg C A and Logas D (1999). Treatment of Culicoides hypersensitive horses with high dose n-3 fatty acids: a doubleblinded crossover study, Veterinary Dermatology 10: 117-122. Ginel P J, Hernandez E, Lucena R, Blanco B, Novales M, Mozos E (2014). Allergenspecific immunotherapy in horses with insect bite hypersensitivity: a double-blind, randomized, placebo-controlled study, Veterinary Dermatology 25: 29-34. Hanna C J, Eyre P, Wells W and McBeath D J (1982). Equine immunology 2: immunopharmacology biochemical basis of hypersensitivity, Equine Veterinary Journal 14 (1): 16-24. Kolm-Stark G and Wagner R (2002). Intradermal skin testing in Icelandic horses in Austria, Equine Veterinary Journal 34: 405-410. Kolm G and Wagner R (2004). FC-53 Serologic allergy testing in horses with insect hypersensitivity, Veterinary Dermatology 15 (4): 37. Lebis C, Bourdeau P and Marzin- Keller F (2002). Intradermal skin tests in equine dermatology: a study of 83 horses, Equine Veterinary Journal 34 (7): 666-672. Littlewood J D (1998). Incidence of recurrent seasonal pruritus (sweet itch) in British and German shire horses, Veterinary Record 142: 66-67. Pascoe R and Knottenbelt D (1999). Manual of Equine Dermatology, W B Saunders. Patterson S and Ball C (2013). A practical approach to equine dermatology, In Practice 35: 190-196. Sloet van Oldruitenborgh-Oosterbaan M M, Van Poppel M, De Raat I J, VanDenBroom R and Savelkoul H F J (2009). Intradermal testing of horses with and without insect bite hypersensitivity in the Netherlands using an extract of native Culicoides species, Veterinary Dermatology 20 (5-6): 607-614. Wilson A D, Harwood L H, Bjornsdottir S (2001). Detection of IgG and IgE serum antibodies to Culicoides salivary gland antigens in horses with insect derma hypersensitivity (sweet itch) Equine Veterinary Journal 33 (7) 707-713. 5 / 15
Figure 1 (inset). The Culicoides midge. 6 / 15
Figure 2. Trees and a slow-moving stream a classic environment to get sweet itch. However, this pony did not have an insect bite allergy and was comfortable in this area despite high numbers of midges appearing at dawn and dusk feeding on it. 7 / 15
Figure 3. Excoriation of the mane of a pony affected by sweet itch. 8 / 15
Figure 4. Alopecia of the rump of a pony affected by sweet itch. 9 / 15
Figure 5. Horse being covered in a protective sheet. 10 / 15
Figure 6. Alopecia and excoriation of the rump from self trauma. 11 / 15
Table 1. Contagious causes of pruritus 12 / 15
Table 2. Non-contagious causes of pruritus 13 / 15
Table 3. Medical approaches to managing sweet itch 14 / 15
Table 4. Management approaches forcontrolling sweet itch 15 / 15 Powered by TCPDF (www.tcpdf.org)