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Sweet itch care: knowledge is power

Author : SUE PATERSON

Categories : Vets

Date : April 27, 2009

Winter’s frost and snow have given many horses a welcome break from the almost interminable torture of insect hypersensitivity.

The most common insectmediated problem in the UK is sweet itch. Sweet itch is a hypersensitivity reaction involving both type-one and type-four reactions to midges’ salivary antigens.

In the UK, the midge species most commonly associated with sweet itch is Culicoides pulicaris. The adult midges are tiny, with a wing length of less than 2mm (Figure 1), which accounts for one of the other terms used to describe them: “no-see-ums”.

The incidence of sweet itch peaks from March to October in the UK (Mellor and McCaig, 1974), in accordance with the feeding habits of the Culicoides species. However, over the past few years, it is the author’s experience that it has not disappeared completely in the winter months. The mild frost-free winters have allowed the midge breeding season to start much earlier.

Sweet itch affects about three per cent of all horses in the UK (McCaig, 1973, 1975). Although horses and ponies of all breeds and types are susceptible, it has long been recognised that some breeds (such as Icelandic horses) appear to have a higher incidence than others (Brostrom and Larsson, 1987).

Glossy equine magazines will soon start to publish their very own “fail-safe” recipes for a midge repellent, and the wholesalers will be plastered with advertisements for the latest protective coats for this season. Repellents and fly rugs (Figure 2) undoubtedly play an important role in the management of insect-mediated diseases, but they are often not enough for very sensitive horses, which are the ones often presented to the primary care equine practitioner. Unfortunately, many owners turn to their vet when their chosen homespun remedies have failed, leaving them with a severely pruritic horse that needs steroid therapy. The reason why horse owners only turn to their veterinarian when all else has failed is that they may not realise practitioners can offer a range of drugs they cannot buy over the counter at their saddlers. Owner education is absolutely paramount in controlling sweet itch. No single drug can solve the problem, and owners should be advised about a ma n a g e - ment strategy that involves midge avoidance and hypersensitivity therapy.

The tables below cite the ways in which both avoidance (Table 1) and therapy (Table 2) can be undertaken.

1 / 8 Therapeutic options

Antihistamine therapy can be used to reduce pruritus in some horses. It is important to remember they are best given to prevent pruritus, rather than trying to treat severe ongoing sweet itch. Chlorpheniramine and have long been recognised as useful drugs in allergic individuals. The dose range for both drugs is detailed in Table 2. Both drugs are first-generation and, as such, can have sedative effects. Therefore, horses may be drowsy after administration, and owners should be awa r e that their animals might be slightly less “footsure” after antihistamines have been given. Cetirazine, a second-generation commonly used in humans, has been investigated for use in horses (Olsen et al, 2008). It has the advantage that it is poorly lipid soluble and only minimally passes the bloodbrain barrier, leading to less sedation. Olsen et al (2008) suggest it has good pharmacokinetic and pharmacodynamic properties when given at a dose of 0.2mg/ kg to 0.4mg/kg twice daily.

Topical therapy with emollients and moisturising agents can be useful in sweet itch, as can topical glucocorticoids. Creams and ointments do not lend themselves well to horses, due to the large body area that needs to be treated and the areas predisposed to sweet itch. Applying creams to a horse’s tail and mane is difficult. Topical sprays designed for use in small animals are useful in horses when utilised via the cascade. The author has found hydrocortisone aceponate spray efficacious when applied to the mane and tail of pruritic horses.

Nicotinamide (niacinamide) is the biologically active pyridine-3- carboxylic amide form of niacin (vitamin B3). Niacinamide has been shown to increase the biosynthesis of ceramides, as well as other intercellular lipids, in the stratum corneum (Tanno, 2000). These lipids are very important in maintaining a functional skin barrier to prevent transepidermal water loss (TEWL).

In fact, in-vivo studies have demonstrated that topical niacinamide increases stratum corneum lipids, resulting in decreased TEWL and an increase in skin-surface water content. The mechanism by which niacinamide affects inflammation is unclear, but it has been shown to inhibit the release of mastcell and inflammatory mediators, and neutrophil chemotaxis. Unpublished open uncontrolled studies in horses with sweet itch have shown nicotinamide has some benefit as an adjunct therapy when used in combination with other measures. Therefore, it may show promise in the therapy of sweet itch but, as yet, requires further assessment in the field and placebo-controlled trials.

Immune modulation

One of the newest concepts in the therapy of sweet itch has been investigated over the past six years by a firm called BioEos. This company has looked at the ability of certain bacteria to modulate the immune response in allergic horses.

Simplistically, the aim of the work has been to try to downregulate the TH2 response (which drives

2 / 8 the allergic reaction) and enhance the TH1 response (which pushes the reaction in the opposite direction). Initial studies in 2003 looked at the treatment of sweet itch in horses with three different bacteria: Rhodococcus coprophilus (BE-R102), Gordonia bronchialis (BE-G101) and Tsukamurella inchonensis (BE-T101). This early work indicated that the horses that had been treated with BE- T101 showed the most improvement in condition out of the three bacterial species tested (Stanford, 2007).

Further studies (undertaken in 2004 and 2005) using these bacteria showed that the horses with clinical signs of sweet itch demonstrated a significant improvement compared to control horses. In these initial trials, the bacilli were administered by intradermal injection. However, subsequent work has shown that when fed by mouth in gelatine capsules, breakdown products from the dead bacilli can be picked up by the multifold cells in the small intestine and carried to the T-cells via the mucosal (McIntyre et al, 2005).

Subsequent trials have looked at a combination of an initial course of intradermal injections, followed by oral administration of bacilli. Results suggest that the bacilli is most effective when given by injection, but that good results can be achieved when the oral gelatin capsules are given every two weeks after an initial priming course of injections. Initial data from early trials suggested a low cure rate of only four per cent, although up to 70 per cent of owners described their horses as being improved. To the author’s knowledge, there are no ongoing trials with the vaccine, which is not available commercially at this stage.

References

Brostrom H and Larsson A (1987). Allergic dermatitis (sweet itch) of Icelandic horses in Sweden: an epidemiological study, Equine Veterinary Journal 19(3): 229-236. Craig J M, Lloyd D H and Jones R D (1997). A double-blinded placebocontrolled trial of an evening-primrose and fish oil combination versus hydrogenated coconut oil in the management of recurrent seasonal pruritus in horses, Veterinary Dermatology, 8: 177-182. Friberg C A and Logas D (1999). Treatment of Culicoides hypersensitive horses with high- dose n-3 fatty acids: a double-blinded crossover study, Veterinary Dermatology 10: 117-122. McCaig J (1973). A survey to establish the incidence of sweet itch in ponies in the United Kingdom, The Veterinary Record, 93: 444-446. McCaig J (1975). Recent thoughts on sweet itch, The Veterinary Annual (15th edn), Wright and Sons, Bristol: 204-206. McIntyre G, Stanford J L, Stanford C A and Bottasso O A (2005). Whole bacterial cells as immune modulator, EP1534330 (European patent). Mellor P and McCaig J (1974). The probable cause of sweet itch in England, The Veterinary Record, 95: 411-415. Olsen L, Bondesson U, Bostrum H, Tjalve H, Ingvast-Larsson C (2008). Cetirazine in horses: pharmacokinetics and pharmacodynamics following repeated oral administration,

3 / 8 The Veterinary Journal 177: 242-249. O’Neill W, McKee S and Clarke A F (2002). Flaxseed (Linum usitatissimum) supplementation associated with reduced skin test lesional areas in horses with Culicoides hypersensitivity, The Canadian Journal of Veterinary Research 66: 272-277. Stanford J L (2007). Review of studies on sweet itch in equines with BET101 carried out under the Home Office and subsequent ATCs. Part of a submission to the VMD. Tanno O, Ota Y and Kitamura N (2000). Nicotinamide increases biosynthesis of ceramides as well as other stratum corneum lipids to improve the epidermal permeability barrier, British Journal of Dermatology 143(3): 524-531.

Figure 1. The Culicoides midge.

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Figure 2. Repellents and fly rugs play an important role in the management of insect- mediated diseases. However, the author believes they are often not enough for the very sensitive horses presented to the primary care equine practitioner.

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6 / 8 Table 1. Means of avoiding Culicoides

Table 2. Means of treating the hypersensitivity reaction

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