Sweet itch

Also known as Queensland Itch, Summer Itch or more technically, "Culicoides" Hypersensitivity.

Sweet Itch is a medical condition in equines caused by an allergic response to the bites of "Culicoides" midges, commonly known as "no-see-ums". It is most commonly found in cobs and native British horses and ponies, although it is not unseen in other equines. It is also found in Canada, Australia, the US and many other parts of the world. [GS Anderson, P Belton, & N Kleider (1988) "The Hypersensitivity of Horses to Culicoides Bites in British Columbia", "Can Vet J." 29(9): 718–723]

Causes

A hypersensitivity reaction to specific allergens (protein molecules causing an extreme immune response in sensitised individuals) in the saliva of "Culicoides" midges (juvenile gnats). There are multiple allergens involved [W. Hellberga, A.D. Wilsonb, P. Mellorc, M.G. Doherra, S. Torsteinsdottird, A. Zurbriggena, T. Jungie and E. Marti (2007) "Equine insect bite hypersensitivity: Immunoblot analysis of IgE and IgG subclass responses to Culicoides nubeculosus salivary gland extract", "Veterinary Immunology and Immunopathology" Volume 113, Issues 1-2, Pages 99-112] , although some workers claim that the larger proteins (of molecular weight 65kDa) are the most important. [E. Ferroglio, P. Pregel, A. Accossato, I. Taricco, E. Bollo, L. Rossi, A. Trisciuoglio (2006) "Equine Culicoides Hypersensitivity: Evaluation of a Skin Test and of Humoral Response", "Journal of Veterinary Medicine", Series A 53 (1), 30–33] These allergens appear to be cross-reactive across many species of "Culicoides" - i.e. many different varieties of midges produce similar allergens, giving the same effects upon horses. [Langner, KFA, Darpel, KE, Denison, E, Drolet, BS, Leibold, W, Mellor, PS, Mertens, PPC, Nimtz, M, Greiser-Wilke, I, (2007) "Collection and Analysis of Salivary Proteins from the Biting Midge Culicoides nubeculosus (Diptera: Ceratopogonidae)", "Journal of Medical Entomology", Volume 44, Number 2, March 2007 , pp. 238-248(11)]

The hypersensitivity response is mediated by IgE, an antibody produced by the horse's immune system which binds the allergens, causing a cascade production of histamine and cytokines which make the horse's skin inflamed and itchy. Of these, histamine appears the most important in the initial phase of reaction. [A. P. Foster, J. McKelvie, and F. M. Cunningham (1998) "Inhibition of antigen-induced cutaneous responses of ponies with insect hypersensitivity by the histamine-1 receptor antagonist chlorpheniramine", "The Veterinary Record", Vol 143, Issue 7, 189-193]

ymptoms

* The allergic reaction develops at the site where the insects feed
* The majority of affected horses show skin lesions affecting the mane and tail and dorsal midline.
* Ventral midline symptoms may also occur.
* Lesions around the ears and head are also common.
* These lesions are characterised by intense pruritus (itching), which results in rubbing and considerable self-trauma. This damage is cause by the horse violently rubbing itself against objects, and causes:
** broken hairs
** broken and sometimes bleeding skin
** bald patches.
* Secondary infections may result [Animal Health TrustEquine Clinics [http://www.aht.org.uk/clinics_equ_disord_sweet.html] ] [PT Colahan, IG Mayhew, AM Merrit & JN Moore, Manual of Equine Medicine and Surgery, Copyright Mosby Inc (1999) (pp. 474-475)]

Treatment & Prevention

Few treatments are fully efficacious once lesions have appeared. The only effective form of treatment is preventitive - i.e. prevent further insect bites - so these techniques will also be discussed here. Treatments generally fall into one of the following categories:

1) Insecticides and Repellents: These may be applied to the horse or its environment. The most commonly used and effective are permethrins [PT Colahan, IG Mayhew, AM Merrit & JN Moore, Manual of Equine Medicine and Surgery, Copyright Mosby Inc (1999) (pp. 474) ] and benzyl benzoate [J Littlewood (1999) "Control of ectoparasites in horses", "In Practice" (BVA Publications Ltd) 21: 418-424] . Citronella has been used, with variable effect. Some sources advocate draining of any stagnant water or pools, to reduce the local breeding potential of the insects [PT Colahan, IG Mayhew, AM Merrit & JN Moore, Manual of Equine Medicine and Surgery, Copyright Mosby Inc (1999) (p. 475)] . Midge numbers are often larger near water sources, for example ponds, bogs, and slow moving water. Moving the horse away from these areas may help to prevent further problems.

2) Barrier Techniques: Rugs etc that prevent flies and midges settling on the animal's skin to bite. These include "Boett Rugs" and fly masks. In addition, thin screens may be placed over stable doors and windows to exclude biting insects. [RR Pascoe, DC Knottenbelt, "Manual of Equine Dermatology" Published 1999, Elsevier Health Sciences (p. 131)] Stabling the horse at times of day when the midges are most prevalent is also quite effective.

3) Immunotherapy: A wide variety immunotherapy and desensitisation protocols have been trialled in attempts to reduce or modify the immune response, with variable success rates. So far, there appear to be none that show a benefit in more than a very small, statistically insignificant, number of cases. The most recent trial results have not yet been published, however, BioEos is confident that the results will be positive and has said that the 2007 series will be the "final" trials for the product. The BioEos vaccination protocol is designed to shift the immune system from a Th2 (theoretically allergy-producing) to Th1 (non-allergy producing) mode. [The National Sweet Itch Centre [http://www.sweet-itch.co.uk/trials.html] ] [BioEos is a trading division of the University College London, the vaccine is under contract to Angel Biotechnology. See [http://www.angelbio.com/news.asp?id=82] ; [http://www.simplycreditreports.com/company-number/05175852.html] and [http://www.angelbio.com] ]

4) Nutritional supplements: Various supplements may be effective in individuals, including fatty acid supplemantation and linseed oil. However, although owners perceived an improvement, this was not bourne out by objective statistical analysis [Friberg, Logas (1999) "Treatment of Culicoides hypersensitive horses with high-dose n-3 fatty acids: a double-blinded crossover study" "Veterinary Dermatology" 10 (2), 117–122] .

5) Symptomatic Control: Control of symptoms to some degree can be achieved with antihistamines (especially hydroxyzine [PT Colahan, IG Mayhew, AM Merrit & JN Moore, Manual of Equine Medicine and Surgery, Copyright Mosby Inc (1999) (p. 475)] , and with corticosteroids, although the potential side effects (e.g. laminitis, immune suppression) make this a less preferred option [ RJ Rose & DR Hodgson, Manual of Equine Practice Copyright WB Saunders (2000) (p. 482)] . In addition, antibiotics may be required to manage any secondary infection.

6) Alternative Medicines: A wide variety of herbal, homeopathic and other alternative remedies have been suggested.

Among the natural remedies suggested are sulfur, wild geranium (as the base for a shampoo), Lavender oil, Aloe vera (to reduce the itching) [The Holistic Horse [http://www.holistichorse.com/shari_bitingbugs.html] ] .

The homeopathic remedy "nosode" made from midge saliva has been suggested [Your Horse Complementary Medicine [http://www.yourhorse.co.uk/nav?page=yourhorse.contentspage&view_resource=6508250] ] , although this may in fact represent a form of desensitisation therapy. "Arsenicum album", "Psorinum" and "Thuja" have also been suggested. [Nick Thompson, Homeopath, quoted in Horse & Hound [http://www.horseandhound.co.uk/horsecare/1370/36522.html] ]

In addition, Acupuncture may be effective. [Greenway Veterinary Acupuncture [http://www.acupuncture-vet.co.uk] ]

Overall, the wide variety of treatments proposed leads to the conclusion that no one method is universally effective.

ee also

Ceratopogonidae (The family of which "Culicoides" is a member)

Culicoides imicola (species of midge)

Culicoides Impunctatus (species of midge)

Veterinary parasitology

External links

* [http://www.homeofrestforhorses.co.uk/pdfs/horsehealth/sweetitch.pdf British Horse Society Sweet Itch Datasheet]

* [http://www.solva-icelandics.co.uk/sweet1.htm Examples of rugs used to protect susceptible horses]

* [http://www.horseandhound.co.uk/horsecare/vet/tag/Sweet_itch Expert advice on treating sweet itch] Horse & Hound magazine (UK)

* [http://www.sweet-itch.com The National Sweet Itch Help Line and the Boett Blanket]
* [http://www.specialtyhorsegear.com Flysheets that help to protect horses against Sweet Itch]

References