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Sweet Itch

May 2017

The latest research on this common skin condition

Written by BHS Veterinary Student Champion Mariella Savage

Insect Bite Hypersensitivity, or ‘sweet itch’, as the condition is colloquially known to horse owners, is an allergic dermatitis caused by the bite of the Culicoides midge. This common skin condition is currently the best characterised allergic conditions of horses2, with individual horses’ response to the saliva of the midge being an interesting area of research.
Horse with sweet itch

It is now well understood that the agent behind Insect Bite Hypersensitivity (IBH) are species of the Culicoides biting midge3, one of the smallest blood sucking flies4, with a wingspan of less than 2mm5. Their egg, larval and pupa developmental lifestages require the presence of water to create a warm, moist environment4, making the UK’s summer and autumn months, particularly at dawn and dusk, an ideal environment for their reproduction6. For this reason, sweet itch is often also referred to as ‘Summer seasonal recurrent dermatitis’5, or ‘Seasonal equine allergic disease’7. The Culicoides midge has been an area of growing concern in other areas of veterinary medicine, regarding their role in transmission of arboviruses, causing diseases such as Bluetongue virus which affects livestock8

A key area of recent research has been looking at the allergic immune response of some equines to the salivary antigens of the Culicoides midge, which concluded that certain individuals are genetically predisposed and therefore more likely to develop sweet itch6. These included ponies of  native heritage (e.g. Welsh, Shetland and Connemara), as well as Shire and Icelandic horses9.

Excessive adaptive immune system functions are classified into four different types based on the mechanisms involved10. Therefore, the advancement of identifying 11 Culicoides salivary gland proteins2, which may be responsible for the type 1 and type 4 hypersensitvity6 to allergen antigens, has been a crucial turning point in the development of a potential treatment for this painful and irritating condition. 

Midges biting

Exciting developments, based on the further understanding of the molecular aetiology and pathogenesis of Insect Bite Hypersensitivity, is that the innovation company, Benchmark, is collaborating with the Swiss equine biotech company Evax AG, to develop a vaccine to prevent and treat IBH.  If successful, the vaccine will drastically improve the welfare and quality of life of those horses affected by sweet itch. Although further research and trials need to be completed before the vaccine becomes available, the companies hope to provide this treatment option to European, US and Canadian vets by 2020-202111.

Further advice on sweet itch, including clinical signs and management options, is available from our free leaflets. 

References

1)Bowman, D.D. 2014 Georgis’ Parasitology for Veterinarians. 10th edition. Elsevier Saunders.

2) Schaffartzik, A., Hamza, E., Janda, J., Crameri, R., Marti, E., Rhyner, C. 2012. Equine insect bite hypersensitivity: What do we know? Switzerland. Available from: http://www.sciencedirect.com/science/article/pii/S0165242712000906.

3) Henderson, C. 2010. Your Horse: The Illustrated Handbook to Owning and caring for your horse.  Voyageur Press. Page 106.

4) Carpenter. S. Groschup, M.H., Garros, C., Felippe-Bauer, M.L., Purse, B.V. 2013. Culicoides biting midges, arboviruses and public health in Europe. Available from:
http://www.sciencedirect.com/science/article/pii/S0166354213002076.

5) Moray Coast Veterinary Group. 2017. Sweet itch. Available from: http://www.moraycoastvetgroup.co.uk/services/equine/sweet_itch/

6) Weese, S., Munroe, G.A. 2011.  Equine Clinical Medicine, Surgery and Reproduction. Manson Publishing Page 881.

7) Greenaway, E.C., Sepulveda, M.F., Cunnigham, F.M., Goode, N.T. 2003. Protein kinase C (PKC) isotype profile in eosinophils from ponies with sweet itch and role in histamine-induced eosinophil activation. Available from:
http://www.sciencedirect.com/science/article/pii/S0165242703001612

8) Scott, P. 2017. National Animal Disease Information Service: Bluetongue in cattle and sheep. Available from:
http://www.nadis.org.uk/bulletins/bluetongue-in-cattle-and-sheep.aspx?altTemplate=PDF

9) St Boniface Equine Vets. 2016. What is Sweet itch? Available from:
http://www.stbonifacevets.co.uk/news/what-sweet-itch

10) Tizard, I. and Schubot, R.M. 2016. Excessive Adaptive Responses. Available from: http://www.msdvetmanual.com/immune-system/immunologic-diseases/excessive-adaptive-responses

11) Benchmark holdings. 2016. Benchmark secures innovative new equine vaccine. Available from:
http://www.benchmarkplc.com/articles/benchmark-secures-innovative-new-equine-vaccine/

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